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Multi-stage continuous and intermittent microwave drying of quince fruit coupled with osmotic dehydration and low temperature hot air drying
Accepted Manuscript Multi-stage continuous and intermittent microwave drying of quince fruit coupled with osmotic dehydration and low temperature hot air drying
Jalal Dehghannya, Seyed-Hamed Hosseinlar, Maryam Khakbaz Heshmati PII: DOI: Reference:
S1466-8564(17)30672-0 doi:10.1016/j.ifset.2017.10.007 INNFOO 1868
To appear in:
Innovative Food Science and Emerging Technologies
Received date: Revised date: Accepted date:
15 June 2017 4 September 2017 6 October 2017
Please cite this article as: Jalal Dehghannya, Seyed-Hamed Hosseinlar, Maryam Khakbaz Heshmati , Multi-stage continuous and intermittent microwave drying of quince fruit coupled with osmotic dehydration and low temperature hot air drying. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Innfoo(2017), doi:10.1016/j.ifset.2017.10.007
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ACCEPTED MANUSCRIPT Multi-stage continuous and intermittent microwave drying of quince fruit coupled with osmotic dehydration and low temperature hot air drying Jalal Dehghannyaa,*, Seyed-Hamed Hosseinlara, Maryam Khakbaz Heshmatia
Department of Food Science and Technology, University of Tabriz, Tabriz 51666-16471, Iran
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a
*
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Corresponding author: Tel.: +98 41 33392063; Fax: +98 41 33356005; E-mail address:
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[email protected] (J. Dehghannya)
Abstract
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In recent years, using intermittent microwave (IM) to dry foodstuffs has been taken into consideration as one of the new drying methods in food industry. The aim of this research was to
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dry cubic pieces of pre-treated "quince" fruit by sucrose osmotic solution using IM – hot air (HA) drying at a low temperature (40°C) in order to investigate the effects of this process on
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improving the dried product quality. The variables of the process included sucrose osmotic solution in 5 concentration levels of 0 (control), 10, 30, 50, and 70% (w/w) and microwave at 4
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powers of 0 (control), 360, 600, and 900 W, with 4 pulse ratios of 1, 2, 3, and 4. Findings indicated that compared to control samples, the samples pre-treated by osmotic solution had
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lower effective moisture diffusion coefficient (Deff). However, Deff increased through increase in power and pulse ratio of the microwave. Increasing the concentration of the osmotic solution,
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power, and pulse ratio led to significant decreases in shrinkage. Due to high shrinkage, the quince samples dried by continuous microwave – HA method showed higher bulk density in
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comparison to the samples dried by IM – HA. In addition, samples dried by IM with low power showed the highest rehydration followed by those dried by IM with high power; however, the control samples dried merely by HA had the lowest rehydration. Moreover, a significant decrease in specific energy consumption was noticed through increasing the microwave power. Drying by IM – HA at the power of 900 W and the pulse ratio of 4 had the lowest specific energy consumption, while drying with only HA had the highest specific energy consumption.
Keywords:
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ACCEPTED MANUSCRIPT Continuous and Intermittent microwave, Drying time, Energy Consumption, Quince fruit, Rehydration, Shrinkage
1. Introduction "Quince" fruit, with the scientific name of "Cydonia Oblonga," is from "Rosaceae" family—
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known for its pleasant fragrance and distinctive taste (Doymaz et al., 2015). Like other fruits,
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quince is a perishable fruit with microbiological, chemical, and physical spoilage agents that decrease its shelf life (Akbarian et al., 2014). Quince is an ancient, delicious fruit with a dry and
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fluffy flesh and an almost sour taste. It is generally grown in Argentina, China, Iran, Morocco, Republic of Azerbaijan, Turkey, and Uzbekistan (Doymaz et al., 2015). This fruit is a rich source
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of vitamins A and B, calcareous salts, and tannin. In addition, quince contains glycoside, lipid, and fiber (about 12% on dry basis) and produces 112 kcal per 100 g (Koc et al., 2008). Dried
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"quince" fruit is used in producing jam, marmalade, jelly, and pudding (Noshad et al., 2012). In addition, dried quince is used as an ingredient of traditional dishes in Iran such as quince stew,
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soup and the like (Noshad et al., 2012; Akbarian et al., 2014). Convective HA drying is the most common method used in drying food (Zhao et al., 2014). More than 85% of industrial dryers are
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convective HA dryers (Aghilinategh et al., 2015), but they have big disadvantages such as high energy consumption and longer drying time due to low thermal conductivity of foodstuffs (Zhao
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et al., 2014). Another reason for such disadvantages is the rapid decrease of surface moisture and, as a result, shrinkage of the product, which often results in decrease in moisture and,
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sometimes, heat transfer (Maskan, 2001). Increase in the process time with high temperature results in change in both fragrance and taste of the product. Furthermore, convective HA drying
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leads to reduction of nutritional value and changes the color of foodstuffs (Zhao et al., 2014). Moisture removal during drying is greatly affected by HA conditions and foodstuff characteristics. HA temperature and moisture content of the foodstuff has a significant effect on qualitative characteristics of the foodstuff such as vitamins, smell, taste, color, tissue or nutritional composition during storage (Mota et al., 2010). Drying with HA results in destruction of the compounds which are sensitive to temperature (such as vitamins) and also leads to loss of important sensory features of the dried product. Although high drying temperature decreases the drying time, it reduces the quality of the product, creates thermal damage to the product surface and has higher energy consumption. Using low temperatures in combined microwave – HA 2
ACCEPTED MANUSCRIPT dryer, especially with low microwave powers, increases samples rehydration (Seremet et al., 2016). Although using IM – HA dryer at low temperature prolongs the drying process, it maintains the vitamins, improves the color and tissue of the product and, as a result, the marketability of the final product; for instance, drying temperature considerably affects the content of vitamin C of the dried quince (Mota et al., 2010). In addition, using low temperature results in slow removal of moisture from the product and prevents surface of the samples from
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hardening.
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Using microwave energy in drying foodstuffs is an appropriate method which helps decrease
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defects and overcome the problems which occur during HA drying. Microwave drying improves the drying process without damaging the quality of the final product (Soysal et al., 2009a; Zhao
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et al., 2014; Aghilinategh et al., 2015). It decreases the critical moisture content and increases the drying constant rate period (Tsuruta et al., 2015). Whereas HA drying proves relatively efficient
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in removing the free water from the surface or near the surface of the product, the unique function of microwave energy through volumetric heating is its efficiency in removing the
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interior free water (Andres et al., 2004). In microwave drying, volumetric heating results in moisture transfer from interior parts of the fruit to the surface (Tsuruta et al., 2015). In
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comparison to HA drying, microwave drying has several advantages including faster drying rate and minimum heating in parts with lower moisture content. As a result, overheating decreases in
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the parts in which less heat is required for moisture removal. However, using microwave has some disadvantages such as non-uniform heating, which creates non-uniformity in the drying
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process. Of reasons for non-uniformity in microwave drying, one can refer to lack of samples’ equal exposure to radiation, non-uniformity of the compounds present in the samples, and also
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lack of geometric uniformity of the foodstuffs. A right combination of these two unit operations (microwave heating and hot air drying) may improve the efficiency and economy of the drying process (Andres et al., 2004). There are three methods to combine microwave energy and HA (Andres et al., 2004): 1. Applying microwave energy in the beginning of the drying process: in this method, interior parts of the sample are quickly heated so that its moisture reaches the vaporization temperature. Improving the drying rate in this method is attributed to formation of a porous structure in the foodstuff that facilitates vapor transfer. 2. Applying microwave energy in the middle of the drying process: This is the state in which the drying rate starts to decrease, the foodstuff surface is dry and the moisture is concentrated in the center. Using microwave at this 3
ACCEPTED MANUSCRIPT moment causes volumetric heating and, consequently, vapor pressure. The moisture ultimately reaches the surface and is easily vaporized. 3. Applying microwave energy at the end of the drying process: this is a state during the falling rate drying period or in low moisture content in which vapor outflow can prevent the shrinkage of the structure and tissue of the foodstuff. Using microwave energy in the last stage of drying can be highly effective in removing the bonded water from the product. In contrast, in order to remove the bonded water by HA drying, high
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temperature is required.
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Microwave drying mechanism includes ionic polarization and bipolar rotation. In fact, water
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molecules change their direction, affected by an electrical field, interact with their surrounding molecules, and, as a result, water kinetic energy is converted to thermal energy. In contrast,
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convective energy is the slow transfer of heat from the surface to the internal part of the foodstuff. The effect of microwave energy inside a substance depends on dielectric properties of
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that substance, which can alter heat distribution in the sample. In a small foodstuff sample, the cumulative effect of microwave as a function of time results in overheating in the center of the
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sample (Aghilinategh et al., 2015). One of the important factors in microwave drying is the power density (specific power) of the microwave. It is the power that is applied to the product in
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unit weight. If the power density of the microwave is not controlled, it may damage the product quality. Power density is calculated by using the Ps=P/m equation in which Ps is the specific
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power, P is the fixed power of the microwave and m is the product weight. Product weight is decreased during time and, as a result, specific power increases (Koné et al., 2013). One of the
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advantages of microwave drying in comparison with hot air drying is that due to the formation of porous structure, microwave dryer causes higher rehydration, easier moisture removal, lower
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bulk density and less color change (Aghilinategh et al., 2015). In addition, samples dried with microwave have faster rehydration, which is due to changes in structure and tissue during drying, in comparison to other drying methods (Maskan, 2001). Energy consumption during drying by microwave depends on its power and the moisture content of the foodstuff i.e. by changing moisture content during drying, energy consumption changes as well (Jindarat et al., 2015). In addition, using a microwave with high power decreases drying time, while using a low-power microwave prolongs it. Research findings indicate that both lengthening of the process and using high power damage the foodstuff. Therefore, these two features should be considered with care (Musielak and Kieca, 2014). If microwave drying is not done properly, it results in a low-quality 4
ACCEPTED MANUSCRIPT product (Maskan, 2001). Moreover, since heating by microwave is not uniform, in order to maximize energy efficiency and to overcome some of the limitations of microwave drying, using the two-stage drying process including microwave and then hot drying is useful for producing high-quality dried products. The combined method of microwave – HA results in producing products with low shrinkage and proper rehydration capacity (Maskan, 2001). Microwave – HA drying has been successfully used for some agricultural products such as carrots, apples,
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potatoes, mushrooms, garlics, blueberries, pumpkins, etc. (Soysal et al., 2009b). For example, in
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a study, a new dietary fiber from orange peel was produced by applying two different drying
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methods: HA and HA + microwave. Results showed that 92% reduction in processing time and 77% reduction in energy consumption was achieved with HA + MW (Talens et al., 2017). In
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another study, apple cylinders were dried in a combined HA – microwave system (Andres et al., 2004). Vacuum impregnation with an isotonic solution was used as a pretreatment before drying.
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Dried samples showed different tissue matrices according to the different drying conditions. Vacuum impregnated samples demonstrated higher density, lower porosity and higher volume
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reduction.
Since continuous use of microwave energy results in loss of quality, due to non-uniform
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distribution of heat and moisture, using intermittent microwave (IM) energy has been proposed as a viable solution to this problem (Zhao et al., 2014). IM drying is one of the promising
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solutions for improving energy efficiency and product quality without increasing the drying cost. IM drying is performed by controlling the application of thermal energy. Applying an equal
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amount of energy throughout microwave drying destroys the quality, results in thermal damage to the product surface, and causes thermal energy waste. This happens because in final stages of
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drying, drying rate decreases and, therefore, the sample does not have enough moisture to be removed. This is based on the balance between heat energy produced by water dipoles under microwave radiation (the internal volumetric heat generation) and energy necessary for water evaporation (Campanone and Zaritzky, 2005). The strategy of using IM provides the time for moisture transfer from the center of the sample to its surface during the tempering period. Therefore, both quality and thermal damages can be minimized by using IM drying. The most common type of IM investigated so far is the pulsed (on/off) strategy in which the thermal energy resource is periodically turned on and off. Higher pulse ratio (higher tempering period or off times) results in higher energy saving, but increases the drying time considerably. Increasing 5
ACCEPTED MANUSCRIPT the total drying time may not be suitable for some products. Hence, the degree of pulse ratio should be chosen precisely in order to achieve optimized energy saving. Another important advantage of drying with IM is the uniform distribution of moisture in the sample. Prevention of overheating and damage to dried products, use of limited thermal energy and, as a result, improvement of the efficiency of drying energy are the advantages of intermittent drying (Kowalski et al., 2013). Another way for controlling the quality of dried product and energy
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saving is reducing of microwave power at the final stage of microwave drying process (Andres et
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al., 2004). Due to requiring less drying time, consuming less energy and producing good quality
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products, the combination of IM – HA drying can be an appropriate method in industrial drying applications.
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In addition to the above-mentioned, osmotic dehydration is used to remove a part of water from the foodstuffs such as fruits and vegetables; it is conducted by immersing food in hypertonic
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solutions with high osmotic pressure such as sugar and salt (Dehghannya et al., 2015). When used in osmotic dehydration, sucrose manifests two main beneficial effects that help produce a
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high-quality product. This substance not only acts as polyphenol oxidase effective inhibitor but also prevents the loss of volatile compounds during the osmotic dehydration. Moisture removal
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up to 50% of the fruit’s primary weight is possible through osmotic dehydration; this percent, however, depends on different factors such as temperature, concentration and type of the osmotic
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solution. Osmotic dehydration at room temperature acts as a mild pre-treatment that does not cause cell death except for the cell(s) in the first layers on the tissue surface (Prothon et al.,
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2001). This pre-treatment in the dried ―quince‖ can reduce shrinkage and improve rehydration, and can help obtain a better quality in the final product (Noshad et al., 2012). Moreover, osmotic
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dehydration minimizes the thermal damage to either color and taste, prevents enzymatic browning, and results in better maintenance of the nutrients during drying (Dehghannya et al., 2016b).
Available resources and information indicate that limited studies have generally been conducted on drying ―quince‖ fruit. Literature review show that IM – HA drying during drying the quince fruit pre-treated with sucrose osmotic solution has not yet been investigated. The purpose of this research was to examine the combined effect of the osmotic process and IM – HA drying on the quantitative and qualitative characteristics of quince fruit such as drying kinetics, effective
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ACCEPTED MANUSCRIPT moisture diffusion coefficient, shrinkage, bulk density, rehydration, and specific energy consumption.
2. Material and Methods 2.1. Materials
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Quince fruit (cv. Mulvian) was purchased from a local market in Tabriz, Iran, and before
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performing the experiments, was kept in a cold storage room at 4-6°C. All quinces were bought from the same place and were of the same variety. Sucrose (Miandoab Sucrose Factory, Iran)
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was bought for preparing the osmotic solution and was kept at room temperature. Toluene was used to measure the volume of the samples through liquid displacement method using a 50 ml
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glass pycnometer (Dehghannya et al., 2016a). Distilled water was used for sample blanching, preparing the osmotic solution, and washing the samples after taking them out of the osmotic
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solution and also for measuring rehydration. Absorbent papers were used to dry the surface water of the samples after osmotic dehydration and after taking the samples out of the distilled water to
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measure rehydration.
A combined microwave – HA dryer (LG SolarDom, model SD-3855SCR, Korea) was used with
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internal dimensions of 527×392×480 mm3, the capacity of 38 Liters, and the capability of adjusting the microwave power at 90, 180, 360, 600, and 900 W in the frequency of 2450 MHz.
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The dryer was equipped with air temperature adjusting system between 40 to 230°C and with a turntable tray with turning rate of 2.5 rpm designed for household applications. An oven (Fan
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Azma Gostar Company, Model BM120, Iran) with the capacity of 120 Liters, equipped with an air circulation fan was used to determine moisture content of the samples. A bain-marie (Fan
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Azma Gostar Company, Model WM22, Iran) with the capacity of 22 Liters, equipped with microcontroller with precise temperature control (±1°C) was used for sample blanching.
2.2. Samples Preparation Before performing the experiments, fruits were kept at room temperature to reach equilibrium temperature. Bigger quinces (between 5.5 and 7 cm in diameter) with harder tissues were chosen for the experiments in order to avoid their being crushed when cutting or their tissue breakdown during blanching with hot water. After being washed with tap water, peeling and seed removing, quinces were cut manually into cubic pieces of 1.2 cm with a specially-designed cutter. In order 7
ACCEPTED MANUSCRIPT to prevent moisture loss and color change, samples were kept in a capped Petri dish after cutting and before blanching. After weighing, the samples were blanched inside a beaker in the bainmarie containing hot water at 90°C for 2 min. Then, the samples were immediately cooled at 15°C water for 5 min to remove the extra heat because overheating can result in losing their tissue. Finally, the water remaining on the samples was dried by an absorbent paper (Akbarian et al., 2014; Doymaz et al., 2015). Blanching time and temperature depend on the sample size.
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Therefore, blanching time and temperature of quince pieces were determined through trial and
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error in a way that enzymatic browning does not occur in the given time and temperature.
2.3. Osmotic Dehydration Pre-treatment
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Sucrose osmotic solutions with concentrations of 10, 30, 50, and 70% (w/w) were prepared through dissolving sucrose in distilled water. Quince pieces were then immersed in these
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solutions for 120 minutes at room temperature (25°C). In order to prevent extra dilution of the osmotic solution during time, the ratio of osmotic solution to quince was chosen with the weight
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ratio of 5:1 (w/w). After keeping the samples as long as required, they were taken out of the osmotic solution and washed with distilled water. Their surface water was then dried gently with
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an absorbent paper and then their weight, volume, shrinkage, and moisture content were
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measured on dry basis.
2.4. Drying Experiments
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Drying experiments were performed in order to investigate the effect of IM – HA drying on the quality of dried quince pieces at 4 powers of 0 (control), 360, 600, and 900 W, and 4 pulse ratios
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(PR=1, PR=2, PR=3 and PR=4) as indicated in Table 1. Sample weight (before drying and after osmotic dehydration) was kept constant at 50 g during all the experiments. It is obvious that high microwave power requires shorter time for sample drying. The time required for each microwave power was determined through trial and error in a way that hot spots and, consequently, sample burning were prevented as much as possible. Pulse ratio was calculated using equation 1 (Soysal et al., 2009a; Zhao et al., 2014):
PR
ton toff
(1)
ton
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ACCEPTED MANUSCRIPT where ton is the time when the microwave is on (s) and toff is the time when it is off (s). After drying the samples with microwave, the drying process was continued by HA at 40°C with the entering air velocity of 1 m/s. Drying stopped when the moisture content of quince samples reached 0.2 or lower (g water/ g dry solids). All experiments were conducted with three
2.5. Measurement of quantitative and qualitative characteristics
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2.5.1. Moisture content
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repetitions.
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Quince samples were placed in the oven at 105°C for 24 hours to reach a constant weight. Moisture amount on dry basis was calculated using the following equation (Dehghannya et al.,
MW MS
(2)
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M .C.(d .b.)
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2016c):
where Mw is the water mass (g) and Ms is the mass of sample's dry solids (g). Average initial
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moisture content of quince was 5.33 g water / g dry solids (84% on wet basis).
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2.5.2. Water loss and solid gain during osmotic pre-treatment In order to determine water loss (g water / g product) and solid gain (g solid / g product) of
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samples, 16 quince cubes (4 for each osmotic concentration) were prepared to evaluate the performance of the osmotic pre-treatment. In each of the osmotic solutions (10, 30, 50, and
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70%), 4 quince cubes were immersed. Every half an hour, one sample was taken out of each solution and was weighed after its surface solids were washed away and its surface water was
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dried with an absorbent paper. Water loss and solid gain of the samples were calculated by using equations 3 and 4 (Dehghannya et al., 2017):
WL SG
wi .M i w f .M f
(3)
wi
w f .[1 M f ] wi .[1 M i ]
(4)
wi
where WL is the water loss (g water / g product), SG is the solid gain (g solid / g product), w is the fruit weight (g), M is the moisture content on wet basis (g water / g sample), i is the primary fresh sample before pre-treatment and f is the final sample after pre-treatment. 9
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2.5.3. Effective moisture diffusion coefficient (Deff) Considering the fact that during drying, diffusion is considered as a dominant mechanism of moisture transfer to the surface of the product, moisture ratio was calculated based on the Fick's second law for transient diffusion that is known as Crank equation (Emam-Djomeh et al., 2006): t
(5)
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(2n 1) 2 2 Deff 1 exp 2 ( 2 n 1 ) 4 L2 n 1
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M Me 8 MR t 2 M0 Me
where MR is the moisture ratio (dimensionless), Mt is the moisture content at tth moment on dry
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basis (g water / g dry solids), M0 and Me are, respectively, the initial moisture and equilibrium moisture contents of the sample on dry basis (g water / g dry solids), n is a positive integer, Deff
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is the effective moisture diffusion coefficient (m2/s), L equals half of the sample thickness (m), and t is the drying time (s).
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For long drying periods, only the first part of the series is taken into consideration. Also, regarding the fact that Me is insignificant in comparison with Mt and M0, equation 5 will be as
(6)
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2 De ff Mt 8 MR exp( t) M0 2 4 L2
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follows:
Taking the ln of both sides of the equation 6 gives: t
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2 8 Deff ln( MR) ln 2 2 4 L
(7)
drying time:
Deff
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2 Deff Slope 4L2
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Now, the Deff can be calculated through the slope of the natural logarithm of moisture ratio to
(8)
Slope 4 L2
(9)
2
2.5.4. Shrinkage The volume of the samples was calculated by toluene displacement method, using a 50 ml glass pycnometer and through equations 10 and 11 (Dehghannya et al., 2016a):
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ACCEPTED MANUSCRIPT V Vf
wsf
(10)
s
wsf wt s w f w
(11)
where Vf is the pycnometer volume, wsf is the weight of added toluene to fill the pycnometer, wt+s is the weight of pycnometer along with sample and solvent, wf is the pycnometer's weight, w
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is the sample weight and ρs is the toluene density that is 0.87 g/cm3 at 20°C.
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Shrinkage is the ratio of the initial volume of the fresh sample to the final volume of the dried sample before and after drying, which is expressed through the following equation (Dehghannya
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et al., 2016a):
V S 1 t 100 Vo
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(12)
where S is the percentage of shrinkage (dimensionless), Vt is the sample volume at time t (cm3),
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and V0 is the initial volume of the sample (cm3).
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2.5.5. Bulk density
Weight per unit volume of a substance is called its bulk density. In other words, bulk density is
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the relationship between the weight of the sample and its total volume and is calculated as follows (Dehghannya et al., 2016b):
Mt Vt
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b
(13)
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where Mt is the sample weight (g), and Vt is the sample volume (cm3).
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2.5.6. Rehydration ratio
In order to measure rehydration, dried quince samples were initially weighed and then immersed in 100 ml of distilled water at 25°C for 30 min. Samples were then taken out of the distilled water and the extra water was gently dried using an absorbent paper. Finally, samples were weighed. Rehydration ratio (RR) was calculated through the following equation (Akbarian et al., 2014):
RR
Wr 100 Wd
(14)
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ACCEPTED MANUSCRIPT where Wr is the sample weight after rehydration (g) and Wd is the sample weight before rehydration (g).
