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The effect of high power airborne ultrasound and microwaves on convective drying effectiveness and quality of green pepper
Ultrasonics Sonochemistry 34 (2017) 531–539
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
The effect of high power airborne ultrasound and microwaves on convective drying effectiveness and quality of green pepper Justyna Szadzin´ska, Joanna Łechtan´ska, Stefan Jan Kowalski ⇑, Marcin Stasiak Poznan University of Technology, Institute of Technology and Chemical Engineering, Department of Process Engineering, Berdychowo 4, 60-965 Poznan, Poland
a r t i c l e
i n f o
Article history: Received 4 February 2016 Received in revised form 11 May 2016 Accepted 22 June 2016 Available online 22 June 2016 Keywords: Hybrid drying Ultrasound Microwave Energy Quality Modelling Green pepper
a b s t r a c t The effectiveness of hybrid drying based on convective drying with application of ultrasound and microwave enhancement is the main subject of the studies. The drying kinetics, energy consumption as well as the quality aspect of green pepper is analysed. It was shown that hybrid drying methods shorten significantly the drying time, reduce the energy consumption and affect positively the quality factors. Each of the analysed aspects depend on combination of the convective-ultrasound-microwave drying programs. Besides, based on the drying model elaborated earlier by one of the authors, the effects of ultrasound on convective drying assessed by such phenomena as ‘‘heating effect”, ‘‘vibration effect” and ‘‘synergistic effect” are presented. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Evaporation of water from fruits and vegetables is a highly complex process associated with many negative consequences, such as changes in the internal structure and sensory properties (colour, flavour, aroma), chemical composition and changes in the content of bioactive components. In drying of foods, it is important to reduce the water content (water activity) while preserving the product quality [1,2]. The quality of dried biomaterial is one of the fundamental indicators in the assessment of the drying process effectiveness. The conventional drying techniques, especially hot air drying is still extensively employed as a preservation technique, however, it affects the final quality of dried product adversely. Convective drying is considered rather as highly destructive due to, e.g. shrinkage, discoloration and loss of nutrients, particularly for thermally sensitive materials like fruits and vegetables. Moreover, hot air drying is usually undesirable long-lasting and energy consuming process of food preservation [3]. One of the recommended ways to minimize these adverse features of convective drying is application of hybrid methods, where the energy is provided alternatively by combination of different energy sources, e.g. convection with ultrasound or microwave ⇑ Corresponding author. E-mail addresses: [email protected] (J. Szadzin´ska), Joanna.L. [email protected] (J. Łechtan´ska), [email protected] (S.J. Kowalski), [email protected] (M. Stasiak). http://dx.doi.org/10.1016/j.ultsonch.2016.06.030 1350-4177/Ó 2016 Elsevier B.V. All rights reserved.
radiation, etc. By microwave radiation heat is provided to the entire material volume in a relatively short time, and not only to the surface as in convective drying. The increase of temperature in the material interior involves thermo-diffusion and pressure gradient that cause ‘‘pumping” of the moisture towards material surface. Thus, the moisture transport is more effective and results in shortening of drying time [4]. Kowalski and Mierzwa [5] showed that convective-microwave drying of beetroot increased the drying rate nearly four times compared with pure convective drying. Convective-microwave drying can be also favourable in terms of product quality. Prabhanjan et al. [6] proved that in microwaveassisted convective drying of carrot cubes the product revealed better reconstitution properties by rehydration than in pure convectively dried material. Moreover, Workneh et al. [7] showed that low temperature air ventilation with microwave assisted drying could be considered as an alternative drying method for tomato slices, as it maintains a superior quality in terms of colour. As presented Kowalski and Mierzwa [8], the convective-microwave drying of red bell pepper caused smaller shrinkage and deformation of this product, and reduced energy consumption by about 62%. Application of microwaves reduces also the degree of vitamin C degradation, i.e. by combined microwave–air–drying the ascorbic acid content was retained up to 98% [9]. Application of ultrasound in drying of foods is a relatively new item and one of the emerging technologies. High power ultrasound generates acoustic cavitation, and the absorption of the acoustic energy causes the so-called ‘‘heating effect” and micro-‘‘vibration
´ ska et al. / Ultrasonics Sonochemistry 34 (2017) 531–539 J. Szadzin
532
Nomenclature A A0 ADrE Am AT
a aUD aw a⁄ b b⁄ CDrEs, eff CDrET, eff CDrEv, eff CV CVMW CVUD CV cl cs Dr Dr E EC hm hT L⁄ l MW