2.5.7. Specific energy consumption Specific energy consumption in combined IM – HA dryer (E) is calculated as follows (Singh and
E2
AVa a Ht c m2
(15)
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Ptm PR m1
(16)
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E1
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Heldman, 2014; Chayjan et al., 2015):
H (C p ,a WC p ,v )(Tin Tamb ) W
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(17)
E E1 E2
(18)
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where E1 is the specific energy consumption in microwave (MJ/kg removed water), E2 is the specific energy consumption in the HA dryer (MJ/kg removed water), P is the microwave output
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power (360, 600 or 900 W), tm is the drying time of the sample with microwave (s), PR is pulse ratio, m1 is the amount of moisture removal in microwave drying (kg), A is the area of the
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sample container (m2), Va is the entering air velocity (m/s), ρa is the air bulk density (kg/m3), ΔH is the air enthalpy (kJ/kg dry air), tc is the drying time of the sample with HA (s), m2 is the
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moisture removal during HA drying (kg), Cp,a is the air specific heat (kJ/kg °C), W is the air absolute humidity (kg water vapor /kg dry air), Cp,v is the water vapor specific heat (kJ/kg °C),
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Tin is the temperature inside the dryer (°C), Tamb is the ambient temperature (°C), and λ is the
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latent heat of water evaporation (kJ/kg water vapor).
2.5.8. Statistical analysis A factorial experiment in a randomized complete design 5×4×4 (osmotic solution concentration, microwave power and pulse ratio) was used to investigate the effect of sucrose osmotic solution in five concentrations of 0 (control), 10, 30, 50 and 70%, at 4 microwave powers of 0 (control), 360, 600, and 900 W and 4 pulse ratios of 1, 2, 3, and 4 with 3 repetitions. The effect of each of these variables on qualitative and quantitative characteristics— including effective moisture diffusion coefficient, shrinkage, bulk density, rehydration and specific energy consumption—
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ACCEPTED MANUSCRIPT was analyzed. Mean comparison was performed by using Duncan multiple range test (P < 0.05). Statistical analyses were done by SAS software (version 9.4).
3. Results and Discussion 3.1. Water loss and solid gain during osmotic dehydration pre-treatment In order to accelerate moisture removal and shorten the time needed in drying quince, osmotic
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pre-treatment can be used along with microwave (Noshad et al., 2012). Osmotic dehydration is a
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mild drying method which has been taken into consideration due to its capacity in making use of
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very low temperature and energy (Dehghannya et al., 2006). Since it does not require phase change, osmotic dehydration is considered to be an energy-efficient method. Each kg of water
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requires 2250 kJ heat for vaporization at saturation temperature (100˚C) and pressure (101.3 kPa). Contrary to drying with HA, osmotic dehydration does not need the application of latent
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heat for vaporization and the moisture is removed in liquid form; therefore, it decreases energy consumption.
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The effect of osmotic dehydration on water loss and solid gain of quince samples during the osmotic pre-treatment has been shown in 4 times of 30, 60, 90, and 120 min and in 4
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concentrations of 10, 30, 50, and 70% in Table 2. According to Table 2, increasing the time of samples immersion in the osmotic solution increased water loss and solid gain. In addition, based
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on Table 2, water loss and solid gain increased by increasing the concentration of the osmotic solution. Increasing the sucrose concentration resulted in greater difference in osmotic pressure
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between quince samples and the osmotic solution, which increased water loss (Dehghannya et al., 2006). Moreover, increasing solid gain within the samples by increasing the osmotic solution
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concentration can be the result of membrane swelling that increases cell membrane permeability (Akbarian et al., 2014). Increasing the permeability of the cellular membrane of quince samples by increasing the concentration of the osmotic solution resulted in increasing moisture diffusion from within the samples towards the osmotic solution; it also increased sucrose penetration from the osmotic solution into the samples. Based on Table 2, the highest solid (sucrose) gain (0.183 g solid / g product) belonged to the 70% osmotic solution after 120 min while the least sucrose gain (0.006 g solid / g product) belonged to the 10% osmotic solution after 30 min. In addition, the highest water loss (0.271 g water / g product) belonged to the 70% osmotic solution after 120
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ACCEPTED MANUSCRIPT min while the lowest water loss (0.032 g water / g product) was for the 10% osmotic solution 30 min. In a similar study, Torringa et al. (2001), dried the mushroom samples pre-treated with sodium chloride osmotic solution by using microwave – HA drying. The purpose of this research was to investigate the effect of using sodium chloride solution on the samples dried by the abovementioned method. The concentrations of salt solution were 10 and 15% (w/w), its temperatures
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were 20 and 45°C, and the immersion times were 10, 30, 50, 70 and 110 min. The microwave
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specific power was maintained at about 4 W/g of the sample during the experiments. The ratio of
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sample to osmotic solution was 1:5. Increasing the solution temperature had the highest impact on increasing the rate of moisture removal during osmotic dehydration. Using osmotic solution
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through entering salt ions to the sample modified the dielectric characteristics and resulted in more uniform heating of the product, features which improved through increasing salt
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concentration. Findings indicated that during the immersion of mushroom in salt solution on the basis of the above-mentioned conditions, 30% of the samples water was lost. Subsequently,
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samples drying time by microwave – HA decreased. Use of salt solution decreased shrinkage and increased the porosity of the final product in comparison to non-pretreated samples. Rehydration
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also increased in these samples.
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3.2. Drying kinetics
Fig. 1a shows kinetics of drying the control quince samples (without application of microwave
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energy) in different osmotic concentrations (0 (control), 10, 30, 50, and 70%) using HA. In addition, Fig. 1b (a,b,c,d) shows kinetics of drying quince samples in osmotic concentrations of 0
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(without osmotic pretreatment) 10, 30, 50 and 70%, at power of 360 W and pulse ratios of 1, 2, 3 and 4. As Fig. 1a and Fig. 1b (a,b,c,d) show, initial moisture content decreased by increasing the concentration of the osmotic solution from 0 (control) to 70% (Table 4). This can be attributed to more moisture removal due to increasing the concentration of the osmotic solution during the osmotic pretreatment (Table 2). Moreover, according to Table 4, moisture content of the samples decreased after applying microwave and increasing pulse ratio from 1 to 4. This result could be due to moisture and temperature redistribution in the tissue when the microwave was off and, consequently, facility of moisture removal when the microwave was on (Aghilinategh et al., 2015). In addition, based on Table 4, drying time of the samples with osmotic concentrations of 14
ACCEPTED MANUSCRIPT 10, 30, 50 and 70% was lower in comparison with the control sample, which was due to removal of a part of moisture during the osmotic pre-treatment (Table 2). Soysal et al. (2009b) investigated microwave – HA drying kinetics and its effect on sensory and physical qualities (color and texture) of chili pepper. In this research, microwave drying was performed in two modes of intermittent and continuous with two output powers of 567.20 and 697.87 W. Overall, though continuous microwave – HA drying needed shorter drying time, it led to production of a
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low-quality product in terms of color and texture. However, IM – HA drying resulted in a high-
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quality product. Microwave – HA drying at a temperature lower than 35°C and with the pulse
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ratio of 3 and power of 597.20 W resulted in a significant decrease in drying time in comparison to HA drying. In another research, Kowalski et al. (2013) studied the effect of intermittent drying
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on the quality of dried carrot. Findings indicated that in case constant HA drying was replaced by intermittent drying, a significant impact on the final quality of carrot was observed. In
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comparison with constant HA drying conditions, intermittent drying of carrot proved more effective in improving the quality of the dried product. In intermittent drying, temperature and
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moisture differences were decreased in different parts of the product and their diffusion became more uniform, significantly decreasing the damage to dried products.
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Fig. 1b shows kinetics of drying in osmotic concentrations of 0 (without osmotic treatment), 10, 30, 50 and 70% of sucrose, at powers of 600 (e,f,g,h) and 900 W (i,j,k,l) and pulse ratios of 1, 2,
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3 and 4. Moisture content decreased by increasing the microwave power (Table 4). This can be attributed to the increase of internal vapor pressure through increasing microwave power and,
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consequently, quicker moisture removal (Sharma and Prasad, 2004). In a similar study, the effect of varying microwave power during microwave – vacuum drying on drying time and quality of
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beet root and carrot slices was investigated (Musielak and Kieca, 2014). The first series of experiments included drying at a low, constant microwave power. During the second series of experiments, periodical increase of microwave power was taken into consideration. However, such increases, which occurred in specific times, did not have a considerable effect on the process time and could not hand in dried products of proper quality. During the third series of experiments, initial drying was done with higher microwave power. The process, then, continued with lower applied power. Findings indicated that using a microwave dryer with varying power (the third method) reduced drying time, improved product in terms of less damage to products, less color change and better maintenance of beta carotene. 15
ACCEPTED MANUSCRIPT In addition, the experimental data were fitted with some empirical models for thin-layer drying (Table 3). Performance of the models were evaluated in terms of R2 and RMSE, which represent the coefficient of determination and the root mean square error of non-linear regression analysis, respectively (Tzempelikos et al., 2015). The higher the R2 and lower the RMSE, the better is the goodness of fit. Table 3 shows that among the selected models, the Page model gave the highest R2 with an average value of 0.9897 and the lowest RMSE with an average value of 0.0311,
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considering all the 65 treatments. Therefore, the Page model was selected as a suitable drying
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model to represent the drying behavior of quince slices in a combined microwave – hot air
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convective dryer within the experimental range.
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3.3. Effective moisture diffusion coefficient (Deff)
Deff is one of the significant factors in drying which is dependent on the characteristics of the
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foodstuffs. This parameter describes all possible mechanisms of moisture movement in the foodstuffs including liquid diffusion, vapor diffusion, surface diffusion, capillary flow, and
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hydrostatic flow. The drying is mostly done during falling rate period and moisture transfer in drying is controlled through internal diffusion. Fick's second law that is considered for moisture
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diffusion in transient state is used for describing the drying process in falling rate drying period. Drying with HA decreases Deff due to increase in shrinkage (Sharma and Prasad,
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2004). Awareness of the changes of Deff during drying is required for modeling mass transfer. Fig. 2 shows the moisture ratio on the natural logarithm basis against drying time in drying the
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control samples at 0 power (without microwave application), and in osmotic concentrations of 0 (without osmotic pre-treatment), 10, 30, 50 and 70%. Generally, the slope of the natural
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logarithm of moisture ratio to drying time had a falling rate for all treatments and in the equations of all curves, negative slope was obtained (Fig. 2). Considering the direct relationship between the slope and Deff (equation 9), decreasing the slope reduces Deff. The Deff resulting from drying curve slopes is shown in Table 5. Generally, by increasing the concentration of the osmotic solution from 10 to 70%, significant decreases were noticed in Deff of quince samples in comparison to control samples. This result was in line with findings of Prothon et al. (2001). The low Deff in the tissue of treated samples can be attributed to the presence of sucrose in the tissue and surface of quince cubes that result in resistance against moisture removal. Prothon et al. (2001) reported similar results for samples of apple cubes pre-treated by sucrose solution. 16
ACCEPTED MANUSCRIPT Changes in Deff of garlic cloves during microwave – HA drying were examined in another research in which microwave powers were 10, 20, 30, and 40 W, the temperature was 40, 50, 60, and 70°C, and the air velocity was 1 and 2 m/s (Sharma and Prasad, 2004). Deff varied between 1.29 to 31.68×10-10 m2/s. A 3rd degree polynomial equation was obtained for establishing a correlation between Deff and moisture content. Deff increased by increasing the temperature and microwave power in constant air velocity. In addition, microwave – HA drying resulted in
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decreasing the drying time by about 80-90% in comparison to HA drying. Noshad et al. (2012)
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investigated the effect of ultrasound- assisted osmotic dehydration pre-treatment on Deff during
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quince drying. Sucrose osmotic solution with the concentration of 50.52% was used (the ratio of osmotic solution to fruit was 20:1). The contact time of osmotic solution with quince slices was
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120 min. Deff of quince for non-pretreated samples ranged from 8.114×10-11 to 2.020×10-10 m2/s, and 6.085×10-11 to 1.308×10-10 m2/s for pre-treated samples. This difference was attributed to
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structural changes caused by pre-treatment. Tzempelikos et al. (2014) investigated the effect of drying air conditions on HA drying of quince fruit. Increase in both temperature and air circulation rate decreased the drying time. Deff obtained in different samples was between
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2.67×10-10 and 8.17×10-10 m2/s. Another similar research was an investigation of the effect of
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different pre-treatments on Deff of quince slices in a cabinet dryer (tray dryer) (Doymaz et al., 2015). The shortest drying time was obtained for quince slices pre-treated with citric acid
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solution. Deff obtained was between 6.97×10-11 and 2.38×10-10 m2/s. In another study, drying kinetics of quince slices during HA drying was evaluated (Tzempelikos et al., 2015). Findings
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indicated that the whole process occurred during the falling rate period and that the air temperature had a significant effect on the drying curve. Deff varied between 3.23×10-10 and
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7.82×10-10 m2/s.
In addition, by increasing the pulse ratio from 1 to 4, Deff in quince samples increased (Table 5). The reason for this phenomenon can be the decrease of internal stress which subsequently provides for facility of moisture removal (Aghilinategh et al., 2015). Moreover, by increasing the microwave power from 360 to 900 W, Deff increased (Table 5). This result was in agreement with researches of Sharma and Prasad (2004). The reason for such a result can be attributed to creation of higher internal vapor pressure at the power of 900 W and quicker moisture removal (Sharma and Prasad, 2004). In another research, Dak and Pareek (2014) investigated the effect of microwave power, vacuum pressure and sample mass on changes in Deff of pomegranate seeds 17
ACCEPTED MANUSCRIPT during microwave – vacuum drying. Deff increased by increasing the microwave power. Decreasing the sample weight—due to decrease in the moisture content of the sample— led to increasing Deff; by contrast, changing the vacuum rate had no effect on it. In total, in this study, the increase in Deff, compared to the literature, may be related to both quince porous texture and also applying intermittent microwave (especially at higher pulse ratios). In addition, most of the literature results, reported in this manuscript, are for continuous
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but not for intermittent microwave. As mentioned above, continuous microwave considerably
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decreases the Deff. Moreover, the lower Deffs for intermittent microwave in the literature could be
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related to the specific product texture investigated.
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3.4. Shrinkage
Shrinkage of the foodstuffs during dehydration and drying is a significant problem in the food
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industry and has a remarkably negative effect on the quality of the dried product. Moisture removal caused by applying heat results in tension in the cellular structure of the foodstuffs and,
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consequently, leads to deformation and shrinkage of the product. When shrinkage is not uniform during drying, the surface of the dried product may crack. Shrinkage occurs simultaneously with
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moisture diffusion during drying and may, consequently, affect moisture removal. Shrinkage occurs due to moisture removal and tension in the cellular structure of the foodstuffs. In food
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systems, shrinkage can be rarely ignored; it is, therefore, necessary to take shrinkage into consideration when predicting moisture profile during drying process (Koc et al., 2008). The
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shrinkage created in foodstuffs during drying often results in decreasing the heat transfer and reduces bulk density and rehydration capacity (Tsuruta et al., 2015).
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Fig. 3a shows shrinkage kinetics in control samples (without applying microwave) and in osmotic concentrations of 0 (without osmotic treatment), 10, 30, 50 and 70%. In addition, Fig. 3b (a,b,c,d) shows shrinkage kinetics in different concentrations of the osmotic solution and at the power of 360 W with the pulse ratios of 1, 2, 3 and 4. Considering Fig. 3a and Fig. 3b (a,b,c,d), it was observed that shrinkage after osmotic dehydration pre-treatment and before the drying process (the starting point in the figures) in samples pre-treated with the 70% osmotic solution was more than other samples. This can be the result of higher moisture removal from the 70% sample during the osmotic pre-treatment (Table 2) and, consequently, creation of more tension during moisture removal and, subsequently, more shrinkage in comparison to other samples. 18
ACCEPTED MANUSCRIPT However, at the end of the drying process, a reverse result was obtained in a way that the 70% sample had the minimum shrinkage. Generally, through increasing the osmotic solution concentration from 10% to 70%, shrinkage had significant decrease compared to the control sample. This result corresponded with findings of Prothon et al. (2001) and Koc et al. (2008). In fact, increasing the concentration of the osmotic solution—due to entrance of more sucrose molecules to the quince tissue and filling the interstitial spaces— prevented not only extra
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volume reduction but also shrinkage. Moreover, by increasing the pulse ratio from 1 to 4,
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shrinkage of quince samples, in comparison to control sample, was reduced. In fact, through
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continuous application of microwave energy (with pulse ratio of 1), shrinkage increased due to production of higher amounts of heat and acceleration of moisture removal from the samples'
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tissue (Maskan, 2001). This can also be attributed to decreasing the internal stress along with increasing the pulse ratio (Aghilinategh et al., 2015).
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Fig. 3b shows kinetics of shrinkage of quince samples in osmotic solution with concentrations of 0 (without application of osmotic pre-treatment), 10, 30, 50 and 70% and at powers of 600
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(e,f,g,h) and 900 W (i,j,k,l) and pulse ratios of 1, 2, 3 and 4, respectively. By increasing the microwave power from 360 to 900 W and simultaneously decreasing microwave energy
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application time (Table 1), shrinkage decreased. This result corresponds with findings of Maskan
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(2001).
3.5. Bulk density
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Bulk density is another physical property that is affected by different drying methods of foodstuffs with high moisture content such as fruits and vegetables. Bulk density affects the
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thermo-physical and transfer characteristics including heat and mass transfer (Dehghannya et al., 2016b). Depending on shrinkage, bulk density is changed by decreasing the moisture content in dried products. This change is, in itself, seriously affected by drying methods and can cause considerable changes in chemical compounds, structure and physical characteristics of the foodstuff (Koc et al., 2008). Changes in the bulk density of the foodstuffs which directly depend on different drying conditions have significant effects on the performance of the drying process such as drying rate and also structure of the dried products. Therefore, research on bulk density is of utmost importance for the development of a drying technology (Dehghannya et al., 2016b).
19
ACCEPTED MANUSCRIPT Investigation of the bulk density is important in storage, transportation, and packaging of the food products. Fig. 4a shows kinetics of bulk density in control samples (without microwave application) and in osmotic concentrations of 0 (without osmotic treatment), 10, 30, 50 and 70%. Fig. 4b (a,b,c,d) also shows kinetics of bulk density in osmotic concentrations of 0 (without osmotic pretreatment), 10, 30, 50 and 70% and at microwave power of 360 W and pulse ratios of 1, 2, 3 and
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4. Results showed that the 70% sample had the highest bulk density after the osmotic pre-
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treatment (in the starting point of the diagram), which can be attributed to greater shrinkage in
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the 70% sample in the stage after the osmotic pre-treatment (the starting point of Fig. 3a and Fig. 3b) (Witrowa-Rajchert and Rząca, 2009). However, at the end of the drying process, the 70%
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sample had the lowest bulk density in comparison to other samples. Overall, increasing the concentration from 0 (control) to 70% decreased bulk density. This result corresponds with the
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research findings of Prothon et al. (2001). Decrease in bulk density can be attributed to filling of intracellular spaces of quince samples with sucrose molecules with high molecular weight and the subsequent increase in the tissue resistance against volume and shrinkage reduction
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(Witrowa-Rajchert and Rząca, 2009). Prothon et al. (2001) obtained similar results during apple
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drying. In another research, bulk density and shrinkage of quince fruit were investigated in different moisture contents during drying (Koc et al., 2008). Four methods for drying cubic
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pieces of quince were used as follows: 1) HA drying with two dryers of fluidized bed and tray dryers, 2) infrared and HA drying, 3) Osmotic dehydration pre-treatment before tray dryer, and
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4) Freeze drying. In the first three methods, bulk density increased by reducing the moisture content. In freeze drying method, however, bulk density of quince pieces decreased by reducing
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the moisture content. In contrast to the lowest bulk density noticed in freeze drying, osmotic dehydration obtained the highest bulk density. In freeze drying, ice was sublimated without creating considerable shrinkage in the product. In this research, osmotic dehydration pretreatment was conducted by immersing the samples in a 50% sucrose solution at 40°C for 6 hours, which led to removal of 40 to 50% of the moisture during osmotic dehydration. In addition, bulk density decreased through increasing pulse ratio from 1 to 4. This result, which is in line with findings of Aghilinategh et al. (2015), can be attributed to reduction in shrinkage caused by increasing the pulse ratio (Fig. 3b).
20
ACCEPTED MANUSCRIPT Fig. 4b shows kinetics of the bulk density in osmotic concentrations of 0 (without osmotic treatment), 10, 30, 50 and 70% at microwave powers of 600 (e,f,g,h) and 900 W (i,j,k,l) and pulse ratios of 1, 2, 3 and 4. By increasing the microwave power from 0 to 900 W, bulk density decreased. This decrease corresponds with research findings of Aghilinategh et al. (2015). Decrease in bulk density by increasing the power can be attributed to lower shrinkage (less
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volume reduction) of the samples (Fig. 3b).
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3.6. Rehydration ratio
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Rehydration is a complicated process which aims at reconstructing dried foodstuffs characteristics and can be considered as a criterion for damages to foodstuffs tissue during drying
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(Noshad et al., 2012). Rehydration enables the product to restore the raw material characteristics it has lost during drying. Pre-treatment and drying conditions affect not only the structure and
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composition of the foodstuffs but also their rehydration. Rehydration capacity depends on tissue characteristics, rehydration method, and the pre-treatments applied to the product. Rehydration in
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dried plant tissues includes three simultaneous processes: 1) Water absorption into the dried product, 2) Product swelling, 3) Solid soluble substances leaking to the environment. The aim of
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research on rehydration is to obtain products with natural characteristics at the maximum possible rate (Noshad et al., 2012).
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Fig. 5a shows rehydration kinetics in control samples (without microwave application) in osmotic concentrations of 0 (without osmotic treatment), 10, 30, 50 and 70%. In addition, Fig. 5b
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(a,b,c,d) shows kinetics of rehydration in osmotic concentrations of 0 (without applying osmotic pre-treatment), 10, 30, 50 and 70% and at power of 360 W and pulse ratios of 1, 2, 3 and 4. By
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increasing the concentration of the osmotic solution from 10 to 70%, rehydration increased. Since rehydration and shrinkage have a negative relationship (Krokida and Philippopoulos, 2005), reduction of shrinkage due to increasing the concentration of the osmotic solution (Fig. 3b) resulted in increase in rehydration. In a similar study, optimization of osmotic solution during dehydration of quince fruit was investigated by response surface methodology (Akbarian et al., 2014). Findings indicated that fructose concentration had a significant effect on water loss. The solution containing 47.68% fructose, 3.99% calcium chloride, and 3.49% citric acid was chosen as the optimized osmotic solution. Electron microscope images showed that osmotic dehydration increased porosity. Osmotically dehydrated samples showed higher rehydration in 21
ACCEPTED MANUSCRIPT comparison to non-treated samples. In another study, Maskan (2001) investigated the effect of HA drying, microwave drying and a combination of these two on shrinkage and rehydration of kiwi fruit during drying. In this research, air velocity of 1.29 m/s, temperature of 60°C, and power of 210 W were used. Findings indicated that drying kiwi by microwave is faster than HA drying or microwave – HA. In comparison with HA method, drying time decreased by 89% in microwave, and by 40% in microwave – HA drying. Kiwi shrinkage during microwave drying
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was more than HA drying, a fact which was due to overheating and consequent acceleration of
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moisture removal from the sample tissue during microwave energy application. The lowest
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shrinkage was observed in microwave – HA drying, which was due to increase in drying rate and considerable decrease in drying time. Kiwi slices dried by microwave showed lower rehydration
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capacity in comparison to other drying methods, which was because of changes in the structure and tissue of the samples during drying. Furthermore, due to low shrinkage, rehydration capacity
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of the samples dried by microwave – HA was the highest. Overall, microwave – HA drying handed in the best product quality with the lowest shrinkage and highest rehydration.
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In addition, increasing the pulse ratio from 1 to 4 increased rehydration. Increase in rehydration can be attributed to reduction of shrinkage through increase in the pulse ratio (Fig. 3b). This
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result is in line with the findings of Aghilinategh et al. (2015). Fig. 5b shows kinetics of rehydration in osmotic concentrations of 0 (without osmotic treatment),
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10, 30, 50 and 70% and at powers of 600 (e,f,g,h) and 900 W (i,j,k,l) and pulse ratios of 1, 2, 3, and 4. By increasing the microwave power from 360 to 900 W, rehydration in quince samples
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increased, compared to the control sample. This result was in line with the findings of Seremet et al. (2016). The reason for this can be attributed to the increase of internal vapor pressure which
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occurs through increase in the microwave power and creation of a more porous structure due to vapor movement within the food product (Aghilinategh et al., 2015). In a similar study, Aghilinategh et al. (2015) investigated the effect of IM and continuous microwave – HA on kinetics of drying apple slices as well as physicochemical characteristics (Deff, bulk density, rehydration, and color change) of the dried product. The variables studied in this research included temperature (between 40 to 80ºC), microwave power (200-600 W), air velocity (0.5-2 m/s) and pulse ratio (2-6). Findings indicated that in comparison with HA, intermittent and continuous microwave dryings improved both drying kinetics and physicochemical characteristics. The lowest Deff belonged to HA drying. Continuous microwave drying had the 22
ACCEPTED MANUSCRIPT shortest drying time and the highest Deff. Since continuous microwave drying had a faster drying process in comparison to other methods, samples produced by this method manifested the lowest amount of color change. The drying time in IM drying was about three times more than continuous microwave drying. The bulk density decreased significantly by increasing microwave power, pulse ratio, temperature and HA velocity. Samples produced by IM drying had the lowest bulk density and the highest rehydration. In another research, optimization of process parameters
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to produce instant brown rice (easily cooked rice) was done by microwave – HA drying (Le and
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Jittanit, 2015). Findings indicated that high drying rate resulted in production of a desirably
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spongy and porous structure in the rice seed which, in turn, improved rehydration in the product. The microwave power level had a significant impact on the process in a way that the highest
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porosity and, consequently, the highest rehydration were obtained at the power of 595 W.
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3.7. Specific energy consumption
Another important aspect in evaluating the drying process is the specific energy consumption.
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Successful drying processes result in product quality maintenance and minimize energy consumption. During microwave heating, energy consumption changes by changing the moisture
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content and microwave power. Specific energy consumption is the energy required for removing 1 kg water from wet products (Tarhan et al., 2011). In other words, specific energy consumption
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is defined as the total energy applied during drying to the moisture content removed in the process (Jindarat et al., 2015). Physical and chemical characteristics of wet products (such as size
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and moisture content) and air conditions (the temperature used in drying and air circulation rate) affect specific energy consumption (Tarhan et al., 2011).
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Various magnitudes of specific energy consumption in drying control and pre-treated samples with HA and the combined method of microwave – HA at different powers and pulse ratios are shown in Table 6. By increasing the concentration of the osmotic solution from 10 (without osmotic pre-treatment) to 70%, specific energy consumption generally increased compared to the control sample (0% concentration). This can be attributed to the increase of sucrose concentration in the fruit tissue through increasing the concentration of the osmotic solution (Table 2). In other words, through increase of sucrose in the fruit tissue and increase in internal resistance against moisture removal, specific energy consumption increased. Moreover, by increasing the pulse ratio from 1 to 4, specific energy consumption decreased. This can be 23
ACCEPTED MANUSCRIPT attributed to decrease in drying time through increasing pulse ratio (Table 4). This result corresponds with findings of Soysal et al. (2009a). Furthermore, by increasing the microwave power from 360 to 900 W, specific energy consumption decreased compared to the power of 0 (control). This result is in line with findings of Jindarat et al. (2015). The reason for this result was decreasing microwave application time by increasing its power (Table 1). In a similar study, Zhao et al. (2014) applied the five following methods for drying carrot slices: 1) HA drying
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(60°C), 2) Drying with low microwave power (140 W) after HA drying (60°C), 3) HA drying
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(60°C) after drying with high microwave power (175 W), 4) Drying with low microwave power
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(140 W) after drying with high microwave power (175 W), and 5) IM – HA drying (60°C). Findings indicated that method 5 needed shorter drying time and lower energy consumption and
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produced the highest quality product in terms of color and rehydration in comparison to the other
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four methods.
3.8. Images of dried samples
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Fig. 6 shows, as samples, images of dried control and pre-treated quinces with different osmotic solution concentrations when drying with IM at powers of 360 (with pulse ratio of 1) and 900 W
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(with pulse ratio of 4), respectively. Images showed that the surface structure of the samples that were pre-treated in low sucrose concentrations was considerably damaged during drying.
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However, the structure of the samples pre-treated with osmotic solution of higher concentrations manifested less appearance damage. Moreover, as these images show, increasing the
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concentration of the osmotic solution decreased shrinkage and improved the appearance of the samples. This might be due to the beneficial effects of sucrose during osmotic dehydration that
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help produce a high-quality product. Sucrose acts as polyphenol oxidase inhibitor and also prevents the loss of volatile compounds during osmotic pretreatment (Prothon et al., 2001). Moreover, increasing solid gain by increasing the sucrose concentration (Table 2) can be the result of cellular membrane swelling of quince samples (Akbarian et al., 2014). This, in turn, prevented extensive shrinkage (Fig. 3b). The lower shrinkage at the higher sucrose concentrations could also be related to the entrance of more sucrose molecules to the quince tissue and filling the interstitial spaces.
4. Conclusion 24
ACCEPTED MANUSCRIPT A multi-stage continuous and intermittent microwave (IM) drying of quince fruit coupled with osmotic dehydration and low temperature HA (HA) drying was developed. Increasing the concentration of the osmotic solution and samples’ immersion time led to increase in water loss and solid gain. Quince samples pre-treated in sucrose osmotic solution had a shorter drying time in most of the treatments in comparison to control samples. Effective moisture diffusion coefficient (Deff) increased through increasing the microwave power and pulse ratio, while it
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decreased by increasing the concentration of the osmotic solution. Moreover, samples shrinkage
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decreased by increasing the concentration of the osmotic solution, microwave power and pulse
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ratio. In addition, samples dried by low-power IM showed the highest rehydration followed by samples dried by high-power IM. The control samples dried by only HA had the lowest
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rehydration. Furthermore, bulk density decreased by increasing the concentration of the osmotic solution, microwave power and pulse ratio. Due to the long time needed for drying, the highest
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specific energy consumption was for drying the control sample, which had undergone no osmotic pre-treatment or IM. Drying with IM – HA at the power of 900 W and the pulse ratio of 4 had
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the lowest specific energy consumption, while drying with merely HA, required the highest energy consumption. Overall, specific energy consumption decreased through increasing
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microwave power and pulse ratio.
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Le, T. Q., & Jittanit, W. (2015). Optimization of operating process parameters for instant brown rice
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production with microwave-followed by convective hot air drying. Journal of Stored Products Research, 61, 1-8.
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Maskan, M. (2001). Drying, shrinkage and rehydration characteristics of kiwifruits during hot air and microwave drying. Journal of Food Engineering, 48(2), 177-182.
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Mota, C. L., Luciano, C., Dias, A., Barroca, M. J., & Guiné, R. P. F. (2010). Convective drying of onion: Kinetics and nutritional evaluation. Food and Bioproducts Processing, 88(2), 115-123.
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Musielak, G., & Kieca, A. (2014). Influence of varying microwave power during microwavevacuum drying on the drying time and quality of beetroot and carrot slices. Drying Technology,
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32(11), 1326-1333.
Noshad, M., Mohebbi, M., Shahidi, F., & Mortazavi, S. A. (2012). Kinetic modeling of rehydration
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in air-dried quinces pretreated with osmotic dehydration and ultrasonic. Journal of Food Processing and Preservation, 36(5), 383-392.
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Prothon, F., Ahrné, L. l. M., Funebo, T., Kidman, S., Langton, M., & Sjöholm, I. (2001). Effects of combined osmotic and microwave dehydration of apple on texture, microstructure and rehydration characteristics. LWT - Food Science and Technology, 34(2), 95-101. Seremet, L., Botez, E., Nistor, O.-V., Andronoiu, D. G., & Mocanu, G.-D. (2016). Effect of different drying methods on moisture ratio and rehydration of pumpkin slices. Food Chemistry, 195, 104-109. Sharma, G. P., & Prasad, S. (2004). Effective moisture diffusivity of garlic cloves undergoing microwave-convective drying. Journal of Food Engineering, 65(4), 609-617.
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ACCEPTED MANUSCRIPT Singh, R. P., & Heldman, D. R. (2014). Introduction to Food Engineering (Fifth ed.). New York: Academic Press. Soysal, Y., Arslan, M., & Keskin, M. (2009a). Intermittent microwave-convective air drying of oregano. Food Science and Technology International, 15(4), 397-406. Soysal, Y., Ayhan, Z., Eştürk, O., & Arıkan, M. F. (2009b). Intermittent microwave-convective drying of red pepper: Drying kinetics, physical (colour and texture) and sensory quality.
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Biosystems Engineering, 103(4), 455-463.
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Tarhan, S., Telci, İ., Taner Tuncay, M., & Polatci, H. (2011). Peppermint drying performance of
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contact dryer in terms of product quality, energy consumption, and drying duration. Drying Technology, 29(6), 642-651.
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Torringa, E., Esveld, E., Scheewe, I., van den Berg, R., & Bartels, P. (2001). Osmotic dehydration as a pre-treatment before combined microwave-hot-air drying of mushrooms. Journal of Food
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Engineering, 49(2), 185-191.
Tsuruta, T., Tanigawa, H., & Sashi, H. (2015). Study on shrinkage deformation of food in
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microwave-vacuum drying. Drying Technology, 33(15-16), 1830-1836. Tzempelikos, D. A., Vouros, A. P., Bardakas, A. V., Filios, A. E., & Margaris, D. P. (2014). Case
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studies on the effect of the air drying conditions on the convective drying of quinces. Case Studies in Thermal Engineering, 3, 79-85.
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Tzempelikos, D. A., Vouros, A. P., Bardakas, A. V., Filios, A. E., & Margaris, D. P. (2015). Experimental study on convective drying of quince slices and evaluation of thin-layer drying
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models. Engineering in Agriculture, Environment and Food, 8(3), 169-177. Witrowa-Rajchert, D., & Rząca, M. (2009). Effect of drying method on the microstructure and
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physical properties of dried apples. Drying Technology, 27(7-8), 903-909. Zhao, D., An, K., Ding, S., Liu, L., Xu, Z., & Wang, Z. (2014). Two-stage intermittent microwave coupled with hot-air drying of carrot slices: Drying kinetics and physical quality. Food and Bioprocess Technology, 7(8), 2308-2318.
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ACCEPTED MANUSCRIPT Table 1 On / off times at different microwave powers and pulse ratios 360 W 600 W Pulse Time (min) ratio 1 10 min on (10 min total) 4 min on (4 min total) 1 min on / 1 min off (20 min 1 min on / 1 min off (8 min 2 total) total) 1 min on / 2 min off (30 min 1 min on / 2 min off (12 min 3 total) total) 1 min on / 3 min off (40 min 1 min on / 3 min off (16 min 4 total) total)
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Table 2 Water loss and solid gain during osmotic dehydration of samples at different concentrations of osmotic solution and immersion times Concentration of osmotic solution Immersion time Water loss (g water / g Solid gain (g solid / g (% w/w) (min) product) product) i i 30 0.032 ± 0.010 0.006 ± 0.001 i i 60 0.040 ± 0.009 0.011 ± 0.009 10 i i 90 0.050 ± 0.007 0.016 ± 0.011 hi i 120 0.060 ± 0.010 0.019 ± 0.014 hi h 30 0.064 ± 0.049 0.067 ± 0.016 gh gh 60 0.100 ± 0.049 0.088 ± 0.016 30 fg fg 90 0.119 ± 0.054 0.098 ± 0.015 efg efg 120 0.138 ± 0.058 0.107 ± 0.012 efg gh 30 0.140 ± 0.029 0.088 ± 0.004 cde def 60 0.179 ± 0.062 0.121 ± 0.007 50 bc cde 90 0.212 ± 0.048 0.133 ± 0.004 ab bcd 120 0.240 ± 0.029 0.150 ± 0.019 def efg 30 0.157 ± 0.049 0.110 ± 0.023 bcd abc 60 0.202 ± 0.052 0.157 ± 0.049 70 ab ab 90 0.245 ± 0.039 0.169 ± 0.046 a a 120 0.271 ± 0.037 0.183 ± 0.052 * Different letters in the same column indicate a significant difference (p < 0.05)
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ACCEPTED MANUSCRIPT Table 3 Drying kinetics modeling at different concentrations of osmotic solution [OS (% w/w)], microwave pulse ratios [PR] and powers [P (W)] Variables Coefficient(s) and Statistical Parameters of Various Drying Kinetics Models
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0.0109 0.0100 1 0.0074 84 0.0090 38 0.0099 29 0.0092 88 0.0117 2 0.0093 5 0.0100 6 0.0126 3 0.0107 2
0.1047 0.208 0.2452
1.28 7 1.1 1.04 9 0.90 89 0.63 23 0.68 95 0.54 56 0.41 12 0.35 79 0.55 35 0.71 2 0.55 17 0.40 71 0.40 24 0.75 46 0.88 74 0.71 02 0.37 18 0.29 65 0.69 99 0.71 83 0.63 13
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0.0461 4 0.0959 5 0.1843 0.1644 0.0308 2 0.0172 4 0.0389 5 0.2392 0.3183 0.0436 8 0.0468 4 0.0625 9
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RMSE 0.0121 9 0.0070 03 0.0140 5 0.0125 8 0.0107 3 0.0521 1 0.0512 2 0.0181 1 0.0274 6 0.0298 8 0.0555 9 0.0539 4 0.0391 7 0.0373 5 0.0370 7 0.0626 1 0.0439 3 0.0390 5 0.0347 1 0.0263 6 0.0306 6 0.0307 9 0.0121 8
a 1.05 2 1.05 1 1.02 8 1.01 3 0.98 27 0.88 4 0.90 71 0.86 59 0.81 63 0.77 02 0.86 45 0.90 96 0.87 19 0.81 38 0.79 58 0.90 27 0.93 91 0.91 42 0.80 38 0.73 33 0.90 57 0.92 31 0.89 65
k 0.0057 7 0.0070 09 0.0059 46 0.0062 86 0.0058 08 0.0097 53 0.0117 2 0.0102 8 0.0106 9 0.0077 24 0.0098 49 0.0105 1 0.0093 12 0.0077 28 0.0056 05 0.0081 42 0.0093 34 0.0084 37 0.0088 29 0.0059 19 0.0089 73 0.0114 9 0.0093 32
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RMSE 0.030 56 0.038 48 0.018 72 0.014 13 0.018 38 0.067 89 0.062 55 0.073 11 0.116 2 0.117 9 0.067 84 0.050 81 0.064 56 0.093 03 0.088 41 0.051 91 0.034 97 0.041 87 0.100 3 0.112 8 0.051 91 0.053 23 0.062 74
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0.83 92 0.81 72 0.9 0.92 07 0.86 88 0.81 82 0.75 04 0.89 39 0.94 44 0.90 37 0.81 42 0.74 8 0.95 26 0.96 55 0.91 23 0.84 32 0.81 03 0.93 34 0.92 77 0.90 13 0.82 22 0.78 12 0.92 33 0.95 09 0.89 79 0.83
0.0198 0.0387 0.0542 8 0.0276 0.0216 1 0.0257 9 0.0544 1 0.0321 8 0.0295 1 0.0401
0.0105 2 0.0084 64 0.0088 73 0.0112 9 0.0078 59 0.0076 53 0.0053 4 0.0084 99 0.0094 87 0.0089 14 0.0069 2 0.0058 72 0.0093 05 0.0098 58 0.0091 65 0.0078 53 0.0066 78 0.0092 16 0.0101 4 0.0090 45 0.0074 09 0.0057 19 0.0078 28 0.0094 86 0.0089 29 0.0094
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0.111 1 0.107 3 0.052 23 0.051 21 0.060 95 0.088 93 0.105 5 0.061 87 0.042 01 0.049 31 0.085 82 0.108 2 0.039 77 0.030 22 0.057 35 0.097 05 0.103 9 0.039 83 0.055 3 0.056 57 0.088 34 0.093 38 0.051 24 0.030 06 0.052 97 0.097
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0 7 43 9 90 0.0104 0.66 0.148 70 3 0.2614 0 3 22 7 36 0.0095 0.98 0.040 0.0149 0 4 0 79 5 47 6 36 0.0098 0.99 0.023 0.0096 10 4 0 73 52 57 07 36 0.0087 0.98 0.039 0.0236 30 4 0 6 31 97 3 36 0.89 0.098 50 4 0.0145 0.1846 0 18 5 36 0.0098 0.88 0.095 70 4 0.1114 0 76 3 48 60 0.0090 0.98 0.043 0.0224 0 4 0 83 23 47 6 60 0.0116 0.97 0.052 0.0364 10 4 0 1 51 84 4 60 0.0103 0.96 0.058 0.0449 30 4 0 2 49 9 4 60 0.0099 0.87 0.098 50 4 0.1039 0 46 69 68 60 0.0086 0.72 0.131 70 4 0.1778 0 98 97 9 90 0.0090 0.96 0.063 0.0315 0 4 0 5 12 9 4 90 0.0104 0.98 0.041 0.0202 10 4 0 6 56 93 5 90 0.0087 0.95 0.061 0.0429 30 4 0 83 96 83 3 90 0.0105 0.85 0.110 50 4 0.1263 0 8 52 5 90 0.0096 0.69 0.141 70 4 0.2221 0 17 31 5 1 Concentration of osmotic solution (% w/w) 2 Pulse ratio 3 Power (W) 4 Newton model (Tzempelikos et al., 2015) 5 Page model (Tzempelikos et al., 2015) 6 Henderson and Pabis model (Tzempelikos et al., 2015)
26 0.81 39 0.98 62 0.99 52 0.98 69 0.90 22 0.90 7 0.98 9 0.98 37 0.97 8 0.92 33 0.84 68 0.97 67 0.99 03 0.97 78 0.91 71 0.83 03