surface [m2] initial surface of sample [m2] ratio of drying rate enhancement [%] surface of mass exchange [m2] surface of heat exchange [m2] microwave energy absorbed by the skeleton [W] absorption coefficient of ultrasonic wave [] water activity [] colour parameter from red to green [] microwave energy absorbed by the moisture [W] colour parameter from yellow to blue [] contribution ratio of ‘‘vibration effect” [%] contribution ratio of ‘‘heating effect” [%] contribution ratio of ‘‘vibration effect” [%] convection convective-microwave drying convective-ultrasound drying convective drying specific heat for liquid [J/kgK] specific heat for liquid [J/kgK] drying rate [g/h] drying rate enhancement [g/h] energy consumption [kWh] mass transfer coefficient [kg/m2 h] heat transfer coefficient [W/m2K] lightness [] latent heat of evaporation [J/kg] microwave
effect” in the material surface layer [10]. Therefore, ultrasound action changes the physical, mechanical or chemical/biochemical properties of biomaterials [11]. Riera-Franco de Sarabia et al. [12] showed that material vibrations induced by ultrasound accelerate notably the drying process and reduces the duration of the process three times. As presented by Rodríguez et al. [13], the ultrasound application in convective drying involved lower total polyphenol and flavonoid content losses in comparison to air dried apples. Gallego-Juárez et al. [14] showed that by dehydration of carrot with application of ultrasound the energy consumption was lower, and the sample rehydration was higher by about 70% than in processing without ultrasound. Deng and Zhao [15] observed that the water activity after the drying of apples with ultrasonic pretreatment is lower as compared to pulsed vacuum pre-treatment. Ultrasound assisted drying can also leads to energy savings. One can state a number of advantages while combining various drying techniques such as convection, microwave and ultrasound in an appropriate way. By combination of several energy sources in one drying process a significant improvement of drying efficiency can be achieved. Then, hybrid drying can be a very attractive and promising solution from the drying kinetics, energy consumption and as well as the product quality point of view. A number of reports in the literature allow to state that use of microwaves and ultrasound to enhance convective drying usually positively affects many properties of dried fruits and vegetables. Among some benefits of such an approach one can mention, e.g. elimination of shrinkage (higher porosity), retention of colour and aromas as well as bioactive components, while shortening processing time and reducing energy consumption [16,17]. As reported García-Pérez et al. [18], high-intensity ultrasound application in convective drying of orange peel reveals energy saving ranging from 12% to 20% in comparison with pure convective hot air drying.
mRt ms mt m0 PUD pvs t te Ta Tm UD
va
(xn) X Xeq Xi Xcr
aV
DE DQ DQMW DQUD dMW ua uj@B
vUD
sample mass during rehydration at time t [kg] mass of dry sample [kg] mass of sample at time t [kg] mass of sample before rehydration [kg] power of ultrasonic generator [W] vapor partial pressure for the saturated state at given temperature [Pa] time [min] drying time at which the moisture content reaches equilibrium [min] air temperature [°C] material temperature [°C] ultrasound air flow velocity [m/s] distance of wave propagation [m] moisture content (dry basis) [kg/kg] equilibrium moisture content (dry basis) [kg/kg] initial moisture content (dry basis) [kg/kg] critical moisture content (dry basis) [kg/kg] volumetric shrinkage coefficient [] total colour change [] heat source [W] microwave heat source [W] ultrasound heat source [W] decay factor of microwave energy [] relative air humidity [%] relative air humidity near the material surface [%] working efficiency of the ultrasonic transducer []
Dried pepper has a lot of application. It is a rich source of valuable nutrients such as ascorbic acid, carotenoids and flavonoids. It is mainly used as a compound for soups, sauces and dry salad mixes [19]. Therefore, the main purpose of this study was to examine the effect of hybrid drying based on convection with ultrasound action and microwave radiation on drying kinetics, energy consumption and final quality of green pepper, i.e. total colour change, water activity, vitamin C retention and the ability to rehydration. In addition, the mathematical modelling is applied to assess the electiveness of ultrasound-convective drying, and to determine some positive effects followed from ultrasound action in drying processes. 2. Material and methods 2.1. Material and apparatus Fresh green pepper (Capsicum annuum L.) imported from Spain were purchased at a local market. The biological material with an average initial water content of 13.19 kg/kg db was washed in tap water, drained with blotting paper and cut into 6 slices (45 mm length, 30 cm width, 2 mm thick), each about 7.5 g. The samples were placed on the pan in the form of ring, skin-side down to allow the most effective application of ultrasound in the focusing area. Then, the green pepper samples were dried to a final moisture content of 0.03 kg/kg db, on average. Seven different drying tests were carried out including convective drying (CV) as a reference, as presented in Table 1. For this purpose an innovative laboratory hybrid dryer (Promis–Tech, Poland), equipped with a microwave and airborne ultrasound generator (Pusonics, Spain) was used. The scheme and photo of the experimental set-up is presented in Figs. 1a and 1b.