84 0.115 8 0.041 96 0.025 31 0.037 59 0.101 2 0.091 0.036 31 0.045 68 0.049 43 0.082 12 0.104 2 0.052 47 0.037 27 0.048 35 0.088 7 0.110 3
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Table 4 Magnitudes of moisture content after osmotic dehydration [MC-OD (g water / g dry solid)], moisture content after microwave drying [MC-MD (g water / g dry solid)], final moisture content [MC-F (g water / g dry solid)], drying time considering microwave "on" time [DT-ON (min)] and drying time considering microwave "on" and "off" times [DT-ON-OFF (min)] of samples as influenced by concentration of osmotic solution [OS (% w/w)], microwave pulse ratio [PR] and power [P (W)] 1 2 3 4 5 6 7 8 OS PR P MC-OD MC-MD MC-F DT-ON DT-ON-OFF defghi abcde 0 0 5.921 ± 0.120 0.190 ± 0.048 585 bcde gf 10 0 6.070 ± 0.080 0.140 ± 0.030 450 j abcde 30 0 3.500 ± 0.033 0.189 ± 0.019 495 k abcd 50 0 2.238 ± 0.024 0.206 ± 0.014 405 l abcde 70 0 1.928 ± 0.033 0.188 ± 0.013 450 bcdef ijkl abcdef 0 1 360 6.055 ± 0.167 4.076 ± 0.167 0.181 ± 0.025 325 325 hi ijkl gf 10 1 360 5.860 ± 0.030 4.035 ± 0.110 0.140 ± 0.030 280 280 j qrs abcde 30 1 360 3.479 ± 0.034 2.306 ± 0.050 0.196 ± 0.010 415 415 k tuvw abcde 50 1 360 2.238 ± 0.011 1.246 ± 0.076 0.191 ± 0.009 280 280 l wxy abcdef 70 1 360 1.877 ± 0.030 0.993 ± 0.012 0.185 ± 0.011 370 370 efghi hijk abcdef 0 1 600 5.903 ± 0.153 4.187 ± 0.334 0.186 ± 0.017 364 364 bcde bcde def 10 1 600 6.071 ± 0.035 4.725 ± 0.190 0.158 ± 0.008 319 319 j op abcde 30 1 600 3.557 ± 0.056 2.639 ± 0.198 0.193 ± 0.018 364 364 k t abc 50 1 600 2.225 ± 0.013 1.481 ± 0.082 0.209 ± 0.023 364 364 l tuv abcdef 70 1 600 1.887 ± 0.009 1.283 ± 0.073 0.183 ± 0.018 454 454 b bc abcde 0 1 900 6.165 ± 0.201 4.840 ± 0.181 0.200 ± 0.029 363 363 a a ef 10 1 900 6.314 ± 0.445 5.329 ± 0.097 0.154 ± 0.032 363 363 j o abcdef 30 1 900 3.574 ± 0.065 2.771 ± 0.049 0.184 ± 0.014 363 363 k t abcd 50 1 900 2.238 ± 0.018 1.454 ± 0.130 0.206 ± 0.015 363 363 l uvwxy abcde 70 1 900 1.861 ± 0.014 1.112 ± 0.034 0.201 ± 0.009 498 498 bcdef imn abcde 0 2 360 6.053 ± 0.053 3.895 ± 0.053 0.193 ± 0.030 415 425 hi n bcdef 10 2 360 5.851 ± 0.043 3.649 ± 0.087 0.167 ± 0.030 325 335 j rs abcde 30 2 360 3.528 ± 0.021 2.263 ± 0.131 0.194 ± 0.013 370 380 k uvwx abcde 50 2 360 2.235 ± 0.026 1.133 ± 0.048 0.195 ± 0.012 325 335 l wxy abcde 70 2 360 1.887 ± 0.010 0.973 ± 0.086 0.198 ± 0.009 370 380 bcde efg abc 0 2 600 6.080 ± 0.187 4.466 ± 0.283 0.210 ± 0.066 364 368 fghi fgh abcd 10 2 600 5.881 ± 0.105 4.414 ± 0.054 0.206 ± 0.046 274 278 j op ab 30 2 600 3.600 ± 0.033 2.600 ± 0.088 0.211 ± 0.019 454 458 k t abcde 50 2 600 2.260 ± 0.020 1.457 ± 0.121 0.195 ± 0.005 409 413 l uvxw ab 70 2 600 1.875 ± 0.021 1.139 ± 0.012 0.215 ± 0.012 499 503 defghi fgh abcdef 0 2 900 5.930 ± 0.030 4.404 ± 0.185 0.175 ± 0.030 318 321 cdefgh b abcdef 10 2 900 5.969 ± 0.192 4.866 ± 0.203 0.172 ± 0.058 318 321 j o abcde 30 2 900 3.569 ± 0.046 2.739 ± 0.018 0.193 ± 0.018 363 366 k t abc 50 2 900 2.266 ± 0.049 1.460 ± 0.039 0.210 ± 0.013 408 411 l vwxy abcde 70 2 900 1.877 ± 0.019 1.068 ± 0.011 0.199 ± 0.013 453 456 bcde imn abcdef 0 3 360 6.061 ± 0.142 3.866 ± 0.205 0.181 ± 0.017 325 345 defghi imn abcdef 10 3 360 5.947 ± 0.053 3.895 ± 0.450 0.175 ± 0.030 325 345 j s abcd 30 3 360 3.569 ± 0.046 2.079 ± 0.039 0.205 ± 0.004 415 435 k vwxy abcde 50 3 360 2.235 ± 0.026 1.071 ± 0.063 0.192 ± 0.007 370 390 l xy abcde 70 3 360 1.848 ± 0.022 0.886 ± 0.029 0.197 ± 0.003 415 435 bcdefg defg abcdef 0 3 600 6.040 ± 0.174 4.526 ± 0.120 0.181 ± 0.025 364 372 cdefgh ghij cdef 10 3 600 5.984 ± 0.135 4.292 ± 0.090 0.164 ± 0.005 274 282 j opq a 30 3 600 3.540 ± 0.020 2.529 ± 0.177 0.218 ± 0.053 319 327 k t abcde 50 3 600 2.262 ± 0.015 1.417 ± 0.156 0.192 ± 0.011 409 417 l uvwx abcde 70 3 600 1.893 ± 0.020 1.122 ± 0.077 0.200 ± 0.011 498 507
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0 3 900 5.947 ± 0.053 4.596 ± 0.030 0.193 ± 0.030 ghi bcd abcd 10 3 900 5.864 ± 0.077 4.745 ± 0.183 0.204 ± 0.006 j op abcde 30 3 900 3.522 ± 0.051 2.600 ± 0.088 0.189 ± 0.019 k tu abcde 50 3 900 2.254 ± 0.019 1.390 ± 0.082 0.199 ± 0.018 l vwxy abcde 70 3 900 1.856 ± 0.019 1.015 ± 0.125 0.192 ± 0.011 bcde mn g 0 4 360 6.073 ± 0.063 3.660 ± 0.021 0.106 ± 0.050 bc klm abcdef 10 4 360 6.141 ± 0.032 3.923 ± 0.153 0.178 ± 0.028 j s abcde 30 4 360 3.622 ± 0.059 2.184 ± 0.111 0.196 ± 0.021 k y abcde 50 4 360 2.190 ± 0.030 0.831 ± 0.013 0.194 ± 0.010 l xy abcde 70 4 360 1.895 ± 0.021 0.906 ± 0.030 0.189 ± 0.009 hi fghi abcdef 0 4 600 5.859 ± 0.031 4.340 ± 0.067 0.187 ± 0.027 defghi ijkl abcdef 10 4 600 5.949 ± 0.138 4.111 ± 0.389 0.172 ± 0.024 j pqr abc 30 4 600 3.511 ± 0.025 2.460 ± 0.184 0.211 ± 0.013 k tu abcde 50 4 600 2.267 ± 0.051 1.387 ± 0.044 0.202 ± 0.011 l vwxy abcd 70 4 600 1.872 ± 0.020 1.022 ± 0.086 0.206 ± 0.013 bcd fgh cdef 0 4 900 6.094 ± 0.089 4.378 ± 0.336 0.164 ± 0.011 i defg abcd 10 4 900 5.781 ± 0.073 4.492 ± 0.128 0.204 ± 0.006 j op abcde 30 4 900 3.561 ± 0.042 2.627 ± 0.012 0.198 ± 0.004 k tu abcdef 50 4 900 2.278 ± 0.027 1.365 ± 0.050 0.183 ± 0.014 l vwxy abcd 70 4 900 1.918 ± 0.053 1.025 ± 0.120 0.203 ± 0.002 * Different letters in the same column indicate a significant difference (p < 0.05) 1 Concentration of osmotic solution (% w/w) 2 Pulse ratio 3 Power (W) 4 Moisture content after osmotic dehydration (g water / g dry solid) 5 Moisture content after microwave drying (g water / g dry solid) 6 Final moisture content (g water / g dry solid) 7 Drying time considering microwave "on" time (min) 8 Drying time considering microwave "on" and "off" times (min)
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Table 5 Magnitudes of effective moisture diffusion coefficient (Deff) at different concentrations of osmotic solution [OS (% w/w)], microwave pulse ratios [PR] and powers [P (W)] 1 2 3 2 OS PR P Deff (m /s) -8opq -9 0 0 8.99732×10 ±3.36947×10 -7j -9 10 0 1.25962×10 ±5.12393×10 -8pqrs -10 30 0 8.70552×10 ±8.42369×10 -8opq -9 50 0 8.90005×10 ±2.52711×10 -8s -9 70 0 7.58693×10 ±2.52711×10 -7def -9 0 1 360 1.45416×10 ±9.49301×10 -7a -9 10 1 360 1.83837×10 ±7.72043×10 -8nop -9 30 1 360 9.43503×10 ±1.68474×10 -7kl -9 50 1 360 1.11372×10 ±2.2287×10 -8rs -9 70 1 360 7.73283×10 ±1.45903×10 -7ghij -9 0 1 600 1.33744×10 ±1.68474×10 -7c -9 10 1 600 1.62438×10 ±2.2287×10 -7kl -9 30 1 600 1.10886×10 ±5.05421×10 -8pqrs -9 50 1 600 8.46235×10 ±1.45903×10 -8t -9 70 1 600 6.41971×10 ±1.45903×10 -7hij -9 0 1 900 1.32771×10 ±6.68609×10 -7de -9 10 1 900 1.47848×10 ±5.12393×10 -7kl -9 30 1 900 1.11859×10 ±3.36947×10 -8pqr -9 50 1 900 8.75415×10 ±2.91805×10 -8u -9 70 1 900 5.30112×10 ±2.2287×10 -7k -9 0 2 360 1.15749×10 ±6.07441×10 -7d -9 10 2 360 1.51739×10 ±5.26059×10 -7lmn -9 30 2 360 1.03591×10 ±2.91805×10 -9 -8opq 50 2 360 ±3.86022×10 9.04596×10 -8s -9 70 2 360 7.58693×10 ±2.52711×10 -7ij -8 0 2 600 1.28394×10 ±1.48075×10 -7bc -8 0 2 600 1.71679×10 ±1.82427×10 -8pqrs -9 30 2 600 8.60825×10 ±6.35974×10 -8rs -9 50 2 600 7.78147×10 ±1.68474×10 -8u -9 70 2 600 5.25249×10 ±1.45903×10 -7defgh -9 0 2 900 1.42498×10 ±2.2287×10 -7c -8 10 2 900 1.62438×10 ±1.24089×10 -7kl -9 30 2 900 1.12831×10 ±3.0372×10 -8rs -9 50 2 900 7.73283×10 ±2.52711×10 -8tu -9 70 2 900 6.07927×10 ±4.4574×10 -7defg -9 0 3 360 1.44443×10 ±8.75415×10 -7de -9 10 3 360 1.47362×10 ±5.26059×10 -8opq -9 30 3 360 9.09459×10 ±4.4574×10 -8qrs -9 50 3 360 8.07327×10 ±1.68474×10 -8t 70 3 360 6.41971×10 ±0 -7efghi -9 0 3 600 1.37635×10 ±3.6718×10 -7bc -10 10 3 600 1.71192×10 ±8.42369×10 -7k -9 30 3 600 1.16236×10 ±8.54911×10 -8rs -9 50 3 600 7.78147×10 ±5.89658×10 -8tu -9 70 3 600 5.6902×10 ±3.86022×10 -7efghi -9 0 3 900 1.37148×10 ±5.26059×10 -7fghij -9 10 3 900 1.35689×10 ±2.52711×10 -7kl -9 30 3 900 1.10886×10 ±6.68609×10
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Table 6 Specific energy consumption of microwave drying [SE-MD (×10 MJ/kg)], specific energy consumption of 3 3 hot-air drying [SE-HA (×10 MJ/kg)] and total specific energy consumption [SE-Total (×10 MJ/kg)] at different concentrations of osmotic solution [OS (% w/w)], microwave pulse ratios [PR] and powers [P (W)] 1 2 3 4 5 6 OS PR P SE-MD SE-HA SE-Total nop nop 0 0 224.060 ±5.090 224.060 ±5.090 pqrstu pqrstu 10 0 192.123 ±3.739 192.123 ±3.739 mno mno 30 0 238.795 ±6.171 238.795 ±6.171 mno mno 50 0 243.135 ±3.799 243.135 ±3.799 hijk hijk 70 0 294.796 ±7.740 294.796 ±7.740 cdefghij tuvwxyz tuvwxy 0 1 360 0.610 ±0.060 172.890 ±8.621 173.500 ±8.586 cdefghi yzαβ xyzα 10 1 360 0.624 ±0.028 140.214 ±3.238 140.838 ±3.212 defghijk mno mno 30 1 360 0.594 ±0.023 237.937 ±4.699 238.531 ±4.681 ghijklmnopq mno mno 50 1 360 0.520 ±0.042 234.835 ±16.468 235.355 ±16.426 ghijklmnopqrs fg fg 70 1 360 0.499 ±0.022 349.107 ±6.694 349.607 ±6.693 opqrs tuvwxyz tuvwxy 0 1 600 0.425 ±0.061 173.672 ±14.648 174.098 ±14.592 efghijklmn yzαβ xyzα 10 1 600 0.569 ±0.070 139.594 ±5.463 140.163 ±5.393 fghijklmno pqrstu pqrstu 30 1 600 0.562 ±0.140 195.856 ±16.116 196.418 ±15.994 lmnopqrs lmn lmn 50 1 600 0.453 ±0.043 253.339 ±11.779 253.791 ±11.738 ghijklmnopqrs gh gh 70 1 600 0.500 ±0.053 328.035 ±16.863 328.536 ±16.810 bcde uvwxyzα uvwxyz 0 1 900 0.695 ±0.136 165.763 ±7.009 166.459 ±6.953 a uvwxyzαβ uvwxyzα 10 1 900 1.049 ±0.324 157.228 ±3.190 158.277 ±3.354 bcd qrstuvw qrstuv 30 1 900 0.706 ±0.105 184.373 ±3.450 185.079 ±3.345 ghijklmnopqr klm klm 50 1 900 0.505 ±0.098 265.660 ±27.259 266.165 ±27.165 lmnopqrs bc bc 70 1 900 0.452 ±0.028 435.805 ±19.638 436.257 ±19.610 ghijklmnopq nopq nopq 0 2 360 0.527 ±0.000 221.246 ±3.667 221.772 ±3.667 hijklmnopqrs tuvwxyz tuvwxy 10 2 360 0.491 ±0.023 173.866 ±5.939 174.357 ±5.918 fghijklmno opqr opqr 30 2 360 0.554 ±0.051 216.085 ±12.073 216.639 ±12.023 klmnopqrs hijk hijk 50 2 360 0.457 ±0.025 300.556 ±11.586 301.012 ±11.562 ghijklmnopqr de de 70 2 360 0.506 ±0.045 383.507 ±44.809 384.012 ±44.765 jklmnopqrs tuvwxyz tuvwxy 0 2 600 0.472 ±0.043 171.812 ±14.719 172.284 ±14.686 ghijklmnopq αβ zα 10 2 600 0.517 ±0.023 129.748 ±1.748 130.265 ±1.770 jklmnopqrs mno mno 30 2 600 0.483 ±0.049 241.543 ±11.093 242.026 ±11.049 mnopqrs hij hij 50 2 600 0.442 ±0.057 302.401 ±28.166 302.843 ±28.110 qrs bc bc 70 2 600 0.408 ±0.007 428.969 ±5.545 429.376 ±5.541 fghijklmno vwxyzαβ vwxyzα 0 2 900 0.563 ±0.059 150.779 ±5.806 151.342 ±5.749 b zαβ yzα 10 2 900 0.777 ±0.074 135.875 ±6.315 136.652 ±6.305 bcdef pqrstuv pqrstuv 30 2 900 0.674 ±0.025 187.359 ±2.639 188.033 ±2.658 ijklmnopqrs hij hij 50 2 900 0.490 ±0.013 303.924 ±12.301 304.414 ±12.305 pqrs cd cd 70 2 900 0.409 ±0.011 405.832 ±11.238 406.241 ±11.232 fghijklmnop rstuvwx rstuvw 0 3 360 0.548 ±0.031 182.875 ±11.026 183.423 ±11.005 efghijklmn tuvwxyz tuvwxy 10 3 360 0.568 ±0.108 173.204 ±23.033 173.772 ±22.928 ghijklmnopqrs ijkl ijkl 30 3 360 0.500 ±0.017 286.262 ±6.316 286.762 ±6.307 nopqrs ef ef 50 3 360 0.432 ±0.022 367.658 ±30.041 368.090 ±30.021 ijklmnopqrs a a 70 3 360 0.489 ±0.026 491.166 ±18.547 491.656 ±18.522 ghijklmnopq stuvwxy stuvwx 0 3 600 0.537 ±0.087 176.903 ±4.635 177.441 ±4.596 jklmnopqrs yzαβ xyzα 10 3 600 0.473 ±0.015 139.608 ±2.896 140.081 ±2.907 ghijklmnopqrs rstuvwx rstuvw 30 3 600 0.500 ±0.087 181.582 ±16.326 182.082 ±16.246 qrs hijk hijk 50 3 600 0.405 ±0.073 299.298 ±42.318 299.703 ±42.248 qrs b b 70 3 600 0.399 ±0.029 441.443 ±42.266 441.842 ±42.237 cdefg xyzαβ wxyzα 0 3 900 0.633 ±0.038 144.635 ±1.002 145.268 ±0.994 bc vwxyzαβ vwxyzα 10 3 900 0.728 ±0.072 152.430 ±6.078 153.158 ±6.006
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30 3 900 0.587 ±0.033 191.437 ±8.319 lmnopqrs opqrst 50 3 900 0.448 ±0.032 207.712 ±11.955 qrs bc 70 3 900 0.398 ±0.054 435.097 ±63.284 ghijklmnopqrs uvwxyzα 0 4 360 0.498 ±0.018 162.104 ±1.422 ghijklmnopq wxyzαβ 10 4 360 0.514 ±0.033 145.892 ±4.858 ghijklmnopq opqrst 30 4 360 0.521 ±0.054 210.335 ±10.792 s ef 50 4 360 0.361 ±0.011 370.130 ±4.201 mnopqrs fg 70 4 360 0.445 ±0.008 345.317 ±18.064 ghijklmnopqrs tuvwxy 0 4 600 0.500 ±0.031 175.300 ±3.737 nopqrs uvwxyzα 10 4 600 0.429 ±0.110 162.919 ±17.494 klmnopqrs opqrst 30 4 600 0.465 ±0.081 205.995 ±17.504 qrs ghi 50 4 600 0.402 ±0.039 320.346 ±9.581 s b 70 4 600 0.363 ±0.033 453.442 ±46.721 ghijklmnopqr tuvwxyz 0 4 900 0.506 ±0.085 173.402 ±12.841 cdefgh β 10 4 900 0.630 ±0.040 120.998 ±3.623 defghijklm opqrs 30 4 900 0.578 ±0.020 213.535 ±0.711 opqrs jkl 50 4 900 0.423 ±0.017 278.858 ±13.324 rs bc 70 4 900 0.372 ±0.029 435.453 ±62.718 * Different letters in the same column indicate a significant difference (p < 0.05) 1 Concentration of osmotic solution (% w/w) 2 Pulse ratio 3 Power (W) 4 3 Specific energy consumption of microwave drying (×10 MJ/kg) 5 3 Specific energy consumption of hot-air drying (×10 MJ/kg) 6 3 Total specific energy consumption (×10 MJ/kg)
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192.024 ±8.290 opqrst 208.160 ±11.923 bc 435.494 ±63.230 uvwxyz 162.602 ±1.437 wxyzα 146.406 ±4.826 opqrst 210.857 ±10.747 ef 370.492 ±4.192 fg 345.763 ±18.057 tuvwx 175.801 ±3.706 uvwxyz 163.348 ±17.397 opqrst 206.460 ±17.426 ghi 320.748 ±9.553 b 453.805 ±46.688 tuvwxy 173.908 ±12.757 α 121.628 ±3.589 opqrs 214.114 ±0.728 jkl 279.281 ±13.308 bc 435.825 ±62.690
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Fig. 1a Drying kinetics of control quince samples (without microwave drying) pretreated with osmotic solutions at concentrations of 0 (control), 10, 30, 50 and 70% during convective hot-air drying
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Fig. 1b Drying kinetics of quince samples pretreated with osmotic solutions at concentrations of 0 (control), 10, 30, 50 and 70% during both microwave and convective hot-air drying at 360 W (a,b,c,d), 600 W (e,f,g,h) and 900 W (i,j,k,l) with different pulse ratios (PR)
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Fig. 2 Ln (MR) versus drying time of control quince samples (without microwave drying) pretreated with osmotic solutions at concentrations of 0 (control), 10, 30, 50 and 70% during convective hot-air drying
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Fig. 3a Shrinkage variations of control quince samples (without microwave drying) pretreated with osmotic solutions at concentrations of 0 (control), 10, 30, 50 and 70% during convective hot-air drying
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Fig. 3b Shrinkage variations of quince samples pretreated with osmotic solutions at concentrations of 0 (control), 10, 30, 50 and 70% during both microwave and convective hot-air drying at 360 W (a,b,c,d), 600 W (e,f,g,h) and 900 W (i,j,k,l) with different pulse ratios (PR)
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Fig. 4a Bulk density variations of control quince samples (without microwave drying) pretreated with osmotic solutions at concentrations of 0 (control), 10, 30, 50 and 70% during convective hot-air drying
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Fig. 4b Bulk density variations of quince samples pretreated with osmotic solutions at concentrations of 0 (control), 10, 30, 50 and 70% during both microwave and convective hot-air drying at 360 W (a,b,c,d), 600 W (e,f,g,h) and 900 W (i,j,k,l) with different pulse ratios (PR)
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Fig. 5a Rehydration ratio variations of control quince samples (without microwave drying) pretreated with osmotic solutions at concentrations of 0 (control), 10, 30, 50 and 70% during convective hot-air drying
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Fig. 5b Rehydration ratio variations of quince samples pretreated with osmotic solutions at concentrations of 0 (control), 10, 30, 50 and 70% during both microwave and convective hot-air drying at 360 W (a,b,c,d), 600 W (e,f,g,h) and 900 W (i,j,k,l) with different pulse ratios (PR)
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(e) Fig. 6 Images of the quince cubes dried using hybrid intermittent microwave – hot air drying technique at 360 W and PR = 1 pretreated at 0 (a), 10 (b), 30 (c), 50 (d) and 70% (e) osmotic solution concentrations
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ACCEPTED MANUSCRIPT Highlights A multi-stage continuous and intermittent microwave (IM) drying was developed.
Osmotic dehydration (OD) and hot air (HA) drying at 40˚C was applied.
OD-IM-HA drying considerably reduced drying time and improved product quality.
OD-IM-HA drying increased rehydration and decreased shrinkage and bulk density.
Specific energy consumption was significantly reduced by the OD-IM-HA drying.
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Jalal Dehghannya, Seyed-Hamed Hosseinlar, Maryam Khakbaz Heshmati PII: DOI: Reference:
S1466-8564(17)30672-0 doi:10.1016/j.ifset.2017.10.007 INNFOO 1868
To appear in:
Innovative Food Science and Emerging Technologies
Received date: Revised date: Accepted date:
15 June 2017 4 September 2017 6 October 2017
Please cite this article as: Jalal Dehghannya, Seyed-Hamed Hosseinlar, Maryam Khakbaz Heshmati , Multi-stage continuous and intermittent microwave drying of quince fruit coupled with osmotic dehydration and low temperature hot air drying. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Innfoo(2017), doi:10.1016/j.ifset.2017.10.007
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Multi-stage continuous and intermittent microwave drying of quince fruit coupled with osmotic dehydration and low temperature hot air drying Jalal Dehghannyaa,*, Seyed-Hamed Hosseinlara, Maryam Khakbaz Heshmatia
Department of Food Science and Technology, University of Tabriz, Tabriz 51666-16471, Iran
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Corresponding author: Tel.: +98 41 33392063; Fax: +98 41 33356005; E-mail address:
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[email protected] (J. Dehghannya)
Abstract
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In recent years, using intermittent microwave (IM) to dry foodstuffs has been taken into consideration as one of the new drying methods in food industry. The aim of this research was to
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dry cubic pieces of pre-treated "quince" fruit by sucrose osmotic solution using IM – hot air (HA) drying at a low temperature (40°C) in order to investigate the effects of this process on
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improving the dried product quality. The variables of the process included sucrose osmotic solution in 5 concentration levels of 0 (control), 10, 30, 50, and 70% (w/w) and microwave at 4
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powers of 0 (control), 360, 600, and 900 W, with 4 pulse ratios of 1, 2, 3, and 4. Findings indicated that compared to control samples, the samples pre-treated by osmotic solution had
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lower effective moisture diffusion coefficient (Deff). However, Deff increased through increase in power and pulse ratio of the microwave. Increasing the concentration of the osmotic solution,
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power, and pulse ratio led to significant decreases in shrinkage. Due to high shrinkage, the quince samples dried by continuous microwave – HA method showed higher bulk density in
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comparison to the samples dried by IM – HA. In addition, samples dried by IM with low power showed the highest rehydration followed by those dried by IM with high power; however, the control samples dried merely by HA had the lowest rehydration. Moreover, a significant decrease in specific energy consumption was noticed through increasing the microwave power. Drying by IM – HA at the power of 900 W and the pulse ratio of 4 had the lowest specific energy consumption, while drying with only HA had the highest specific energy consumption.