´ ska et al. / Ultrasonics Sonochemistry 34 (2017) 531–539 J. Szadzin Table 1 Programs of drying. Drying program
Total energy consumption EC kWh
CV CVUD100 CVUD200 CVMW100 CVMW10010min CVMW100UD200 CVMW10010minUD200
7.97 ± 0.19 7.28 ± 0.20 7.12 ± 0.16 1.26 ± 0.20 3.52 ± 0.12 1.50 ± 0.13 4.73 ± 0.07
*
CV – convection, MW – microwave, UD – ultrasound.
Fig. 1a. Scheme of the hybrid dryer: 1. Fan, 2. Airborne Ultrasound System (AUS), 3. Ultrasound feeder, 4. Electric heater, 5. Air outlet, 6. Ultrasound transducer AUS, 7. Pyrometer, 8. Rotating sample pan, 9. Drive sample pan, 10. Balance, 11. Microwave generator, 12. Control cupboard [20].
Fig. 1b. Photo of the experimental set-up.
This hybrid dryer enables convective drying with microwave and ultrasound enhancement separately as well as in different combinations. High power ultrasound is generated by the Airborne Ultrasound System – AUS, that enables to achieve the sound pressure about 176 dB. The control cupboard of the dryer allows to measure all of the process parameters online such as inlet and outlet air temperature and humidity, air flow rate, material mass and its surface temperature, microwave and ultrasound power, as well as total energy consumption.
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green pepper was determined on the basis of fresh material dried for 24 h at 60 °C in the chamber dryer, model SN75 (Memmert, Germany). The moisture content X kg/kg db was calculated using the following formula (1):
X¼
mt ms ms
ð1Þ
where mt kg denotes the sample mass measured at time t, and ms kg is the mass of the dry sample [21]. Each drying test was performed in triplicate. Energy consumption EC (in kW h) is defined as the total electric energy consumed in drying process by both the dryer and the control equipment (computer, control cupboard etc.). Moreover, in order to evaluate the effect of hybrid drying on quality of green pepper, several quality factors were assessed such as total colour change, water activity, vitamin C retention and ability to rehydration. Total colour change DE between fresh and dried product was measured 25 times in 5 different points on the sample surface using a colorimeter, model CR-400 (Konica Minolta, Japan), and indicated by CIELab colour space. The value of total colour change was calculated using the following formula (2):
DE ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 ðDL Þ þ ðDa Þ2 þ ðDb Þ
ð2Þ
where L⁄ is lightness, a⁄ is the colour parameter from red to green and b⁄ denotes the colour parameter from yellow to blue [22]. Water activity aw in fresh and dried material was measured with the temperature and humidity converter having the function of water activity measurement, model 650/0628.0024 (Testo, Germany). In this measurement, the randomly selected dried samples were placed in a converter chamber and kept there until equilibrium was reached. Since the moisture profile in the sample has to be aligned after processing, the measurements of aw in dried product started after three hours from the end of the drying process. The DE and aw measurements were carried out in triplicate and the standard deviations were calculated. The content of vitamin C in green pepper was determined using Tillmans method based on titration of the analysed sample with 2,6-dichlorophenolindophenol standard solution in an acid environment [23]. To determine the content of vitamin C, three portions of raw material (in the form of slurry) of 10 g and dried material of 0.5 g were prepared. Then the samples were placed in beakers covered with 3% oxalic acid (50 mL) and held in dark and cool place for 15 min. Next, the solution was filtered with blotting paper and 5 ml of the filtrate was titrated with solution