Keywords:
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ACCEPTED MANUSCRIPT Continuous and Intermittent microwave, Drying time, Energy Consumption, Quince fruit, Rehydration, Shrinkage
1. Introduction "Quince" fruit, with the scientific name of "Cydonia Oblonga," is from "Rosaceae" family—
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known for its pleasant fragrance and distinctive taste (Doymaz et al., 2015). Like other fruits,
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quince is a perishable fruit with microbiological, chemical, and physical spoilage agents that decrease its shelf life (Akbarian et al., 2014). Quince is an ancient, delicious fruit with a dry and
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fluffy flesh and an almost sour taste. It is generally grown in Argentina, China, Iran, Morocco, Republic of Azerbaijan, Turkey, and Uzbekistan (Doymaz et al., 2015). This fruit is a rich source
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of vitamins A and B, calcareous salts, and tannin. In addition, quince contains glycoside, lipid, and fiber (about 12% on dry basis) and produces 112 kcal per 100 g (Koc et al., 2008). Dried
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"quince" fruit is used in producing jam, marmalade, jelly, and pudding (Noshad et al., 2012). In addition, dried quince is used as an ingredient of traditional dishes in Iran such as quince stew,
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soup and the like (Noshad et al., 2012; Akbarian et al., 2014). Convective HA drying is the most common method used in drying food (Zhao et al., 2014). More than 85% of industrial dryers are
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convective HA dryers (Aghilinategh et al., 2015), but they have big disadvantages such as high energy consumption and longer drying time due to low thermal conductivity of foodstuffs (Zhao
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et al., 2014). Another reason for such disadvantages is the rapid decrease of surface moisture and, as a result, shrinkage of the product, which often results in decrease in moisture and,
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sometimes, heat transfer (Maskan, 2001). Increase in the process time with high temperature results in change in both fragrance and taste of the product. Furthermore, convective HA drying
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leads to reduction of nutritional value and changes the color of foodstuffs (Zhao et al., 2014). Moisture removal during drying is greatly affected by HA conditions and foodstuff characteristics. HA temperature and moisture content of the foodstuff has a significant effect on qualitative characteristics of the foodstuff such as vitamins, smell, taste, color, tissue or nutritional composition during storage (Mota et al., 2010). Drying with HA results in destruction of the compounds which are sensitive to temperature (such as vitamins) and also leads to loss of important sensory features of the dried product. Although high drying temperature decreases the drying time, it reduces the quality of the product, creates thermal damage to the product surface and has higher energy consumption. Using low temperatures in combined microwave – HA 2
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hardening.
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Using microwave energy in drying foodstuffs is an appropriate method which helps decrease
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defects and overcome the problems which occur during HA drying. Microwave drying improves the drying process without damaging the quality of the final product (Soysal et al., 2009a; Zhao
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et al., 2014; Aghilinategh et al., 2015). It decreases the critical moisture content and increases the drying constant rate period (Tsuruta et al., 2015). Whereas HA drying proves relatively efficient
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in removing the free water from the surface or near the surface of the product, the unique function of microwave energy through volumetric heating is its efficiency in removing the
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interior free water (Andres et al., 2004). In microwave drying, volumetric heating results in moisture transfer from interior parts of the fruit to the surface (Tsuruta et al., 2015). In
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comparison to HA drying, microwave drying has several advantages including faster drying rate and minimum heating in parts with lower moisture content. As a result, overheating decreases in
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the parts in which less heat is required for moisture removal. However, using microwave has some disadvantages such as non-uniform heating, which creates non-uniformity in the drying
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process. Of reasons for non-uniformity in microwave drying, one can refer to lack of samples’ equal exposure to radiation, non-uniformity of the compounds present in the samples, and also
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lack of geometric uniformity of the foodstuffs. A right combination of these two unit operations (microwave heating and hot air drying) may improve the efficiency and economy of the drying process (Andres et al., 2004). There are three methods to combine microwave energy and HA (Andres et al., 2004): 1. Applying microwave energy in the beginning of the drying process: in this method, interior parts of the sample are quickly heated so that its moisture reaches the vaporization temperature. Improving the drying rate in this method is attributed to formation of a porous structure in the foodstuff that facilitates vapor transfer. 2. Applying microwave energy in the middle of the drying process: This is the state in which the drying rate starts to decrease, the foodstuff surface is dry and the moisture is concentrated in the center. Using microwave at this 3
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Microwave drying mechanism includes ionic polarization and bipolar rotation. In fact, water
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molecules change their direction, affected by an electrical field, interact with their surrounding molecules, and, as a result, water kinetic energy is converted to thermal energy. In contrast,
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convective energy is the slow transfer of heat from the surface to the internal part of the foodstuff. The effect of microwave energy inside a substance depends on dielectric properties of
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sample (Aghilinategh et al., 2015). One of the important factors in microwave drying is the power density (specific power) of the microwave. It is the power that is applied to the product in
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unit weight. If the power density of the microwave is not controlled, it may damage the product quality. Power density is calculated by using the Ps=P/m equation in which Ps is the specific
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power, P is the fixed power of the microwave and m is the product weight. Product weight is decreased during time and, as a result, specific power increases (Koné et al., 2013). One of the
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advantages of microwave drying in comparison with hot air drying is that due to the formation of porous structure, microwave dryer causes higher rehydration, easier moisture removal, lower
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bulk density and less color change (Aghilinategh et al., 2015). In addition, samples dried with microwave have faster rehydration, which is due to changes in structure and tissue during drying, in comparison to other drying methods (Maskan, 2001). Energy consumption during drying by microwave depends on its power and the moisture content of the foodstuff i.e. by changing moisture content during drying, energy consumption changes as well (Jindarat et al., 2015). In addition, using a microwave with high power decreases drying time, while using a low-power microwave prolongs it. Research findings indicate that both lengthening of the process and using high power damage the foodstuff. Therefore, these two features should be considered with care (Musielak and Kieca, 2014). If microwave drying is not done properly, it results in a low-quality 4
ACCEPTED MANUSCRIPT product (Maskan, 2001). Moreover, since heating by microwave is not uniform, in order to maximize energy efficiency and to overcome some of the limitations of microwave drying, using the two-stage drying process including microwave and then hot drying is useful for producing high-quality dried products. The combined method of microwave – HA results in producing products with low shrinkage and proper rehydration capacity (Maskan, 2001). Microwave – HA drying has been successfully used for some agricultural products such as carrots, apples,
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potatoes, mushrooms, garlics, blueberries, pumpkins, etc. (Soysal et al., 2009b). For example, in
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a study, a new dietary fiber from orange peel was produced by applying two different drying
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methods: HA and HA + microwave. Results showed that 92% reduction in processing time and 77% reduction in energy consumption was achieved with HA + MW (Talens et al., 2017). In
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another study, apple cylinders were dried in a combined HA – microwave system (Andres et al., 2004). Vacuum impregnation with an isotonic solution was used as a pretreatment before drying.
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Dried samples showed different tissue matrices according to the different drying conditions. Vacuum impregnated samples demonstrated higher density, lower porosity and higher volume
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reduction.
Since continuous use of microwave energy results in loss of quality, due to non-uniform
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distribution of heat and moisture, using intermittent microwave (IM) energy has been proposed as a viable solution to this problem (Zhao et al., 2014). IM drying is one of the promising
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solutions for improving energy efficiency and product quality without increasing the drying cost. IM drying is performed by controlling the application of thermal energy. Applying an equal
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amount of energy throughout microwave drying destroys the quality, results in thermal damage to the product surface, and causes thermal energy waste. This happens because in final stages of
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drying, drying rate decreases and, therefore, the sample does not have enough moisture to be removed. This is based on the balance between heat energy produced by water dipoles under microwave radiation (the internal volumetric heat generation) and energy necessary for water evaporation (Campanone and Zaritzky, 2005). The strategy of using IM provides the time for moisture transfer from the center of the sample to its surface during the tempering period. Therefore, both quality and thermal damages can be minimized by using IM drying. The most common type of IM investigated so far is the pulsed (on/off) strategy in which the thermal energy resource is periodically turned on and off. Higher pulse ratio (higher tempering period or off times) results in higher energy saving, but increases the drying time considerably. Increasing 5
ACCEPTED MANUSCRIPT the total drying time may not be suitable for some products. Hence, the degree of pulse ratio should be chosen precisely in order to achieve optimized energy saving. Another important advantage of drying with IM is the uniform distribution of moisture in the sample. Prevention of overheating and damage to dried products, use of limited thermal energy and, as a result, improvement of the efficiency of drying energy are the advantages of intermittent drying (Kowalski et al., 2013). Another way for controlling the quality of dried product and energy
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saving is reducing of microwave power at the final stage of microwave drying process (Andres et
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al., 2004). Due to requiring less drying time, consuming less energy and producing good quality
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products, the combination of IM – HA drying can be an appropriate method in industrial drying applications.
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In addition to the above-mentioned, osmotic dehydration is used to remove a part of water from the foodstuffs such as fruits and vegetables; it is conducted by immersing food in hypertonic
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solutions with high osmotic pressure such as sugar and salt (Dehghannya et al., 2015). When used in osmotic dehydration, sucrose manifests two main beneficial effects that help produce a
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high-quality product. This substance not only acts as polyphenol oxidase effective inhibitor but also prevents the loss of volatile compounds during the osmotic dehydration. Moisture removal
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up to 50% of the fruit’s primary weight is possible through osmotic dehydration; this percent, however, depends on different factors such as temperature, concentration and type of the osmotic
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solution. Osmotic dehydration at room temperature acts as a mild pre-treatment that does not cause cell death except for the cell(s) in the first layers on the tissue surface (Prothon et al.,
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2001). This pre-treatment in the dried ―quince‖ can reduce shrinkage and improve rehydration, and can help obtain a better quality in the final product (Noshad et al., 2012). Moreover, osmotic
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dehydration minimizes the thermal damage to either color and taste, prevents enzymatic browning, and results in better maintenance of the nutrients during drying (Dehghannya et al., 2016b).
Available resources and information indicate that limited studies have generally been conducted on drying ―quince‖ fruit. Literature review show that IM – HA drying during drying the quince fruit pre-treated with sucrose osmotic solution has not yet been investigated. The purpose of this research was to examine the combined effect of the osmotic process and IM – HA drying on the quantitative and qualitative characteristics of quince fruit such as drying kinetics, effective
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ACCEPTED MANUSCRIPT moisture diffusion coefficient, shrinkage, bulk density, rehydration, and specific energy consumption.
2. Material and Methods 2.1. Materials
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Quince fruit (cv. Mulvian) was purchased from a local market in Tabriz, Iran, and before
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performing the experiments, was kept in a cold storage room at 4-6°C. All quinces were bought from the same place and were of the same variety. Sucrose (Miandoab Sucrose Factory, Iran)
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was bought for preparing the osmotic solution and was kept at room temperature. Toluene was used to measure the volume of the samples through liquid displacement method using a 50 ml
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glass pycnometer (Dehghannya et al., 2016a). Distilled water was used for sample blanching, preparing the osmotic solution, and washing the samples after taking them out of the osmotic
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solution and also for measuring rehydration. Absorbent papers were used to dry the surface water of the samples after osmotic dehydration and after taking the samples out of the distilled water to
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measure rehydration.
A combined microwave – HA dryer (LG SolarDom, model SD-3855SCR, Korea) was used with
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internal dimensions of 527×392×480 mm3, the capacity of 38 Liters, and the capability of adjusting the microwave power at 90, 180, 360, 600, and 900 W in the frequency of 2450 MHz.
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The dryer was equipped with air temperature adjusting system between 40 to 230°C and with a turntable tray with turning rate of 2.5 rpm designed for household applications. An oven (Fan
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Azma Gostar Company, Model BM120, Iran) with the capacity of 120 Liters, equipped with an air circulation fan was used to determine moisture content of the samples. A bain-marie (Fan
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Azma Gostar Company, Model WM22, Iran) with the capacity of 22 Liters, equipped with microcontroller with precise temperature control (±1°C) was used for sample blanching.
2.2. Samples Preparation Before performing the experiments, fruits were kept at room temperature to reach equilibrium temperature. Bigger quinces (between 5.5 and 7 cm in diameter) with harder tissues were chosen for the experiments in order to avoid their being crushed when cutting or their tissue breakdown during blanching with hot water. After being washed with tap water, peeling and seed removing, quinces were cut manually into cubic pieces of 1.2 cm with a specially-designed cutter. In order 7
ACCEPTED MANUSCRIPT to prevent moisture loss and color change, samples were kept in a capped Petri dish after cutting and before blanching. After weighing, the samples were blanched inside a beaker in the bainmarie containing hot water at 90°C for 2 min. Then, the samples were immediately cooled at 15°C water for 5 min to remove the extra heat because overheating can result in losing their tissue. Finally, the water remaining on the samples was dried by an absorbent paper (Akbarian et al., 2014; Doymaz et al., 2015). Blanching time and temperature depend on the sample size.
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Therefore, blanching time and temperature of quince pieces were determined through trial and
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error in a way that enzymatic browning does not occur in the given time and temperature.
2.3. Osmotic Dehydration Pre-treatment
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Sucrose osmotic solutions with concentrations of 10, 30, 50, and 70% (w/w) were prepared through dissolving sucrose in distilled water. Quince pieces were then immersed in these
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solutions for 120 minutes at room temperature (25°C). In order to prevent extra dilution of the osmotic solution during time, the ratio of osmotic solution to quince was chosen with the weight
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ratio of 5:1 (w/w). After keeping the samples as long as required, they were taken out of the osmotic solution and washed with distilled water. Their surface water was then dried gently with
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an absorbent paper and then their weight, volume, shrinkage, and moisture content were
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measured on dry basis.
2.4. Drying Experiments
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Drying experiments were performed in order to investigate the effect of IM – HA drying on the quality of dried quince pieces at 4 powers of 0 (control), 360, 600, and 900 W, and 4 pulse ratios
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(PR=1, PR=2, PR=3 and PR=4) as indicated in Table 1. Sample weight (before drying and after osmotic dehydration) was kept constant at 50 g during all the experiments. It is obvious that high microwave power requires shorter time for sample drying. The time required for each microwave power was determined through trial and error in a way that hot spots and, consequently, sample burning were prevented as much as possible. Pulse ratio was calculated using equation 1 (Soysal et al., 2009a; Zhao et al., 2014):
PR
ton toff
(1)
ton
8
ACCEPTED MANUSCRIPT where ton is the time when the microwave is on (s) and toff is the time when it is off (s). After drying the samples with microwave, the drying process was continued by HA at 40°C with the entering air velocity of 1 m/s. Drying stopped when the moisture content of quince samples reached 0.2 or lower (g water/ g dry solids). All experiments were conducted with three
2.5. Measurement of quantitative and qualitative characteristics
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2.5.1. Moisture content
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repetitions.
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Quince samples were placed in the oven at 105°C for 24 hours to reach a constant weight. Moisture amount on dry basis was calculated using the following equation (Dehghannya et al.,
MW MS
(2)
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M .C.(d .b.)
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2016c):
where Mw is the water mass (g) and Ms is the mass of sample's dry solids (g). Average initial
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moisture content of quince was 5.33 g water / g dry solids (84% on wet basis).
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2.5.2. Water loss and solid gain during osmotic pre-treatment In order to determine water loss (g water / g product) and solid gain (g solid / g product) of
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samples, 16 quince cubes (4 for each osmotic concentration) were prepared to evaluate the performance of the osmotic pre-treatment. In each of the osmotic solutions (10, 30, 50, and
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70%), 4 quince cubes were immersed. Every half an hour, one sample was taken out of each solution and was weighed after its surface solids were washed away and its surface water was
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dried with an absorbent paper. Water loss and solid gain of the samples were calculated by using equations 3 and 4 (Dehghannya et al., 2017):
WL SG
wi .M i w f .M f
(3)
wi
w f .[1 M f ] wi .[1 M i ]
(4)
wi
where WL is the water loss (g water / g product), SG is the solid gain (g solid / g product), w is the fruit weight (g), M is the moisture content on wet basis (g water / g sample), i is the primary fresh sample before pre-treatment and f is the final sample after pre-treatment. 9
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2.5.3. Effective moisture diffusion coefficient (Deff) Considering the fact that during drying, diffusion is considered as a dominant mechanism of moisture transfer to the surface of the product, moisture ratio was calculated based on the Fick's second law for transient diffusion that is known as Crank equation (Emam-Djomeh et al., 2006): t
(5)
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(2n 1) 2 2 Deff 1 exp 2 ( 2 n 1 ) 4 L2 n 1
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M Me 8 MR t 2 M0 Me
where MR is the moisture ratio (dimensionless), Mt is the moisture content at tth moment on dry
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basis (g water / g dry solids), M0 and Me are, respectively, the initial moisture and equilibrium moisture contents of the sample on dry basis (g water / g dry solids), n is a positive integer, Deff
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is the effective moisture diffusion coefficient (m2/s), L equals half of the sample thickness (m), and t is the drying time (s).
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For long drying periods, only the first part of the series is taken into consideration. Also, regarding the fact that Me is insignificant in comparison with Mt and M0, equation 5 will be as
(6)
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2 De ff Mt 8 MR exp( t) M0 2 4 L2
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follows:
Taking the ln of both sides of the equation 6 gives: t
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2 8 Deff ln( MR) ln 2 2 4 L
(7)
drying time:
Deff
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2 Deff Slope 4L2
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Now, the Deff can be calculated through the slope of the natural logarithm of moisture ratio to
(8)
Slope 4 L2
(9)
2
2.5.4. Shrinkage The volume of the samples was calculated by toluene displacement method, using a 50 ml glass pycnometer and through equations 10 and 11 (Dehghannya et al., 2016a):
10
ACCEPTED MANUSCRIPT V Vf
wsf
(10)
s
wsf wt s w f w
(11)
where Vf is the pycnometer volume, wsf is the weight of added toluene to fill the pycnometer, wt+s is the weight of pycnometer along with sample and solvent, wf is the pycnometer's weight, w
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is the sample weight and ρs is the toluene density that is 0.87 g/cm3 at 20°C.
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Shrinkage is the ratio of the initial volume of the fresh sample to the final volume of the dried sample before and after drying, which is expressed through the following equation (Dehghannya
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et al., 2016a):
V S 1 t 100 Vo
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(12)
where S is the percentage of shrinkage (dimensionless), Vt is the sample volume at time t (cm3),
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and V0 is the initial volume of the sample (cm3).
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2.5.5. Bulk density
Weight per unit volume of a substance is called its bulk density. In other words, bulk density is
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the relationship between the weight of the sample and its total volume and is calculated as follows (Dehghannya et al., 2016b):
Mt Vt
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b
(13)
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where Mt is the sample weight (g), and Vt is the sample volume (cm3).
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2.5.6. Rehydration ratio
In order to measure rehydration, dried quince samples were initially weighed and then immersed in 100 ml of distilled water at 25°C for 30 min. Samples were then taken out of the distilled water and the extra water was gently dried using an absorbent paper. Finally, samples were weighed. Rehydration ratio (RR) was calculated through the following equation (Akbarian et al., 2014):
RR
Wr 100 Wd
(14)
11
ACCEPTED MANUSCRIPT where Wr is the sample weight after rehydration (g) and Wd is the sample weight before rehydration (g).
2.5.7. Specific energy consumption Specific energy consumption in combined IM – HA dryer (E) is calculated as follows (Singh and
E2
AVa a Ht c m2
(15)
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Ptm PR m1
(16)
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E1
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Heldman, 2014; Chayjan et al., 2015):
H (C p ,a WC p ,v )(Tin Tamb ) W
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(17)
E E1 E2
(18)
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where E1 is the specific energy consumption in microwave (MJ/kg removed water), E2 is the specific energy consumption in the HA dryer (MJ/kg removed water), P is the microwave output
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power (360, 600 or 900 W), tm is the drying time of the sample with microwave (s), PR is pulse ratio, m1 is the amount of moisture removal in microwave drying (kg), A is the area of the
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sample container (m2), Va is the entering air velocity (m/s), ρa is the air bulk density (kg/m3), ΔH is the air enthalpy (kJ/kg dry air), tc is the drying time of the sample with HA (s), m2 is the
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moisture removal during HA drying (kg), Cp,a is the air specific heat (kJ/kg °C), W is the air absolute humidity (kg water vapor /kg dry air), Cp,v is the water vapor specific heat (kJ/kg °C),
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Tin is the temperature inside the dryer (°C), Tamb is the ambient temperature (°C), and λ is the
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latent heat of water evaporation (kJ/kg water vapor).
2.5.8. Statistical analysis A factorial experiment in a randomized complete design 5×4×4 (osmotic solution concentration, microwave power and pulse ratio) was used to investigate the effect of sucrose osmotic solution in five concentrations of 0 (control), 10, 30, 50 and 70%, at 4 microwave powers of 0 (control), 360, 600, and 900 W and 4 pulse ratios of 1, 2, 3, and 4 with 3 repetitions. The effect of each of these variables on qualitative and quantitative characteristics— including effective moisture diffusion coefficient, shrinkage, bulk density, rehydration and specific energy consumption—
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ACCEPTED MANUSCRIPT was analyzed. Mean comparison was performed by using Duncan multiple range test (P < 0.05). Statistical analyses were done by SAS software (version 9.4).
3. Results and Discussion 3.1. Water loss and solid gain during osmotic dehydration pre-treatment In order to accelerate moisture removal and shorten the time needed in drying quince, osmotic
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pre-treatment can be used along with microwave (Noshad et al., 2012). Osmotic dehydration is a
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mild drying method which has been taken into consideration due to its capacity in making use of
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very low temperature and energy (Dehghannya et al., 2006). Since it does not require phase change, osmotic dehydration is considered to be an energy-efficient method. Each kg of water
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requires 2250 kJ heat for vaporization at saturation temperature (100˚C) and pressure (101.3 kPa). Contrary to drying with HA, osmotic dehydration does not need the application of latent
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heat for vaporization and the moisture is removed in liquid form; therefore, it decreases energy consumption.
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The effect of osmotic dehydration on water loss and solid gain of quince samples during the osmotic pre-treatment has been shown in 4 times of 30, 60, 90, and 120 min and in 4
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concentrations of 10, 30, 50, and 70% in Table 2. According to Table 2, increasing the time of samples immersion in the osmotic solution increased water loss and solid gain. In addition, based
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on Table 2, water loss and solid gain increased by increasing the concentration of the osmotic solution. Increasing the sucrose concentration resulted in greater difference in osmotic pressure
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between quince samples and the osmotic solution, which increased water loss (Dehghannya et al., 2006). Moreover, increasing solid gain within the samples by increasing the osmotic solution
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concentration can be the result of membrane swelling that increases cell membrane permeability (Akbarian et al., 2014). Increasing the permeability of the cellular membrane of quince samples by increasing the concentration of the osmotic solution resulted in increasing moisture diffusion from within the samples towards the osmotic solution; it also increased sucrose penetration from the osmotic solution into the samples. Based on Table 2, the highest solid (sucrose) gain (0.183 g solid / g product) belonged to the 70% osmotic solution after 120 min while the least sucrose gain (0.006 g solid / g product) belonged to the 10% osmotic solution after 30 min. In addition, the highest water loss (0.271 g water / g product) belonged to the 70% osmotic solution after 120
13
ACCEPTED MANUSCRIPT min while the lowest water loss (0.032 g water / g product) was for the 10% osmotic solution 30 min. In a similar study, Torringa et al. (2001), dried the mushroom samples pre-treated with sodium chloride osmotic solution by using microwave – HA drying. The purpose of this research was to investigate the effect of using sodium chloride solution on the samples dried by the abovementioned method. The concentrations of salt solution were 10 and 15% (w/w), its temperatures
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were 20 and 45°C, and the immersion times were 10, 30, 50, 70 and 110 min. The microwave
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specific power was maintained at about 4 W/g of the sample during the experiments. The ratio of
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sample to osmotic solution was 1:5. Increasing the solution temperature had the highest impact on increasing the rate of moisture removal during osmotic dehydration. Using osmotic solution
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through entering salt ions to the sample modified the dielectric characteristics and resulted in more uniform heating of the product, features which improved through increasing salt
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concentration. Findings indicated that during the immersion of mushroom in salt solution on the basis of the above-mentioned conditions, 30% of the samples water was lost. Subsequently,
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samples drying time by microwave – HA decreased. Use of salt solution decreased shrinkage and increased the porosity of the final product in comparison to non-pretreated samples. Rehydration
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also increased in these samples.
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3.2. Drying kinetics
Fig. 1a shows kinetics of drying the control quince samples (without application of microwave
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energy) in different osmotic concentrations (0 (control), 10, 30, 50, and 70%) using HA. In addition, Fig. 1b (a,b,c,d) shows kinetics of drying quince samples in osmotic concentrations of 0
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(without osmotic pretreatment) 10, 30, 50 and 70%, at power of 360 W and pulse ratios of 1, 2, 3 and 4. As Fig. 1a and Fig. 1b (a,b,c,d) show, initial moisture content decreased by increasing the concentration of the osmotic solution from 0 (control) to 70% (Table 4). This can be attributed to more moisture removal due to increasing the concentration of the osmotic solution during the osmotic pretreatment (Table 2). Moreover, according to Table 4, moisture content of the samples decreased after applying microwave and increasing pulse ratio from 1 to 4. This result could be due to moisture and temperature redistribution in the tissue when the microwave was off and, consequently, facility of moisture removal when the microwave was on (Aghilinategh et al., 2015). In addition, based on Table 4, drying time of the samples with osmotic concentrations of 14
ACCEPTED MANUSCRIPT 10, 30, 50 and 70% was lower in comparison with the control sample, which was due to removal of a part of moisture during the osmotic pre-treatment (Table 2). Soysal et al. (2009b) investigated microwave – HA drying kinetics and its effect on sensory and physical qualities (color and texture) of chili pepper. In this research, microwave drying was performed in two modes of intermittent and continuous with two output powers of 567.20 and 697.87 W. Overall, though continuous microwave – HA drying needed shorter drying time, it led to production of a
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low-quality product in terms of color and texture. However, IM – HA drying resulted in a high-
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quality product. Microwave – HA drying at a temperature lower than 35°C and with the pulse
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ratio of 3 and power of 597.20 W resulted in a significant decrease in drying time in comparison to HA drying. In another research, Kowalski et al. (2013) studied the effect of intermittent drying
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on the quality of dried carrot. Findings indicated that in case constant HA drying was replaced by intermittent drying, a significant impact on the final quality of carrot was observed. In
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comparison with constant HA drying conditions, intermittent drying of carrot proved more effective in improving the quality of the dried product. In intermittent drying, temperature and
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moisture differences were decreased in different parts of the product and their diffusion became more uniform, significantly decreasing the damage to dried products.
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Fig. 1b shows kinetics of drying in osmotic concentrations of 0 (without osmotic treatment), 10, 30, 50 and 70% of sucrose, at powers of 600 (e,f,g,h) and 900 W (i,j,k,l) and pulse ratios of 1, 2,
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3 and 4. Moisture content decreased by increasing the microwave power (Table 4). This can be attributed to the increase of internal vapor pressure through increasing microwave power and,
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consequently, quicker moisture removal (Sharma and Prasad, 2004). In a similar study, the effect of varying microwave power during microwave – vacuum drying on drying time and quality of
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beet root and carrot slices was investigated (Musielak and Kieca, 2014). The first series of experiments included drying at a low, constant microwave power. During the second series of experiments, periodical increase of microwave power was taken into consideration. However, such increases, which occurred in specific times, did not have a considerable effect on the process time and could not hand in dried products of proper quality. During the third series of experiments, initial drying was done with higher microwave power. The process, then, continued with lower applied power. Findings indicated that using a microwave dryer with varying power (the third method) reduced drying time, improved product in terms of less damage to products, less color change and better maintenance of beta carotene. 15
ACCEPTED MANUSCRIPT In addition, the experimental data were fitted with some empirical models for thin-layer drying (Table 3). Performance of the models were evaluated in terms of R2 and RMSE, which represent the coefficient of determination and the root mean square error of non-linear regression analysis, respectively (Tzempelikos et al., 2015). The higher the R2 and lower the RMSE, the better is the goodness of fit. Table 3 shows that among the selected models, the Page model gave the highest R2 with an average value of 0.9897 and the lowest RMSE with an average value of 0.0311,
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considering all the 65 treatments. Therefore, the Page model was selected as a suitable drying
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model to represent the drying behavior of quince slices in a combined microwave – hot air
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convective dryer within the experimental range.
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3.3. Effective moisture diffusion coefficient (Deff)
Deff is one of the significant factors in drying which is dependent on the characteristics of the
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foodstuffs. This parameter describes all possible mechanisms of moisture movement in the foodstuffs including liquid diffusion, vapor diffusion, surface diffusion, capillary flow, and
M
hydrostatic flow. The drying is mostly done during falling rate period and moisture transfer in drying is controlled through internal diffusion. Fick's second law that is considered for moisture
ED
diffusion in transient state is used for describing the drying process in falling rate drying period. Drying with HA decreases Deff due to increase in shrinkage (Sharma and Prasad,
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2004). Awareness of the changes of Deff during drying is required for modeling mass transfer. Fig. 2 shows the moisture ratio on the natural logarithm basis against drying time in drying the
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control samples at 0 power (without microwave application), and in osmotic concentrations of 0 (without osmotic pre-treatment), 10, 30, 50 and 70%. Generally, the slope of the natural
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logarithm of moisture ratio to drying time had a falling rate for all treatments and in the equations of all curves, negative slope was obtained (Fig. 2). Considering the direct relationship between the slope and Deff (equation 9), decreasing the slope reduces Deff. The Deff resulting from drying curve slopes is shown in Table 5. Generally, by increasing the concentration of the osmotic solution from 10 to 70%, significant decreases were noticed in Deff of quince samples in comparison to control samples. This result was in line with findings of Prothon et al. (2001). The low Deff in the tissue of treated samples can be attributed to the presence of sucrose in the tissue and surface of quince cubes that result in resistance against moisture removal. Prothon et al. (2001) reported similar results for samples of apple cubes pre-treated by sucrose solution. 16
ACCEPTED MANUSCRIPT Changes in Deff of garlic cloves during microwave – HA drying were examined in another research in which microwave powers were 10, 20, 30, and 40 W, the temperature was 40, 50, 60, and 70°C, and the air velocity was 1 and 2 m/s (Sharma and Prasad, 2004). Deff varied between 1.29 to 31.68×10-10 m2/s. A 3rd degree polynomial equation was obtained for establishing a correlation between Deff and moisture content. Deff increased by increasing the temperature and microwave power in constant air velocity. In addition, microwave – HA drying resulted in
T
decreasing the drying time by about 80-90% in comparison to HA drying. Noshad et al. (2012)
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investigated the effect of ultrasound- assisted osmotic dehydration pre-treatment on Deff during
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quince drying. Sucrose osmotic solution with the concentration of 50.52% was used (the ratio of osmotic solution to fruit was 20:1). The contact time of osmotic solution with quince slices was
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120 min. Deff of quince for non-pretreated samples ranged from 8.114×10-11 to 2.020×10-10 m2/s, and 6.085×10-11 to 1.308×10-10 m2/s for pre-treated samples. This difference was attributed to
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structural changes caused by pre-treatment. Tzempelikos et al. (2014) investigated the effect of drying air conditions on HA drying of quince fruit. Increase in both temperature and air circulation rate decreased the drying time. Deff obtained in different samples was between
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2.67×10-10 and 8.17×10-10 m2/s. Another similar research was an investigation of the effect of
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different pre-treatments on Deff of quince slices in a cabinet dryer (tray dryer) (Doymaz et al., 2015). The shortest drying time was obtained for quince slices pre-treated with citric acid
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solution. Deff obtained was between 6.97×10-11 and 2.38×10-10 m2/s. In another study, drying kinetics of quince slices during HA drying was evaluated (Tzempelikos et al., 2015). Findings
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indicated that the whole process occurred during the falling rate period and that the air temperature had a significant effect on the drying curve. Deff varied between 3.23×10-10 and
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7.82×10-10 m2/s.
In addition, by increasing the pulse ratio from 1 to 4, Deff in quince samples increased (Table 5). The reason for this phenomenon can be the decrease of internal stress which subsequently provides for facility of moisture removal (Aghilinategh et al., 2015). Moreover, by increasing the microwave power from 360 to 900 W, Deff increased (Table 5). This result was in agreement with researches of Sharma and Prasad (2004). The reason for such a result can be attributed to creation of higher internal vapor pressure at the power of 900 W and quicker moisture removal (Sharma and Prasad, 2004). In another research, Dak and Pareek (2014) investigated the effect of microwave power, vacuum pressure and sample mass on changes in Deff of pomegranate seeds 17
ACCEPTED MANUSCRIPT during microwave – vacuum drying. Deff increased by increasing the microwave power. Decreasing the sample weight—due to decrease in the moisture content of the sample— led to increasing Deff; by contrast, changing the vacuum rate had no effect on it. In total, in this study, the increase in Deff, compared to the literature, may be related to both quince porous texture and also applying intermittent microwave (especially at higher pulse ratios). In addition, most of the literature results, reported in this manuscript, are for continuous
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but not for intermittent microwave. As mentioned above, continuous microwave considerably
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decreases the Deff. Moreover, the lower Deffs for intermittent microwave in the literature could be
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related to the specific product texture investigated.
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3.4. Shrinkage
Shrinkage of the foodstuffs during dehydration and drying is a significant problem in the food
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industry and has a remarkably negative effect on the quality of the dried product. Moisture removal caused by applying heat results in tension in the cellular structure of the foodstuffs and,
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consequently, leads to deformation and shrinkage of the product. When shrinkage is not uniform during drying, the surface of the dried product may crack. Shrinkage occurs simultaneously with
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moisture diffusion during drying and may, consequently, affect moisture removal. Shrinkage occurs due to moisture removal and tension in the cellular structure of the foodstuffs. In food
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systems, shrinkage can be rarely ignored; it is, therefore, necessary to take shrinkage into consideration when predicting moisture profile during drying process (Koc et al., 2008). The
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shrinkage created in foodstuffs during drying often results in decreasing the heat transfer and reduces bulk density and rehydration capacity (Tsuruta et al., 2015).
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Fig. 3a shows shrinkage kinetics in control samples (without applying microwave) and in osmotic concentrations of 0 (without osmotic treatment), 10, 30, 50 and 70%. In addition, Fig. 3b (a,b,c,d) shows shrinkage kinetics in different concentrations of the osmotic solution and at the power of 360 W with the pulse ratios of 1, 2, 3 and 4. Considering Fig. 3a and Fig. 3b (a,b,c,d), it was observed that shrinkage after osmotic dehydration pre-treatment and before the drying process (the starting point in the figures) in samples pre-treated with the 70% osmotic solution was more than other samples. This can be the result of higher moisture removal from the 70% sample during the osmotic pre-treatment (Table 2) and, consequently, creation of more tension during moisture removal and, subsequently, more shrinkage in comparison to other samples. 18
ACCEPTED MANUSCRIPT However, at the end of the drying process, a reverse result was obtained in a way that the 70% sample had the minimum shrinkage. Generally, through increasing the osmotic solution concentration from 10% to 70%, shrinkage had significant decrease compared to the control sample. This result corresponded with findings of Prothon et al. (2001) and Koc et al. (2008). In fact, increasing the concentration of the osmotic solution—due to entrance of more sucrose molecules to the quince tissue and filling the interstitial spaces— prevented not only extra
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volume reduction but also shrinkage. Moreover, by increasing the pulse ratio from 1 to 4,
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shrinkage of quince samples, in comparison to control sample, was reduced. In fact, through
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continuous application of microwave energy (with pulse ratio of 1), shrinkage increased due to production of higher amounts of heat and acceleration of moisture removal from the samples'
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tissue (Maskan, 2001). This can also be attributed to decreasing the internal stress along with increasing the pulse ratio (Aghilinategh et al., 2015).
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Fig. 3b shows kinetics of shrinkage of quince samples in osmotic solution with concentrations of 0 (without application of osmotic pre-treatment), 10, 30, 50 and 70% and at powers of 600
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(e,f,g,h) and 900 W (i,j,k,l) and pulse ratios of 1, 2, 3 and 4, respectively. By increasing the microwave power from 360 to 900 W and simultaneously decreasing microwave energy
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application time (Table 1), shrinkage decreased. This result corresponds with findings of Maskan
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(2001).
3.5. Bulk density
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Bulk density is another physical property that is affected by different drying methods of foodstuffs with high moisture content such as fruits and vegetables. Bulk density affects the
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thermo-physical and transfer characteristics including heat and mass transfer (Dehghannya et al., 2016b). Depending on shrinkage, bulk density is changed by decreasing the moisture content in dried products. This change is, in itself, seriously affected by drying methods and can cause considerable changes in chemical compounds, structure and physical characteristics of the foodstuff (Koc et al., 2008). Changes in the bulk density of the foodstuffs which directly depend on different drying conditions have significant effects on the performance of the drying process such as drying rate and also structure of the dried products. Therefore, research on bulk density is of utmost importance for the development of a drying technology (Dehghannya et al., 2016b).
19
ACCEPTED MANUSCRIPT Investigation of the bulk density is important in storage, transportation, and packaging of the food products. Fig. 4a shows kinetics of bulk density in control samples (without microwave application) and in osmotic concentrations of 0 (without osmotic treatment), 10, 30, 50 and 70%. Fig. 4b (a,b,c,d) also shows kinetics of bulk density in osmotic concentrations of 0 (without osmotic pretreatment), 10, 30, 50 and 70% and at microwave power of 360 W and pulse ratios of 1, 2, 3 and
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4. Results showed that the 70% sample had the highest bulk density after the osmotic pre-
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treatment (in the starting point of the diagram), which can be attributed to greater shrinkage in
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the 70% sample in the stage after the osmotic pre-treatment (the starting point of Fig. 3a and Fig. 3b) (Witrowa-Rajchert and Rząca, 2009). However, at the end of the drying process, the 70%
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sample had the lowest bulk density in comparison to other samples. Overall, increasing the concentration from 0 (control) to 70% decreased bulk density. This result corresponds with the
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research findings of Prothon et al. (2001). Decrease in bulk density can be attributed to filling of intracellular spaces of quince samples with sucrose molecules with high molecular weight and the subsequent increase in the tissue resistance against volume and shrinkage reduction
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(Witrowa-Rajchert and Rząca, 2009). Prothon et al. (2001) obtained similar results during apple
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drying. In another research, bulk density and shrinkage of quince fruit were investigated in different moisture contents during drying (Koc et al., 2008). Four methods for drying cubic
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pieces of quince were used as follows: 1) HA drying with two dryers of fluidized bed and tray dryers, 2) infrared and HA drying, 3) Osmotic dehydration pre-treatment before tray dryer, and
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4) Freeze drying. In the first three methods, bulk density increased by reducing the moisture content. In freeze drying method, however, bulk density of quince pieces decreased by reducing
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the moisture content. In contrast to the lowest bulk density noticed in freeze drying, osmotic dehydration obtained the highest bulk density. In freeze drying, ice was sublimated without creating considerable shrinkage in the product. In this research, osmotic dehydration pretreatment was conducted by immersing the samples in a 50% sucrose solution at 40°C for 6 hours, which led to removal of 40 to 50% of the moisture during osmotic dehydration. In addition, bulk density decreased through increasing pulse ratio from 1 to 4. This result, which is in line with findings of Aghilinategh et al. (2015), can be attributed to reduction in shrinkage caused by increasing the pulse ratio (Fig. 3b).
20
ACCEPTED MANUSCRIPT Fig. 4b shows kinetics of the bulk density in osmotic concentrations of 0 (without osmotic treatment), 10, 30, 50 and 70% at microwave powers of 600 (e,f,g,h) and 900 W (i,j,k,l) and pulse ratios of 1, 2, 3 and 4. By increasing the microwave power from 0 to 900 W, bulk density decreased. This decrease corresponds with research findings of Aghilinategh et al. (2015). Decrease in bulk density by increasing the power can be attributed to lower shrinkage (less
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volume reduction) of the samples (Fig. 3b).
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3.6. Rehydration ratio
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Rehydration is a complicated process which aims at reconstructing dried foodstuffs characteristics and can be considered as a criterion for damages to foodstuffs tissue during drying
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(Noshad et al., 2012). Rehydration enables the product to restore the raw material characteristics it has lost during drying. Pre-treatment and drying conditions affect not only the structure and
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composition of the foodstuffs but also their rehydration. Rehydration capacity depends on tissue characteristics, rehydration method, and the pre-treatments applied to the product. Rehydration in
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dried plant tissues includes three simultaneous processes: 1) Water absorption into the dried product, 2) Product swelling, 3) Solid soluble substances leaking to the environment. The aim of
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research on rehydration is to obtain products with natural characteristics at the maximum possible rate (Noshad et al., 2012).
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Fig. 5a shows rehydration kinetics in control samples (without microwave application) in osmotic concentrations of 0 (without osmotic treatment), 10, 30, 50 and 70%. In addition, Fig. 5b
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(a,b,c,d) shows kinetics of rehydration in osmotic concentrations of 0 (without applying osmotic pre-treatment), 10, 30, 50 and 70% and at power of 360 W and pulse ratios of 1, 2, 3 and 4. By
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increasing the concentration of the osmotic solution from 10 to 70%, rehydration increased. Since rehydration and shrinkage have a negative relationship (Krokida and Philippopoulos, 2005), reduction of shrinkage due to increasing the concentration of the osmotic solution (Fig. 3b) resulted in increase in rehydration. In a similar study, optimization of osmotic solution during dehydration of quince fruit was investigated by response surface methodology (Akbarian et al., 2014). Findings indicated that fructose concentration had a significant effect on water loss. The solution containing 47.68% fructose, 3.99% calcium chloride, and 3.49% citric acid was chosen as the optimized osmotic solution. Electron microscope images showed that osmotic dehydration increased porosity. Osmotically dehydrated samples showed higher rehydration in 21
ACCEPTED MANUSCRIPT comparison to non-treated samples. In another study, Maskan (2001) investigated the effect of HA drying, microwave drying and a combination of these two on shrinkage and rehydration of kiwi fruit during drying. In this research, air velocity of 1.29 m/s, temperature of 60°C, and power of 210 W were used. Findings indicated that drying kiwi by microwave is faster than HA drying or microwave – HA. In comparison with HA method, drying time decreased by 89% in microwave, and by 40% in microwave – HA drying. Kiwi shrinkage during microwave drying
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was more than HA drying, a fact which was due to overheating and consequent acceleration of
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moisture removal from the sample tissue during microwave energy application. The lowest
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shrinkage was observed in microwave – HA drying, which was due to increase in drying rate and considerable decrease in drying time. Kiwi slices dried by microwave showed lower rehydration
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capacity in comparison to other drying methods, which was because of changes in the structure and tissue of the samples during drying. Furthermore, due to low shrinkage, rehydration capacity
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of the samples dried by microwave – HA was the highest. Overall, microwave – HA drying handed in the best product quality with the lowest shrinkage and highest rehydration.
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In addition, increasing the pulse ratio from 1 to 4 increased rehydration. Increase in rehydration can be attributed to reduction of shrinkage through increase in the pulse ratio (Fig. 3b). This
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result is in line with the findings of Aghilinategh et al. (2015). Fig. 5b shows kinetics of rehydration in osmotic concentrations of 0 (without osmotic treatment),
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10, 30, 50 and 70% and at powers of 600 (e,f,g,h) and 900 W (i,j,k,l) and pulse ratios of 1, 2, 3, and 4. By increasing the microwave power from 360 to 900 W, rehydration in quince samples
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increased, compared to the control sample. This result was in line with the findings of Seremet et al. (2016). The reason for this can be attributed to the increase of internal vapor pressure which
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occurs through increase in the microwave power and creation of a more porous structure due to vapor movement within the food product (Aghilinategh et al., 2015). In a similar study, Aghilinategh et al. (2015) investigated the effect of IM and continuous microwave – HA on kinetics of drying apple slices as well as physicochemical characteristics (Deff, bulk density, rehydration, and color change) of the dried product. The variables studied in this research included temperature (between 40 to 80ºC), microwave power (200-600 W), air velocity (0.5-2 m/s) and pulse ratio (2-6). Findings indicated that in comparison with HA, intermittent and continuous microwave dryings improved both drying kinetics and physicochemical characteristics. The lowest Deff belonged to HA drying. Continuous microwave drying had the 22
ACCEPTED MANUSCRIPT shortest drying time and the highest Deff. Since continuous microwave drying had a faster drying process in comparison to other methods, samples produced by this method manifested the lowest amount of color change. The drying time in IM drying was about three times more than continuous microwave drying. The bulk density decreased significantly by increasing microwave power, pulse ratio, temperature and HA velocity. Samples produced by IM drying had the lowest bulk density and the highest rehydration. In another research, optimization of process parameters
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to produce instant brown rice (easily cooked rice) was done by microwave – HA drying (Le and
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Jittanit, 2015). Findings indicated that high drying rate resulted in production of a desirably
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spongy and porous structure in the rice seed which, in turn, improved rehydration in the product. The microwave power level had a significant impact on the process in a way that the highest
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porosity and, consequently, the highest rehydration were obtained at the power of 595 W.
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3.7. Specific energy consumption
Another important aspect in evaluating the drying process is the specific energy consumption.
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Successful drying processes result in product quality maintenance and minimize energy consumption. During microwave heating, energy consumption changes by changing the moisture
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content and microwave power. Specific energy consumption is the energy required for removing 1 kg water from wet products (Tarhan et al., 2011). In other words, specific energy consumption
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is defined as the total energy applied during drying to the moisture content removed in the process (Jindarat et al., 2015). Physical and chemical characteristics of wet products (such as size
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and moisture content) and air conditions (the temperature used in drying and air circulation rate) affect specific energy consumption (Tarhan et al., 2011).
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Various magnitudes of specific energy consumption in drying control and pre-treated samples with HA and the combined method of microwave – HA at different powers and pulse ratios are shown in Table 6. By increasing the concentration of the osmotic solution from 10 (without osmotic pre-treatment) to 70%, specific energy consumption generally increased compared to the control sample (0% concentration). This can be attributed to the increase of sucrose concentration in the fruit tissue through increasing the concentration of the osmotic solution (Table 2). In other words, through increase of sucrose in the fruit tissue and increase in internal resistance against moisture removal, specific energy consumption increased. Moreover, by increasing the pulse ratio from 1 to 4, specific energy consumption decreased. This can be 23
ACCEPTED MANUSCRIPT attributed to decrease in drying time through increasing pulse ratio (Table 4). This result corresponds with findings of Soysal et al. (2009a). Furthermore, by increasing the microwave power from 360 to 900 W, specific energy consumption decreased compared to the power of 0 (control). This result is in line with findings of Jindarat et al. (2015). The reason for this result was decreasing microwave application time by increasing its power (Table 1). In a similar study, Zhao et al. (2014) applied the five following methods for drying carrot slices: 1) HA drying
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(60°C), 2) Drying with low microwave power (140 W) after HA drying (60°C), 3) HA drying
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(60°C) after drying with high microwave power (175 W), 4) Drying with low microwave power
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(140 W) after drying with high microwave power (175 W), and 5) IM – HA drying (60°C). Findings indicated that method 5 needed shorter drying time and lower energy consumption and
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produced the highest quality product in terms of color and rehydration in comparison to the other
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four methods.