of 2,6-dichlorophenolindophenol until the light pink colour occurred, which was sustained for 30 s. By rehydration tests the dried pepper slices were immersed in 50 cm3 of a distilled water at room temperature of 20 ± 0.2 °C. The kinetics of rehydration process was analysed in time intervals of 10, 20, 30, 60, 90, 120, 150, 180, 210, 240, 270, and 300 min. The experiments were carried out in triplicate. The relative increase in mass of dried green pepper sample was calculated as the ratio of mRt to m0, where mRt is the sample mass during rehydration at time t and m0 is the sample mas before rehydration [24]. 3. Results and discussion
2.2. Methodology
3.1. Drying kinetics
The process parameters used in these studies were as follows: average air temperature of 54 °C, air flow velocity va = 2 m/s, microwave power of 100 W, ultrasound power of 100 and 200 W, respectively. The drying kinetics as well as the air temperature Ta and material temperature Tm plots, and the total energy consumption EC were determined. The initial moisture content Xi of the
First, the convective drying test CV (Fig. 2a) as a reference was carried out at constant air temperature of 54 °C. In this case, the material temperature was maintained below the air temperature for the period of drying time. The final moisture content was reached after te = 759 min, on average. Due to long lasting hot air drying the dried biomaterial was strongly deformed and shrunken.
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´ ska et al. / Ultrasonics Sonochemistry 34 (2017) 531–539 J. Szadzin
Fig. 2. The drying curves and the temperature profiles: a) convective drying (CV), b) convective-ultrasound drying with power of 100 W (CVUD100), c) convective-ultrasound drying with power of 200 W (CVUD200).
Furthermore, as it follows from Fig. 2a the convective drying was found to be the longest process. The second test was convective drying with ultrasonic assistance with UD power of 100 and 200 W. The total drying time of green pepper samples in this case was te = 514 min (100 W) and te = 475 min (200 W) on average, thus, the UD application reduced the overall drying time by about 32% and 37% in CVUD100 and CVUD200 test with respect to CV. Similar observations have been noticed by Gamboa-Santos et al. [25], where the applied acoustic power in drying of strawberries caused a significant reduction of drying time (13–44%). In hybrid drying, an additional energy in the form of ultrasound caused a slight increase in the material temperature, i.e. the ‘‘heating effect” (Fig. 2b and c). However, the heating phenomenon was different in the CVUD200 drying. Due to higher ultrasound power the material
temperature raised already after about 180 min and rose above the air temperature by 3 °C (Fig. 2c). In these two cases the dried green pepper samples were less deformed as compared to pure convective drying. Thus, modification of classical convective drying method allows to eliminate or limit its defects. Another tests concerned the convective drying with microwave assistance (Fig. 3a and b). In the convective-microwave drying (CVMW100), the microwave power of 100 W was applied through the whole process. This value is a minimal stable microwave power generated by the magnetron and also has been selected to provide a suitable radiation to the sample volume. In the CVMW100 drying, by the action of microwave, the heat produced inside the biological material caused a rapid temperature increase, high above the air temperature (Fig. 3a).
Fig. 3. The drying curves and the temperature profiles: a) convective-microwave drying with power of 100 W (CVMW100), b) convective drying assisted with microwave with power of 100 W for the first 10 min (CVMW10010min).