3.8. Images of dried samples
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Fig. 6 shows, as samples, images of dried control and pre-treated quinces with different osmotic solution concentrations when drying with IM at powers of 360 (with pulse ratio of 1) and 900 W
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(with pulse ratio of 4), respectively. Images showed that the surface structure of the samples that were pre-treated in low sucrose concentrations was considerably damaged during drying.
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However, the structure of the samples pre-treated with osmotic solution of higher concentrations manifested less appearance damage. Moreover, as these images show, increasing the
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concentration of the osmotic solution decreased shrinkage and improved the appearance of the samples. This might be due to the beneficial effects of sucrose during osmotic dehydration that
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help produce a high-quality product. Sucrose acts as polyphenol oxidase inhibitor and also prevents the loss of volatile compounds during osmotic pretreatment (Prothon et al., 2001). Moreover, increasing solid gain by increasing the sucrose concentration (Table 2) can be the result of cellular membrane swelling of quince samples (Akbarian et al., 2014). This, in turn, prevented extensive shrinkage (Fig. 3b). The lower shrinkage at the higher sucrose concentrations could also be related to the entrance of more sucrose molecules to the quince tissue and filling the interstitial spaces.
4. Conclusion 24
ACCEPTED MANUSCRIPT A multi-stage continuous and intermittent microwave (IM) drying of quince fruit coupled with osmotic dehydration and low temperature HA (HA) drying was developed. Increasing the concentration of the osmotic solution and samples’ immersion time led to increase in water loss and solid gain. Quince samples pre-treated in sucrose osmotic solution had a shorter drying time in most of the treatments in comparison to control samples. Effective moisture diffusion coefficient (Deff) increased through increasing the microwave power and pulse ratio, while it
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decreased by increasing the concentration of the osmotic solution. Moreover, samples shrinkage
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decreased by increasing the concentration of the osmotic solution, microwave power and pulse
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ratio. In addition, samples dried by low-power IM showed the highest rehydration followed by samples dried by high-power IM. The control samples dried by only HA had the lowest
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rehydration. Furthermore, bulk density decreased by increasing the concentration of the osmotic solution, microwave power and pulse ratio. Due to the long time needed for drying, the highest
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specific energy consumption was for drying the control sample, which had undergone no osmotic pre-treatment or IM. Drying with IM – HA at the power of 900 W and the pulse ratio of 4 had
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the lowest specific energy consumption, while drying with merely HA, required the highest energy consumption. Overall, specific energy consumption decreased through increasing
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microwave power and pulse ratio.
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ACCEPTED MANUSCRIPT Chayjan, R. A., Kaveh, M., & Khayati, S. (2015). Modeling drying characteristics of hawthorn fruit under microwave-convective conditions. Journal of Food Processing and Preservation, 39(3), 239-253. Dak, M., & Pareek, N. K. (2014). Effective moisture diffusivity of pomegranate arils under going microwave-vacuum drying. Journal of Food Engineering, 122, 117-121. Dehghannya, J., Emam-Djomeh, Z., Sotudeh-Gharebagh, R., & Ngadi, M. (2006). Osmotic
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ACCEPTED MANUSCRIPT Koc, B., Eren, I., & Ertekin, F. K. (2008). Modelling bulk density, porosity and shrinkage of quince during drying: The effect of drying method. Journal of Food Engineering, 85(3), 340-349. Koné, K. Y., Druon, C., Gnimpieba, E. Z., Delmotte, M., Duquenoy, A., & Laguerre, J.-C. (2013). Power density control in microwave assisted air drying to improve quality of food. Journal of Food Engineering, 119(4), 750-757. Kowalski, S. J., Szadzińska, J., & Łechtańska, J. (2013). Non-stationary drying of carrot: Effect on
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Maskan, M. (2001). Drying, shrinkage and rehydration characteristics of kiwifruits during hot air and microwave drying. Journal of Food Engineering, 48(2), 177-182.
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Prothon, F., Ahrné, L. l. M., Funebo, T., Kidman, S., Langton, M., & Sjöholm, I. (2001). Effects of combined osmotic and microwave dehydration of apple on texture, microstructure and rehydration characteristics. LWT - Food Science and Technology, 34(2), 95-101. Seremet, L., Botez, E., Nistor, O.-V., Andronoiu, D. G., & Mocanu, G.-D. (2016). Effect of different drying methods on moisture ratio and rehydration of pumpkin slices. Food Chemistry, 195, 104-109. Sharma, G. P., & Prasad, S. (2004). Effective moisture diffusivity of garlic cloves undergoing microwave-convective drying. Journal of Food Engineering, 65(4), 609-617.
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ACCEPTED MANUSCRIPT Singh, R. P., & Heldman, D. R. (2014). Introduction to Food Engineering (Fifth ed.). New York: Academic Press. Soysal, Y., Arslan, M., & Keskin, M. (2009a). Intermittent microwave-convective air drying of oregano. Food Science and Technology International, 15(4), 397-406. Soysal, Y., Ayhan, Z., Eştürk, O., & Arıkan, M. F. (2009b). Intermittent microwave-convective drying of red pepper: Drying kinetics, physical (colour and texture) and sensory quality.
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Tarhan, S., Telci, İ., Taner Tuncay, M., & Polatci, H. (2011). Peppermint drying performance of
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Torringa, E., Esveld, E., Scheewe, I., van den Berg, R., & Bartels, P. (2001). Osmotic dehydration as a pre-treatment before combined microwave-hot-air drying of mushrooms. Journal of Food
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Engineering, 49(2), 185-191.
Tsuruta, T., Tanigawa, H., & Sashi, H. (2015). Study on shrinkage deformation of food in
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microwave-vacuum drying. Drying Technology, 33(15-16), 1830-1836. Tzempelikos, D. A., Vouros, A. P., Bardakas, A. V., Filios, A. E., & Margaris, D. P. (2014). Case
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Tzempelikos, D. A., Vouros, A. P., Bardakas, A. V., Filios, A. E., & Margaris, D. P. (2015). Experimental study on convective drying of quince slices and evaluation of thin-layer drying
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ACCEPTED MANUSCRIPT Table 1 On / off times at different microwave powers and pulse ratios 360 W 600 W Pulse Time (min) ratio 1 10 min on (10 min total) 4 min on (4 min total) 1 min on / 1 min off (20 min 1 min on / 1 min off (8 min 2 total) total) 1 min on / 2 min off (30 min 1 min on / 2 min off (12 min 3 total) total) 1 min on / 3 min off (40 min 1 min on / 3 min off (16 min 4 total) total)
900 W
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3 min on (3 min total) 1 min on / 1 min off (6 min total) 1 min on / 2 min off (9 min total) 1 min on / 3 min off (12 min total)
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ACCEPTED MANUSCRIPT
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Table 2 Water loss and solid gain during osmotic dehydration of samples at different concentrations of osmotic solution and immersion times Concentration of osmotic solution Immersion time Water loss (g water / g Solid gain (g solid / g (% w/w) (min) product) product) i i 30 0.032 ± 0.010 0.006 ± 0.001 i i 60 0.040 ± 0.009 0.011 ± 0.009 10 i i 90 0.050 ± 0.007 0.016 ± 0.011 hi i 120 0.060 ± 0.010 0.019 ± 0.014 hi h 30 0.064 ± 0.049 0.067 ± 0.016 gh gh 60 0.100 ± 0.049 0.088 ± 0.016 30 fg fg 90 0.119 ± 0.054 0.098 ± 0.015 efg efg 120 0.138 ± 0.058 0.107 ± 0.012 efg gh 30 0.140 ± 0.029 0.088 ± 0.004 cde def 60 0.179 ± 0.062 0.121 ± 0.007 50 bc cde 90 0.212 ± 0.048 0.133 ± 0.004 ab bcd 120 0.240 ± 0.029 0.150 ± 0.019 def efg 30 0.157 ± 0.049 0.110 ± 0.023 bcd abc 60 0.202 ± 0.052 0.157 ± 0.049 70 ab ab 90 0.245 ± 0.039 0.169 ± 0.046 a a 120 0.271 ± 0.037 0.183 ± 0.052 * Different letters in the same column indicate a significant difference (p < 0.05)
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ACCEPTED MANUSCRIPT Table 3 Drying kinetics modeling at different concentrations of osmotic solution [OS (% w/w)], microwave pulse ratios [PR] and powers [P (W)] Variables Coefficient(s) and Statistical Parameters of Various Drying Kinetics Models
-
0
50
-
0
70
-
0
0
1
10
1
30
1
50
1
70
1
0
1
10
1
30
1
50
1
70
1
0
1
10
1
30
1
50
1
70
1
0
2
10
2
30
2
36 0 36 0 36 0 36 0 36 0 60 0 60 0 60 0 60 0 60 0 90 0 90 0 90 0 90 0 90 0 36 0 36 0 36 0
0.0109 0.0100 1 0.0074 84 0.0090 38 0.0099 29 0.0092 88 0.0117 2 0.0093 5 0.0100 6 0.0126 3 0.0107 2
0.1047 0.208 0.2452
1.28 7 1.1 1.04 9 0.90 89 0.63 23 0.68 95 0.54 56 0.41 12 0.35 79 0.55 35 0.71 2 0.55 17 0.40 71 0.40 24 0.75 46 0.88 74 0.71 02 0.37 18 0.29 65 0.69 99 0.71 83 0.63 13
0.1006
0.0461 4 0.0959 5 0.1843 0.1644 0.0308 2 0.0172 4 0.0389 5 0.2392 0.3183 0.0436 8 0.0468 4 0.0625 9
31
RMSE 0.0121 9 0.0070 03 0.0140 5 0.0125 8 0.0107 3 0.0521 1 0.0512 2 0.0181 1 0.0274 6 0.0298 8 0.0555 9 0.0539 4 0.0391 7 0.0373 5 0.0370 7 0.0626 1 0.0439 3 0.0390 5 0.0347 1 0.0263 6 0.0306 6 0.0307 9 0.0121 8
a 1.05 2 1.05 1 1.02 8 1.01 3 0.98 27 0.88 4 0.90 71 0.86 59 0.81 63 0.77 02 0.86 45 0.90 96 0.87 19 0.81 38 0.79 58 0.90 27 0.93 91 0.91 42 0.80 38 0.73 33 0.90 57 0.92 31 0.89 65
k 0.0057 7 0.0070 09 0.0059 46 0.0062 86 0.0058 08 0.0097 53 0.0117 2 0.0102 8 0.0106 9 0.0077 24 0.0098 49 0.0105 1 0.0093 12 0.0077 28 0.0056 05 0.0081 42 0.0093 34 0.0084 37 0.0088 29 0.0059 19 0.0089 73 0.0114 9 0.0093 32
T
30
1.23
R 0.99 87 0.99 96 0.99 82 0.99 85 0.99 88 0.97 77 0.98 14 0.99 67 0.99 3 0.98 94 0.97 39 0.97 91 0.98 66 0.98 52 0.98 24 0.96 86 0.98 62 0.98 76 0.98 75 0.98 99 0.99 1 0.99 23 0.99 86
MR a exp(kt)
IP
0
n
CR
-
k 0.0016 04 0.0015 17 0.0034 09 0.0048 13 0.0095 61 0.0658 6 0.0555 9
6
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RMSE 0.034 65 0.041 33 0.020 61 0.014 24 0.018 65 0.083 47 0.073 52 0.087 68 0.137 9 0.148 7 0.089 9 0.065 34 0.085 24 0.123 6 0.123 4 0.068 2 0.044 12 0.057 16 0.130 9 0.152 7 0.061 4 0.059 16 0.072 06
2
AN
0
R 0.98 85 0.98 51 0.99 57 0.99 79 0.99 6 0.93 47 0.95 52 0.91 5 0.79 48 0.70 47 0.92 33 0.96 5 0.92 84 0.81 71 0.78 48 0.95 81 0.98 44 0.97 01 0.80 04 0.63 13 0.96 01 0.96 75 0.94 32
MR exp( kt n )
M
-
k 0.0054 98 0.0066 95 0.0057 85 0.0062 05 0.0059 18 0.0112 1 0.0130 6 0.0124 7 0.0143 6 0.0112 7 0.0115 4 0.0115 8
2
5
ED
0
MR exp(kt)
PT
4 3
P
CE
PR 2
1
AC
OS
2
R 0.99 17 0.98 84 0.99 68 0.99 82 0.99 65 0.96 22 0.97 22 0.94 68 0.87 5 0.83 49 0.96 12 0.98 15 0.96 35 0.90 79 0.89 96 0.97 84 0.99 13 0.98 58 0.89 59 0.81 55 0.97 43 0.97 7 0.96 17
RMSE 0.030 56 0.038 48 0.018 72 0.014 13 0.018 38 0.067 89 0.062 55 0.073 11 0.116 2 0.117 9 0.067 84 0.050 81 0.064 56 0.093 03 0.088 41 0.051 91 0.034 97 0.041 87 0.100 3 0.112 8 0.051 91 0.053 23 0.062 74
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3
0.0098 0.0102 3 0.0102 6 0.0098 8 0.0089 73 0.0099 18 0.0109 9 0.0102 2 0.0096 32 0.0080 01 0.0085 28 0.0099 97 0.0101 1 0.0117
0.0449 8 0.0444 4 0.0737 5
0.116
0.1451
0.148 3 0.042 03 0.031 59 0.062 7 0.108 7 0.119 8 0.047 72 0.062 38 0.069 77 0.111 2 0.123 4 0.061 07 0.036 05 0.067 72 0.120
0.145 0.2111 0.0483 9 0.0158 4 0.0487 7
0.2518 0.0218 4 0.0185 2 0.0546 0.1275 0.1558 0.0249 8 0.0309 9 0.0535 7 0.1271 0.1529 0.0242 6 0.0187 0.0543 0.1469
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0.83 92 0.81 72 0.9 0.92 07 0.86 88 0.81 82 0.75 04 0.89 39 0.94 44 0.90 37 0.81 42 0.74 8 0.95 26 0.96 55 0.91 23 0.84 32 0.81 03 0.93 34 0.92 77 0.90 13 0.82 22 0.78 12 0.92 33 0.95 09 0.89 79 0.83
0.0198 0.0387 0.0542 8 0.0276 0.0216 1 0.0257 9 0.0544 1 0.0321 8 0.0295 1 0.0401
0.0105 2 0.0084 64 0.0088 73 0.0112 9 0.0078 59 0.0076 53 0.0053 4 0.0084 99 0.0094 87 0.0089 14 0.0069 2 0.0058 72 0.0093 05 0.0098 58 0.0091 65 0.0078 53 0.0066 78 0.0092 16 0.0101 4 0.0090 45 0.0074 09 0.0057 19 0.0078 28 0.0094 86 0.0089 29 0.0094
T
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0.067 2 0.061 91 0.079 37 0.114 3 0.141 8 0.078 39 0.048 02 0.064 45
0.0096 52 0.0144 6 0.0463 5 0.0440 3 0.0262 7 0.0253 5 0.0182 6 0.0619 3 0.0493 9 0.0390 2 0.0435 3 0.0358 5 0.0326 4 0.0252 7 0.0131 4 0.0278 1
IP
30
0.1735
0.99 9 0.99 75 0.98 21 0.98 66 0.99 29 0.99 27 0.99 5 0.96 9 0.98 32 0.98 73 0.97 85 0.98 22 0.99 11 0.99 49 0.99 82 0.99 08 0.99 47 0.98 82 0.97 88 0.99 34 0.99 46 0.99 06 0.97 62 0.99 23 0.99 24 0.98
CR
2
0.123
0.43 3 0.42 64 0.69 34 0.72 76 0.58 43 0.45 48 0.36 08 0.67 21 0.90 67 0.67 58 0.44 47 0.33 54 0.83 56 0.87 68 0.65 38 0.47 67 0.42 68 0.81 15 0.78 33 0.65 74 0.47 63 0.42 32 0.79 09 0.87 15 0.65 54 0.46
US
10
0.1819
AN
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0.120 9
M
0
0.82 4 0.79 89 0.95 76 0.96 9 0.92 88 0.83 61 0.67 4 0.94 33 0.98 18 0.96 09 0.83 04 0.66 56 0.98 32 0.99 08 0.95 34 0.84 25 0.78 34 0.97 97 0.96 74 0.95 19 0.84 07 0.76 58 0.96 58 0.98 91 0.95 49 0.84
ED
2
0.0140 2 0.0114 9 0.0099 48 0.0123 5 0.0093 09 0.0099 77 0.0080 42 0.0095 81 0.0100 5 0.0099 65 0.0089 2 0.0087 17
PT
70
36 0 36 0 60 0 60 0 60 0 60 0 60 0 90 0 90 0 90 0 90 0 90 0 36 0 36 0 36 0 36 0 36 0 60 0 60 0 60 0 60 0 60 0 90 0 90 0 90 0 90
CE
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AC
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0.87 01 0.86 41 0.97 72 0.98 18 0.96 18 0.91 07 0.83 46 0.96 91 0.98 78 0.97 97 0.91 64 0.83 81 0.98 68 0.99 26 0.96 49 0.88 83 0.85 36 0.98 75 0.97 8 0.97 24 0.90 96 0.87 71 0.97 89 0.99 33 0.97 54 0.91
0.111 1 0.107 3 0.052 23 0.051 21 0.060 95 0.088 93 0.105 5 0.061 87 0.042 01 0.049 31 0.085 82 0.108 2 0.039 77 0.030 22 0.057 35 0.097 05 0.103 9 0.039 83 0.055 3 0.056 57 0.088 34 0.093 38 0.051 24 0.030 06 0.052 97 0.097
ACCEPTED MANUSCRIPT
0.86 23 0.68 23 0.48 83 0.36 54
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18 0.0066 3 0.0093 2 0.0098 14 0.0083 14 0.0128 8 0.0084 39 0.0084 77 0.0106 9 0.0092 76 0.0079 44 0.0060 3 0.0081 8 0.0099 22 0.0077 97 0.0083 26 0.0063 81
T
81 0.74 57 0.97 32 0.99 39 0.95 24 0.91 74 0.88 45 0.93 83 0.92 9 0.91 38 0.84 59 0.77 41 0.91 02 0.95 19 0.90 22 0.83 17 0.75 71
IP
7 0.0188 9 0.0403 6 0.0254 4 0.0207 7 0.0108 3 0.0128 5 0.0319 7 0.0323 7 0.0194 6 0.0195 2 0.0202 8 0.0506 6 0.0379 2 0.0245 8 0.0334 9
CR
53 0.99 5 0.98 72 0.99 52 0.99 6 0.99 89 0.99 81 0.99 15 0.99 18 0.99 66 0.99 57 0.99 42 0.97 83 0.98 99 0.99 43 0.98 82 0.99 19
US
51 0.33 97 0.90 86 1.00 6 0.79 97 0.44 01 0.50 22 0.81 73 0.75 85 0.69 52 0.51 88 0.39 88
AN
0 7 43 9 90 0.0104 0.66 0.148 70 3 0.2614 0 3 22 7 36 0.0095 0.98 0.040 0.0149 0 4 0 79 5 47 6 36 0.0098 0.99 0.023 0.0096 10 4 0 73 52 57 07 36 0.0087 0.98 0.039 0.0236 30 4 0 6 31 97 3 36 0.89 0.098 50 4 0.0145 0.1846 0 18 5 36 0.0098 0.88 0.095 70 4 0.1114 0 76 3 48 60 0.0090 0.98 0.043 0.0224 0 4 0 83 23 47 6 60 0.0116 0.97 0.052 0.0364 10 4 0 1 51 84 4 60 0.0103 0.96 0.058 0.0449 30 4 0 2 49 9 4 60 0.0099 0.87 0.098 50 4 0.1039 0 46 69 68 60 0.0086 0.72 0.131 70 4 0.1778 0 98 97 9 90 0.0090 0.96 0.063 0.0315 0 4 0 5 12 9 4 90 0.0104 0.98 0.041 0.0202 10 4 0 6 56 93 5 90 0.0087 0.95 0.061 0.0429 30 4 0 83 96 83 3 90 0.0105 0.85 0.110 50 4 0.1263 0 8 52 5 90 0.0096 0.69 0.141 70 4 0.2221 0 17 31 5 1 Concentration of osmotic solution (% w/w) 2 Pulse ratio 3 Power (W) 4 Newton model (Tzempelikos et al., 2015) 5 Page model (Tzempelikos et al., 2015) 6 Henderson and Pabis model (Tzempelikos et al., 2015)
26 0.81 39 0.98 62 0.99 52 0.98 69 0.90 22 0.90 7 0.98 9 0.98 37 0.97 8 0.92 33 0.84 68 0.97 67 0.99 03 0.97 78 0.91 71 0.83 03
84 0.115 8 0.041 96 0.025 31 0.037 59 0.101 2 0.091 0.036 31 0.045 68 0.049 43 0.082 12 0.104 2 0.052 47 0.037 27 0.048 35 0.088 7 0.110 3