´ ska et al. / Ultrasonics Sonochemistry 34 (2017) 531–539 J. Szadzin
Due to more effective internal heat and moisture transport the drying material reached the final moisture content after te = 87 min, on average. Because, the dried product was characterized by some dark spots, in the next test it was decided to apply microwave at the beginning of the drying process, but only for the first 10 min. The overall drying time in CVMW10010min was te = 300 min, on average, and was shorter by about 60% in comparison with CV. A similar finding was made by Maskan [26], in reference to hot air-microwave combination drying of kiwi fruit. In those studies, the drying time was reduced to about 40%, as compared to hot air drying. In this case, when the microwave energy was applied the material temperature raised up to about Tm = 65 °C, but after 10-min period it decreased and then remained at the level close to air temperature (Fig. 3b). Although, a longer drying time was gained as compared to CVMW100 program, however, after CVMW10010min drying a better quality product was obtained. Next series of experiments concerned convective-microwaveultrasound drying. In the combination of CVMW100UD200 (Fig. 4a) the total drying time was about te = 80 min, on average. In this case the shortest total drying time was obtained. It can be explained by the collision of water molecules and acoustic cavitation, which caused the ‘‘heating effect” and ‘‘vibration effect”. For this reason, the material temperature increased rapidly and much above the air temperature, i.e. up to about Tm = 70 °C (Fig. 4a). In drying of green pepper Łechtan´ska et al. [27] demonstrated that the fastest drying technique was by hybrid drying. In that case the convection was combined with microwave and infrared radiation, and the drying time reduction amounted to about 70%, as compared to convective drying. It proved that hybrid methods consisted of several different processes of moisture evaporation enhances the drying efficiency. The last drying test carried out in these studies was CVMW10010minUD200 (Fig. 4b). In this drying program the green pepper samples reached the final moisture content after te = 263 min, on average. Similarly to CVMW10010min program, when the microwave application was shortened to only 10 min, the total drying time increased considerably. However, the dried material avoided an excessive heating and revealed a better visual appearance, as compared to CVMW100UD200 drying. 3.2. Total energy consumption Comparison of total electric energy consumed in different drying programs was shown in Table 1. The most energy consuming was CV process, as the electric energy usage equalled to 7.97 kW h, on average. It was found that
535
the application of microwave or ultrasound in convective drying as additional energy sources can significantly reduce the total electric energy consumption. Such a low value of energy consumed in CVMW100UD200 is due to simultaneous application of MW and UD. Depending on the applied ultrasound power, the energy usage amounted to about 7.28 and 7.12 kW h for CVUD100 and CVUD200, respectively. As demonstrated Gallego-Juárez et al. [28], by direct contact of ultrasonic vibration the dehydration of carrot slices is quicker, less energy consuming and more powerful than the convective forced-air drying. However, the lowest EC value was observed after convective-microwave drying (CVMW100), i.e. 1.26 kW h. Varith et al. [29] showed that combined microwave-hot air drying greatly reduced specific energy consumption up to 48% as compared to conventional hot-air treatment. A quite similar energy consumption was obtained for CVMW10010minUD200 program. In the case of short microwave application followed by convective drying (CVMW10010min), the EC decreased over two times in comparison with CV. Experimental results of Motevali et al. [30] have also revealed that the use of microwave pre-treatment (for the first 20 min/100 W) in drying pomegranate arils decreased energy consumption maximum up to about 2 times in comparison with pure convective drying.
3.3. Quality Table 2 shows the results of the total colour difference DE between fresh green pepper samples and those after drying, and also the vitamin C retention. Data obtained from the pure convective drying process (CV) served as a reference. The highest value of DE was observed for the green pepper samples dried solely convectively. It proves that a long-lasting hot air drying causes a meaningful colour alteration. Natural dyes present in vegetables and fruits readily undergo destruction. The colour of natural pigment is unstable and susceptible to various factors such as light, temperature, oxidation, pH, metal ions etc. Application of microwave and ultrasound shortened effectively the drying time, thus the green pepper dried in CVMW100UD200 program revealed the lowest colour change. Satisfactory results were also obtained in the case of convective-ultrasound drying, whereas the samples dried with microwave application (CVMW100) were characterized by the highest DE value among all the hybrid drying programs. As noted Contreras et al. [31], shorter process time associated with higher air temperature or microwave application results in lesser colour changes in the dried samples. It is necessary to point out that despite of lower values in total colour change after MW and UD application, the obtained values are still high, since above
Fig. 4. The drying curves and the temperature profiles: a) convective-microwave-ultrasound drying with microwave power of 100 W and ultrasound power of 200 W (CVMW100UD200), b) convective-ultrasound drying with power of 200 W assisted with microwave with power of 100 W for the first 10 min (CVMW10010minUD200).