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Table 4 Magnitudes of moisture content after osmotic dehydration [MC-OD (g water / g dry solid)], moisture content after microwave drying [MC-MD (g water / g dry solid)], final moisture content [MC-F (g water / g dry solid)], drying time considering microwave "on" time [DT-ON (min)] and drying time considering microwave "on" and "off" times [DT-ON-OFF (min)] of samples as influenced by concentration of osmotic solution [OS (% w/w)], microwave pulse ratio [PR] and power [P (W)] 1 2 3 4 5 6 7 8 OS PR P MC-OD MC-MD MC-F DT-ON DT-ON-OFF defghi abcde 0 0 5.921 ± 0.120 0.190 ± 0.048 585 bcde gf 10 0 6.070 ± 0.080 0.140 ± 0.030 450 j abcde 30 0 3.500 ± 0.033 0.189 ± 0.019 495 k abcd 50 0 2.238 ± 0.024 0.206 ± 0.014 405 l abcde 70 0 1.928 ± 0.033 0.188 ± 0.013 450 bcdef ijkl abcdef 0 1 360 6.055 ± 0.167 4.076 ± 0.167 0.181 ± 0.025 325 325 hi ijkl gf 10 1 360 5.860 ± 0.030 4.035 ± 0.110 0.140 ± 0.030 280 280 j qrs abcde 30 1 360 3.479 ± 0.034 2.306 ± 0.050 0.196 ± 0.010 415 415 k tuvw abcde 50 1 360 2.238 ± 0.011 1.246 ± 0.076 0.191 ± 0.009 280 280 l wxy abcdef 70 1 360 1.877 ± 0.030 0.993 ± 0.012 0.185 ± 0.011 370 370 efghi hijk abcdef 0 1 600 5.903 ± 0.153 4.187 ± 0.334 0.186 ± 0.017 364 364 bcde bcde def 10 1 600 6.071 ± 0.035 4.725 ± 0.190 0.158 ± 0.008 319 319 j op abcde 30 1 600 3.557 ± 0.056 2.639 ± 0.198 0.193 ± 0.018 364 364 k t abc 50 1 600 2.225 ± 0.013 1.481 ± 0.082 0.209 ± 0.023 364 364 l tuv abcdef 70 1 600 1.887 ± 0.009 1.283 ± 0.073 0.183 ± 0.018 454 454 b bc abcde 0 1 900 6.165 ± 0.201 4.840 ± 0.181 0.200 ± 0.029 363 363 a a ef 10 1 900 6.314 ± 0.445 5.329 ± 0.097 0.154 ± 0.032 363 363 j o abcdef 30 1 900 3.574 ± 0.065 2.771 ± 0.049 0.184 ± 0.014 363 363 k t abcd 50 1 900 2.238 ± 0.018 1.454 ± 0.130 0.206 ± 0.015 363 363 l uvwxy abcde 70 1 900 1.861 ± 0.014 1.112 ± 0.034 0.201 ± 0.009 498 498 bcdef imn abcde 0 2 360 6.053 ± 0.053 3.895 ± 0.053 0.193 ± 0.030 415 425 hi n bcdef 10 2 360 5.851 ± 0.043 3.649 ± 0.087 0.167 ± 0.030 325 335 j rs abcde 30 2 360 3.528 ± 0.021 2.263 ± 0.131 0.194 ± 0.013 370 380 k uvwx abcde 50 2 360 2.235 ± 0.026 1.133 ± 0.048 0.195 ± 0.012 325 335 l wxy abcde 70 2 360 1.887 ± 0.010 0.973 ± 0.086 0.198 ± 0.009 370 380 bcde efg abc 0 2 600 6.080 ± 0.187 4.466 ± 0.283 0.210 ± 0.066 364 368 fghi fgh abcd 10 2 600 5.881 ± 0.105 4.414 ± 0.054 0.206 ± 0.046 274 278 j op ab 30 2 600 3.600 ± 0.033 2.600 ± 0.088 0.211 ± 0.019 454 458 k t abcde 50 2 600 2.260 ± 0.020 1.457 ± 0.121 0.195 ± 0.005 409 413 l uvxw ab 70 2 600 1.875 ± 0.021 1.139 ± 0.012 0.215 ± 0.012 499 503 defghi fgh abcdef 0 2 900 5.930 ± 0.030 4.404 ± 0.185 0.175 ± 0.030 318 321 cdefgh b abcdef 10 2 900 5.969 ± 0.192 4.866 ± 0.203 0.172 ± 0.058 318 321 j o abcde 30 2 900 3.569 ± 0.046 2.739 ± 0.018 0.193 ± 0.018 363 366 k t abc 50 2 900 2.266 ± 0.049 1.460 ± 0.039 0.210 ± 0.013 408 411 l vwxy abcde 70 2 900 1.877 ± 0.019 1.068 ± 0.011 0.199 ± 0.013 453 456 bcde imn abcdef 0 3 360 6.061 ± 0.142 3.866 ± 0.205 0.181 ± 0.017 325 345 defghi imn abcdef 10 3 360 5.947 ± 0.053 3.895 ± 0.450 0.175 ± 0.030 325 345 j s abcd 30 3 360 3.569 ± 0.046 2.079 ± 0.039 0.205 ± 0.004 415 435 k vwxy abcde 50 3 360 2.235 ± 0.026 1.071 ± 0.063 0.192 ± 0.007 370 390 l xy abcde 70 3 360 1.848 ± 0.022 0.886 ± 0.029 0.197 ± 0.003 415 435 bcdefg defg abcdef 0 3 600 6.040 ± 0.174 4.526 ± 0.120 0.181 ± 0.025 364 372 cdefgh ghij cdef 10 3 600 5.984 ± 0.135 4.292 ± 0.090 0.164 ± 0.005 274 282 j opq a 30 3 600 3.540 ± 0.020 2.529 ± 0.177 0.218 ± 0.053 319 327 k t abcde 50 3 600 2.262 ± 0.015 1.417 ± 0.156 0.192 ± 0.011 409 417 l uvwx abcde 70 3 600 1.893 ± 0.020 1.122 ± 0.077 0.200 ± 0.011 498 507
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318 363 363 273 453 280 280 325 280 325 364 319 364 409 454 363 273 408 363 453
AC
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defghi
0 3 900 5.947 ± 0.053 4.596 ± 0.030 0.193 ± 0.030 ghi bcd abcd 10 3 900 5.864 ± 0.077 4.745 ± 0.183 0.204 ± 0.006 j op abcde 30 3 900 3.522 ± 0.051 2.600 ± 0.088 0.189 ± 0.019 k tu abcde 50 3 900 2.254 ± 0.019 1.390 ± 0.082 0.199 ± 0.018 l vwxy abcde 70 3 900 1.856 ± 0.019 1.015 ± 0.125 0.192 ± 0.011 bcde mn g 0 4 360 6.073 ± 0.063 3.660 ± 0.021 0.106 ± 0.050 bc klm abcdef 10 4 360 6.141 ± 0.032 3.923 ± 0.153 0.178 ± 0.028 j s abcde 30 4 360 3.622 ± 0.059 2.184 ± 0.111 0.196 ± 0.021 k y abcde 50 4 360 2.190 ± 0.030 0.831 ± 0.013 0.194 ± 0.010 l xy abcde 70 4 360 1.895 ± 0.021 0.906 ± 0.030 0.189 ± 0.009 hi fghi abcdef 0 4 600 5.859 ± 0.031 4.340 ± 0.067 0.187 ± 0.027 defghi ijkl abcdef 10 4 600 5.949 ± 0.138 4.111 ± 0.389 0.172 ± 0.024 j pqr abc 30 4 600 3.511 ± 0.025 2.460 ± 0.184 0.211 ± 0.013 k tu abcde 50 4 600 2.267 ± 0.051 1.387 ± 0.044 0.202 ± 0.011 l vwxy abcd 70 4 600 1.872 ± 0.020 1.022 ± 0.086 0.206 ± 0.013 bcd fgh cdef 0 4 900 6.094 ± 0.089 4.378 ± 0.336 0.164 ± 0.011 i defg abcd 10 4 900 5.781 ± 0.073 4.492 ± 0.128 0.204 ± 0.006 j op abcde 30 4 900 3.561 ± 0.042 2.627 ± 0.012 0.198 ± 0.004 k tu abcdef 50 4 900 2.278 ± 0.027 1.365 ± 0.050 0.183 ± 0.014 l vwxy abcd 70 4 900 1.918 ± 0.053 1.025 ± 0.120 0.203 ± 0.002 * Different letters in the same column indicate a significant difference (p < 0.05) 1 Concentration of osmotic solution (% w/w) 2 Pulse ratio 3 Power (W) 4 Moisture content after osmotic dehydration (g water / g dry solid) 5 Moisture content after microwave drying (g water / g dry solid) 6 Final moisture content (g water / g dry solid) 7 Drying time considering microwave "on" time (min) 8 Drying time considering microwave "on" and "off" times (min)
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324 369 369 279 459 310 310 355 310 355 376 331 376 421 466 372 282 417 372 462
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Table 5 Magnitudes of effective moisture diffusion coefficient (Deff) at different concentrations of osmotic solution [OS (% w/w)], microwave pulse ratios [PR] and powers [P (W)] 1 2 3 2 OS PR P Deff (m /s) -8opq -9 0 0 8.99732×10 ±3.36947×10 -7j -9 10 0 1.25962×10 ±5.12393×10 -8pqrs -10 30 0 8.70552×10 ±8.42369×10 -8opq -9 50 0 8.90005×10 ±2.52711×10 -8s -9 70 0 7.58693×10 ±2.52711×10 -7def -9 0 1 360 1.45416×10 ±9.49301×10 -7a -9 10 1 360 1.83837×10 ±7.72043×10 -8nop -9 30 1 360 9.43503×10 ±1.68474×10 -7kl -9 50 1 360 1.11372×10 ±2.2287×10 -8rs -9 70 1 360 7.73283×10 ±1.45903×10 -7ghij -9 0 1 600 1.33744×10 ±1.68474×10 -7c -9 10 1 600 1.62438×10 ±2.2287×10 -7kl -9 30 1 600 1.10886×10 ±5.05421×10 -8pqrs -9 50 1 600 8.46235×10 ±1.45903×10 -8t -9 70 1 600 6.41971×10 ±1.45903×10 -7hij -9 0 1 900 1.32771×10 ±6.68609×10 -7de -9 10 1 900 1.47848×10 ±5.12393×10 -7kl -9 30 1 900 1.11859×10 ±3.36947×10 -8pqr -9 50 1 900 8.75415×10 ±2.91805×10 -8u -9 70 1 900 5.30112×10 ±2.2287×10 -7k -9 0 2 360 1.15749×10 ±6.07441×10 -7d -9 10 2 360 1.51739×10 ±5.26059×10 -7lmn -9 30 2 360 1.03591×10 ±2.91805×10 -9 -8opq 50 2 360 ±3.86022×10 9.04596×10 -8s -9 70 2 360 7.58693×10 ±2.52711×10 -7ij -8 0 2 600 1.28394×10 ±1.48075×10 -7bc -8 0 2 600 1.71679×10 ±1.82427×10 -8pqrs -9 30 2 600 8.60825×10 ±6.35974×10 -8rs -9 50 2 600 7.78147×10 ±1.68474×10 -8u -9 70 2 600 5.25249×10 ±1.45903×10 -7defgh -9 0 2 900 1.42498×10 ±2.2287×10 -7c -8 10 2 900 1.62438×10 ±1.24089×10 -7kl -9 30 2 900 1.12831×10 ±3.0372×10 -8rs -9 50 2 900 7.73283×10 ±2.52711×10 -8tu -9 70 2 900 6.07927×10 ±4.4574×10 -7defg -9 0 3 360 1.44443×10 ±8.75415×10 -7de -9 10 3 360 1.47362×10 ±5.26059×10 -8opq -9 30 3 360 9.09459×10 ±4.4574×10 -8qrs -9 50 3 360 8.07327×10 ±1.68474×10 -8t 70 3 360 6.41971×10 ±0 -7efghi -9 0 3 600 1.37635×10 ±3.6718×10 -7bc -10 10 3 600 1.71192×10 ±8.42369×10 -7k -9 30 3 600 1.16236×10 ±8.54911×10 -8rs -9 50 3 600 7.78147×10 ±5.89658×10 -8tu -9 70 3 600 5.6902×10 ±3.86022×10 -7efghi -9 0 3 900 1.37148×10 ±5.26059×10 -7fghij -9 10 3 900 1.35689×10 ±2.52711×10 -7kl -9 30 3 900 1.10886×10 ±6.68609×10
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-7kl
50 3 900 1.13804×10 ±3.86022×10 -8tu -9 70 3 900 5.93337×10 ±1.68474×10 -7ab -8 0 4 360 1.76542×10 ±1.82815×10 -7bc -9 10 4 360 1.66815×10 ±3.0372×10 -7kl -9 30 4 360 1.1429×10 ±3.36947×10 -8mno -9 50 4 360 9.97001×10 ±3.6718×10 -8qrs -9 70 4 360 8.26781×10 ±3.0372×10 -7ij -9 0 4 600 1.27422×10 ±4.4574×10 -7d -8 10 4 600 1.51739×10 ±1.39182×10 -7klm -9 30 4 600 1.07482×10 ±7.1972×10 -8rs -9 50 4 600 7.6842×10 ±2.2287×10 -8tu -9 70 4 600 5.88473×10 ±2.2287×10 -7hij -9 0 4 900 1.33258×10 ±2.2287×10 -7c -9 10 4 900 1.61952×10 ±2.91805×10 -8nop -10 30 4 900 9.58093×10 ±8.42369×10 -9 -8opq 50 4 900 ±1.45903×10 9.19186×10 -8tu -9 70 4 900 6.03064×10 ±3.6718×10 * Different letters in the same column indicate a significant difference (p < 0.05) 1 Concentration of osmotic solution (% w/w) 2 Pulse ratio 3 Power (W)
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Table 6 Specific energy consumption of microwave drying [SE-MD (×10 MJ/kg)], specific energy consumption of 3 3 hot-air drying [SE-HA (×10 MJ/kg)] and total specific energy consumption [SE-Total (×10 MJ/kg)] at different concentrations of osmotic solution [OS (% w/w)], microwave pulse ratios [PR] and powers [P (W)] 1 2 3 4 5 6 OS PR P SE-MD SE-HA SE-Total nop nop 0 0 224.060 ±5.090 224.060 ±5.090 pqrstu pqrstu 10 0 192.123 ±3.739 192.123 ±3.739 mno mno 30 0 238.795 ±6.171 238.795 ±6.171 mno mno 50 0 243.135 ±3.799 243.135 ±3.799 hijk hijk 70 0 294.796 ±7.740 294.796 ±7.740 cdefghij tuvwxyz tuvwxy 0 1 360 0.610 ±0.060 172.890 ±8.621 173.500 ±8.586 cdefghi yzαβ xyzα 10 1 360 0.624 ±0.028 140.214 ±3.238 140.838 ±3.212 defghijk mno mno 30 1 360 0.594 ±0.023 237.937 ±4.699 238.531 ±4.681 ghijklmnopq mno mno 50 1 360 0.520 ±0.042 234.835 ±16.468 235.355 ±16.426 ghijklmnopqrs fg fg 70 1 360 0.499 ±0.022 349.107 ±6.694 349.607 ±6.693 opqrs tuvwxyz tuvwxy 0 1 600 0.425 ±0.061 173.672 ±14.648 174.098 ±14.592 efghijklmn yzαβ xyzα 10 1 600 0.569 ±0.070 139.594 ±5.463 140.163 ±5.393 fghijklmno pqrstu pqrstu 30 1 600 0.562 ±0.140 195.856 ±16.116 196.418 ±15.994 lmnopqrs lmn lmn 50 1 600 0.453 ±0.043 253.339 ±11.779 253.791 ±11.738 ghijklmnopqrs gh gh 70 1 600 0.500 ±0.053 328.035 ±16.863 328.536 ±16.810 bcde uvwxyzα uvwxyz 0 1 900 0.695 ±0.136 165.763 ±7.009 166.459 ±6.953 a uvwxyzαβ uvwxyzα 10 1 900 1.049 ±0.324 157.228 ±3.190 158.277 ±3.354 bcd qrstuvw qrstuv 30 1 900 0.706 ±0.105 184.373 ±3.450 185.079 ±3.345 ghijklmnopqr klm klm 50 1 900 0.505 ±0.098 265.660 ±27.259 266.165 ±27.165 lmnopqrs bc bc 70 1 900 0.452 ±0.028 435.805 ±19.638 436.257 ±19.610 ghijklmnopq nopq nopq 0 2 360 0.527 ±0.000 221.246 ±3.667 221.772 ±3.667 hijklmnopqrs tuvwxyz tuvwxy 10 2 360 0.491 ±0.023 173.866 ±5.939 174.357 ±5.918 fghijklmno opqr opqr 30 2 360 0.554 ±0.051 216.085 ±12.073 216.639 ±12.023 klmnopqrs hijk hijk 50 2 360 0.457 ±0.025 300.556 ±11.586 301.012 ±11.562 ghijklmnopqr de de 70 2 360 0.506 ±0.045 383.507 ±44.809 384.012 ±44.765 jklmnopqrs tuvwxyz tuvwxy 0 2 600 0.472 ±0.043 171.812 ±14.719 172.284 ±14.686 ghijklmnopq αβ zα 10 2 600 0.517 ±0.023 129.748 ±1.748 130.265 ±1.770 jklmnopqrs mno mno 30 2 600 0.483 ±0.049 241.543 ±11.093 242.026 ±11.049 mnopqrs hij hij 50 2 600 0.442 ±0.057 302.401 ±28.166 302.843 ±28.110 qrs bc bc 70 2 600 0.408 ±0.007 428.969 ±5.545 429.376 ±5.541 fghijklmno vwxyzαβ vwxyzα 0 2 900 0.563 ±0.059 150.779 ±5.806 151.342 ±5.749 b zαβ yzα 10 2 900 0.777 ±0.074 135.875 ±6.315 136.652 ±6.305 bcdef pqrstuv pqrstuv 30 2 900 0.674 ±0.025 187.359 ±2.639 188.033 ±2.658 ijklmnopqrs hij hij 50 2 900 0.490 ±0.013 303.924 ±12.301 304.414 ±12.305 pqrs cd cd 70 2 900 0.409 ±0.011 405.832 ±11.238 406.241 ±11.232 fghijklmnop rstuvwx rstuvw 0 3 360 0.548 ±0.031 182.875 ±11.026 183.423 ±11.005 efghijklmn tuvwxyz tuvwxy 10 3 360 0.568 ±0.108 173.204 ±23.033 173.772 ±22.928 ghijklmnopqrs ijkl ijkl 30 3 360 0.500 ±0.017 286.262 ±6.316 286.762 ±6.307 nopqrs ef ef 50 3 360 0.432 ±0.022 367.658 ±30.041 368.090 ±30.021 ijklmnopqrs a a 70 3 360 0.489 ±0.026 491.166 ±18.547 491.656 ±18.522 ghijklmnopq stuvwxy stuvwx 0 3 600 0.537 ±0.087 176.903 ±4.635 177.441 ±4.596 jklmnopqrs yzαβ xyzα 10 3 600 0.473 ±0.015 139.608 ±2.896 140.081 ±2.907 ghijklmnopqrs rstuvwx rstuvw 30 3 600 0.500 ±0.087 181.582 ±16.326 182.082 ±16.246 qrs hijk hijk 50 3 600 0.405 ±0.073 299.298 ±42.318 299.703 ±42.248 qrs b b 70 3 600 0.399 ±0.029 441.443 ±42.266 441.842 ±42.237 cdefg xyzαβ wxyzα 0 3 900 0.633 ±0.038 144.635 ±1.002 145.268 ±0.994 bc vwxyzαβ vwxyzα 10 3 900 0.728 ±0.072 152.430 ±6.078 153.158 ±6.006
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30 3 900 0.587 ±0.033 191.437 ±8.319 lmnopqrs opqrst 50 3 900 0.448 ±0.032 207.712 ±11.955 qrs bc 70 3 900 0.398 ±0.054 435.097 ±63.284 ghijklmnopqrs uvwxyzα 0 4 360 0.498 ±0.018 162.104 ±1.422 ghijklmnopq wxyzαβ 10 4 360 0.514 ±0.033 145.892 ±4.858 ghijklmnopq opqrst 30 4 360 0.521 ±0.054 210.335 ±10.792 s ef 50 4 360 0.361 ±0.011 370.130 ±4.201 mnopqrs fg 70 4 360 0.445 ±0.008 345.317 ±18.064 ghijklmnopqrs tuvwxy 0 4 600 0.500 ±0.031 175.300 ±3.737 nopqrs uvwxyzα 10 4 600 0.429 ±0.110 162.919 ±17.494 klmnopqrs opqrst 30 4 600 0.465 ±0.081 205.995 ±17.504 qrs ghi 50 4 600 0.402 ±0.039 320.346 ±9.581 s b 70 4 600 0.363 ±0.033 453.442 ±46.721 ghijklmnopqr tuvwxyz 0 4 900 0.506 ±0.085 173.402 ±12.841 cdefgh β 10 4 900 0.630 ±0.040 120.998 ±3.623 defghijklm opqrs 30 4 900 0.578 ±0.020 213.535 ±0.711 opqrs jkl 50 4 900 0.423 ±0.017 278.858 ±13.324 rs bc 70 4 900 0.372 ±0.029 435.453 ±62.718 * Different letters in the same column indicate a significant difference (p < 0.05) 1 Concentration of osmotic solution (% w/w) 2 Pulse ratio 3 Power (W) 4 3 Specific energy consumption of microwave drying (×10 MJ/kg) 5 3 Specific energy consumption of hot-air drying (×10 MJ/kg) 6 3 Total specific energy consumption (×10 MJ/kg)
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192.024 ±8.290 opqrst 208.160 ±11.923 bc 435.494 ±63.230 uvwxyz 162.602 ±1.437 wxyzα 146.406 ±4.826 opqrst 210.857 ±10.747 ef 370.492 ±4.192 fg 345.763 ±18.057 tuvwx 175.801 ±3.706 uvwxyz 163.348 ±17.397 opqrst 206.460 ±17.426 ghi 320.748 ±9.553 b 453.805 ±46.688 tuvwxy 173.908 ±12.757 α 121.628 ±3.589 opqrs 214.114 ±0.728 jkl 279.281 ±13.308 bc 435.825 ±62.690
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Fig. 1a Drying kinetics of control quince samples (without microwave drying) pretreated with osmotic solutions at concentrations of 0 (control), 10, 30, 50 and 70% during convective hot-air drying
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Fig. 1b Drying kinetics of quince samples pretreated with osmotic solutions at concentrations of 0 (control), 10, 30, 50 and 70% during both microwave and convective hot-air drying at 360 W (a,b,c,d), 600 W (e,f,g,h) and 900 W (i,j,k,l) with different pulse ratios (PR)
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Fig. 2 Ln (MR) versus drying time of control quince samples (without microwave drying) pretreated with osmotic solutions at concentrations of 0 (control), 10, 30, 50 and 70% during convective hot-air drying
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Fig. 3a Shrinkage variations of control quince samples (without microwave drying) pretreated with osmotic solutions at concentrations of 0 (control), 10, 30, 50 and 70% during convective hot-air drying
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Fig. 3b Shrinkage variations of quince samples pretreated with osmotic solutions at concentrations of 0 (control), 10, 30, 50 and 70% during both microwave and convective hot-air drying at 360 W (a,b,c,d), 600 W (e,f,g,h) and 900 W (i,j,k,l) with different pulse ratios (PR)
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Fig. 4a Bulk density variations of control quince samples (without microwave drying) pretreated with osmotic solutions at concentrations of 0 (control), 10, 30, 50 and 70% during convective hot-air drying
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Fig. 4b Bulk density variations of quince samples pretreated with osmotic solutions at concentrations of 0 (control), 10, 30, 50 and 70% during both microwave and convective hot-air drying at 360 W (a,b,c,d), 600 W (e,f,g,h) and 900 W (i,j,k,l) with different pulse ratios (PR)
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Fig. 5a Rehydration ratio variations of control quince samples (without microwave drying) pretreated with osmotic solutions at concentrations of 0 (control), 10, 30, 50 and 70% during convective hot-air drying
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Fig. 5b Rehydration ratio variations of quince samples pretreated with osmotic solutions at concentrations of 0 (control), 10, 30, 50 and 70% during both microwave and convective hot-air drying at 360 W (a,b,c,d), 600 W (e,f,g,h) and 900 W (i,j,k,l) with different pulse ratios (PR)
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(e) Fig. 6 Images of the quince cubes dried using hybrid intermittent microwave – hot air drying technique at 360 W and PR = 1 pretreated at 0 (a), 10 (b), 30 (c), 50 (d) and 70% (e) osmotic solution concentrations
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ACCEPTED MANUSCRIPT Highlights A multi-stage continuous and intermittent microwave (IM) drying was developed.
Osmotic dehydration (OD) and hot air (HA) drying at 40˚C was applied.
OD-IM-HA drying considerably reduced drying time and improved product quality.
OD-IM-HA drying increased rehydration and decreased shrinkage and bulk density.
Specific energy consumption was significantly reduced by the OD-IM-HA drying.
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