´ ska et al. / Ultrasonics Sonochemistry 34 (2017) 531–539 J. Szadzin
536 Table 2 Total colour change and vitamin C retention. Drying process
CV
CVUD100
CVUD200
CVMW100
CVMW10010min
CVMW100 UD200
CVMW10010min UD200
Total colour change DE [] Vitamin C retention [%]
15.12 ± 0.40 44 ± 2
11.32 ± 0.67 62 ± 1
11.51 ± 0.16 69 ± 1
12.34 ± 0.29 51 ± 2
11.94 ± 0.59 63 ± 3
10.88 ± 0.22 61 ± 1
12.16 ± 0.64 67 ± 2
DE = 5 the observer may have an impression of two different colours. Vitamin C is the most heat labile of all vitamins. Therefore, drying processes should be carried out in a way ensuring the preservation of the highest amount of vitamins contained in fresh biomaterials [32]. Convective drying contributed to the lowest vitamin C retention from all the analysed drying programs. However, as reported Carvajal et al. [33], the variety of pepper determines the retention of the vitamin C after drying. In most studied varietals, the retention was 24–28%, while in one of them it was comparable with that in Table 2, i.e. about 42%. In turn, the lowest vitamin C retention in hybrid drying was observed after CVMW100 drying. For other programs, a quite similar vitamin C retention was observed, i.e. about 62%, except CVUD200 and CVMW10010minUD200. In those programs, a rather mild conditions were used, so the green pepper retained the highest content of vitamin C. Frias et al. [34] reported about 82–92% retention of vitamin C after ultrasound drying of blanched carrots and less than 50% for air dried carrots. It proves
Fig. 5. Water activity change after different drying programs.
that high power ultrasound drying allows to preserve high amount of vitamin C. However, when UD is combined with MW, the content of vitamin C in dried product is lower. The second important indicator in the quality assessment was water activity (aw). The results of aw for raw and dried product are presented in Fig. 5. Water activity influences the microbial growth fundamentally including: bacteria, yeast and moulds. However, it is worth noting that above the value of 0.4 the relative speed of lipid oxidation, Maillard, hydrolysis, enzymatic and microbiological reactions increases considerably [35]. Thus, aw is a critical parameter that allows to determine the product stability and safety [36]. The fresh material was characterized by water activity of about 0.970, on average. All samples dried in hybrid programs, except the samples dried only convectively, were characterized by water activity less than 0.4. Thus, bacteria, yeast and mould growth was inhibited. Water activity of all green pepper samples decreased significantly after drying, but the lowest value of about aw = 0.208 was reached after convective drying with MW and UD application, i.e. CVMW100UD200. In turn, the samples dried by convectivemicrowave drying (CVMW100) had a visibly lower value of aw, in comparison with the samples dried in CVUD100-200 programs. The last quality parameter evaluated in this work was the degree of rehydration, which determines the dried product ability to irrigation. Fig. 6 shows the rehydration curves of dried green pepper samples as the relative increase in mass of dried material during rehydration in a distilled water. After 300 min of rehydration the relative increase in mass of samples dried only convectively (CV) reached the smallest value. It proves that, hot air drying has a negative influence on structure properties. In turn, the highest mass gain was observed in the case of samples dried in CVMW100 program. It means that hybrid drying consisting of convection and microwave assistance affects the structure of plant tissue to a lesser extent. Feng and Tang [37] found that spouted-bed drying combined with microwave of
Fig. 6. Rehydration curves.
´ ska et al. / Ultrasonics Sonochemistry 34 (2017) 531–539 J. Szadzin
apples increases the rehydration rates compared to that of conventional spouted-bed drying. Satisfactory results were also obtained after rehydration of samples dried with CVMW100UD200 program. On the other hand, hybrid drying with ultrasound enhancement, i.e. CVUD100 and CVUD200, improved the rehydration properties insignificantly, as compared to CV drying. Similarly, Schössler et al. [38] found no differences in rehydration characteristics in studies on freeze-dried and ultrasound assisted freezedried bell pepper. 4. Modelling The effect of ultrasound on drying process can be determined using a mathematical model describing the kinetics of convective drying assisted with ultrasound. For this purpose, the coupled ordinary differential equations by Kowalski and Pawłowski [39] and Kowalski and Pawłowski [20] were used. The proposed global mathematical model of drying kinetics allows the numerical calculation of drying curves and material temperature plots, which should be reflected in the experiments. The mass and heat balance equations provide the basis for the drying kinetics determination. The final form of these equations is as follows:
ms
dX uj p ðT m Þ ¼ Am hm ln @B v s dt ua pv s ðT a Þ
ms
d uj p ðT m Þ ½ðcs þ cl XÞT m ¼ AT hT ðT a T m Þ Am lhm ln @B v s þ DQ dt ua pv s ðT a Þ
ð3Þ
ð4Þ where ms is the mass of dry sample, X is the moisture content (dry basis), Am is the surface of mass exchange, AT is the surface of heat exchange, hm is the mass transfer coefficient, hT is the heat transfer coefficient, uj@B denotes the relative air humidity near the material surface, ua is the relative air humidity, pms is the vapor partial pressure for the saturated state at given temperature, Tm denotes the material temperature, Ta denotes the air temperature, cs and cl denote the specific heat for solid and liquid, l is the latent heat of evaporation, and DQ = {DQUD; DQMW} is the heat source expressing an additional heat supply (e.g. ultrasound UD or microwave MW), and is an auxiliary relation that has to be determined independently. The ultrasound heat source is expressed as DQUD = aUDvUDPUD, where aUD [] denotes the dimensionless absorption coefficient of the ultrasonic wave, vUD [] is the dimensionless working efficiency of the ultrasonic transducer, and PUD [W] is the power of the ultrasonic generator [40]. The microwave heat source DQMW is possible to determine on the basis of Maxwell electromagnetic equations [41]. In practical applications the heat source in microwave drying can be written as DQMW = (a + bX) exp [2dMW(xn)], where a and b express the amount of microwave energy absorbed by the skeleton and moisture, (xn) denotes the distance of microwave propagation in n-direction, and dMW expresses the decay of microwave energy due to its absorption with distance, respectively. When the dried samples are small-sized, then the decay of microwave energy due to absorption can be considered as a small one, and therefore dMW(xn) 0 [42]. It was assumed that the heat and mass exchange occurs on the whole sample surface, and also that the dried material undergoing linear volumetric shrinkage. Hence, the change of surface dimension is a function of moisture content described by the following formula:
Am ¼ AT ¼ AðXÞ ¼ ½1 aV ðX i XÞ2=3 A0
ð5Þ
where aV and Xi denotes the volumetric shrinkage coefficient and initial moisture content, respectively. In the present simulations
537
an averaged geometry for all samples was assumed. It was also assumed that the air relative humidity close to the material surface uj@B is a function of the sample moisture content X:
(
uj@B ¼
1 for
X P X cr
X 1 ð1 ua Þ XXcrcrX eq
for
ð6Þ
X cr P X P X eq
where Xcr and Xeq are the critical and equilibrium values of the sample moisture content. Based on data given in [21], one can stated that the temperature dependent saturated vapor pressure pms can be approximated using the following function:
pms ðTÞ ¼ 9:61966 104 T 4 1:08405264T 3 þ 4:61325529 102 T 2 2:77803513 104 T þ 6:29588464 106
ð7Þ
In order to make use of the above presented drying model for complex hybrid drying of biological materials, the physical heat (hT) and mass (hm) transfer coefficients as well as the heat sources DQ have to be determined first. For this purpose an optimization methodology of assessment of the heat and mass transfer coefficients and the heat source DQ, based on adjusting the drying kinetics plots determined numerically to the experimentally established data, was used. The initial value problem based on the set of kinetic Eqs. (3) and (4) was solved by the Adams–Bashforth non-selfstarting multistep method. Selection of this method was motivated by good convergence and stability in long-term simulations [43]. The physical heat (hT) and mass (hm) transfer coefficients existing in this model were assessed on the basis of drying kinetics obtained from experimental studies. Usually, the convective heat and mass transfer coefficients depend on both the moisture content and the temperature. Below, an example of modelling of ultrasound assisted connective drying is presented. A good fit of experimental curves to the theoretical one (Fig. 7) allows to determine these coefficients. As it follows form Fig. 7, a good adjustment of the numerical drying curves as well as the material temperature plots to the experimental one was achieved, although the calculations were confined to about 500 min of drying time. Drying rate Dr [g/h] expresses the rate of moisture decrease in the drying material as a function of time:
Dr ðtÞ ¼ ms
dX uj p ðT m Þ ¼ Am hm ln @B v s ua pv s ðT a Þ dt
ð8Þ
The average drying rate Dr,ave for a given drying process is determined as:
Dr;av e ¼
1 te
Z
te
Dr ðtÞdt
ð9Þ
0
where te is the drying time at which the moisture content reaches equilibrium Xeq. The average drying time for convective drying of (Fig. 2a) and for green pepper equals t e ¼ tCV e ¼ 759 min convective-ultrasound drying is t e ¼ t UD ¼ 475 min (Fig. 2c). e Drying rate enhancement DrE and the ratio of drying rate enhancement ADrE are used to determine the effectiveness of convective drying assisted with UD, that is: CV Dr E ¼ DUD r;av e Dr;av e
¼ Am ðhm þ Dhm Þ ln
uj@B pv s ðT m þ DT m Þ uj p ðT m Þ Am hm ln @B v s ua pv s ðT a Þ ua pv s ðT a Þ ð10Þ
As it was stated in Chapter 2.1 the average initial water content of the pepper sample was 13.19 kg/kg db, and the single sample slice (45 mm length, 30 cm width, 2 mm thick) about 7.5 g. Then, the green pepper samples were dried to a final moisture content
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538
Fig. 7. Drying curves and material temperature plots determined experimentally (exp) and numerically (num) for: (a) convective drying (CV), (b) and convective-ultrasound drying with power of 200 W (CVUD200).
of 0.03 kg/kg db, on average. The total mass of 6 sample slices was about 45 g, and total mass of moisture removed from 6 samples was about 25.33 g. Thus, the rate of single sample drying amounted about 2.02 g/h for pure convective drying, and about 3.18 g/h for convective drying enhanced with ultrasound. Thus, the drying rate enhancement DrE and the ratio of drying rate enhancement ADrE of 6 samples amount to: CV Dr E ¼ DUD r;av e Dr;av e ¼ 19:09 12:13 ¼ 6:96 g=h
ADr E ¼
CV DUD r;av e Dr;av e
DCV r;av e
100% ¼
6:96 100% ¼ 57% 12:13
ð11Þ
ð12Þ
A high frequency vibration of air with a certain sound pressure near the body surface causes ‘‘vibration effect”, which contributes to the increase of mass transfer. At the same time a part of the ultrasonic energy is absorbed by the material under drying and converted to heat (‘‘heating effect”). This effect causes material temperature increase, which contributes to the vapor pressure increase. Due to ‘‘vibration effect” and ‘‘heating effect” in CVUD process the heat and mass transfer coefficients are greater than in pure CV drying. An interesting phenomenon is the creation of the third effect that is associated with both the ‘‘heating effect” and the ‘‘vibration effect”, i.e. the ‘‘synergistic effect”. The ‘‘synergistic effect” occurs when the ‘‘heating effect” is significant, that is, when Tm + DTm > Ta, and contributes to drying performance improvement. To describe the ‘‘heating effect” (T), ‘‘vibration effect” (v) and the ‘‘synergistic effect” (s) quantitatively, the equation of drying rate enhancement (10), together with the equation of the evaporation [20], is reformulated as follows:
CDr ET;eff ¼
Am hm l 1 1 100% Dr E R T m T m þ DT m
ð13Þ
CDr Ev ;eff ¼
Am hm uj@B 100% ln Dr E ua
ð14Þ
CDr Es;eff ¼
A m Dh m l 1 1 D r E R T a T m þ DT m
100%
ð15Þ
The average air temperature was Ta = 54 °C, the average material temperature during CV drying was Tm = 53 °C, and in CVUD200 drying Tm = 56 °C. Knowing the temperatures of drying medium and dried body, the vapor partial pressures pms for saturated state can be read directly from tables [21]:
pms = 15,002 Pa for air temperature Ta = 54 °C pms = 14,293 Pa for material temperature Tm = 53 °C in CV drying pms = 16,509 Pa for material temperature Tm + DTm = 56 °C in CVUD200 drying The average surface area of 6 green pepper samples equals Am = 0.0081 m2, the average relative air humidity ua = 0.075, the hm coefficients for CV and CVUD200 processes are 1.0067 and 1.2695 kg/m2 h, so that Dhm = 0.2628 kg/m2 h. Therefore, after substituting the above data in formulas (13)–(15), one can obtain the component values of ultrasound action (C) for the ‘‘heating effect” (T), ‘‘vibration effect” (v) and the ‘‘synergistic effect” (s):
Am hm pv s ðT m þ DT m Þ ln 100% pv s ðT m Þ Dr E 8:1 1:0067 16509 ln 100% ¼ 17% ¼¼ 6:96 14293
CDr ET;eff ¼
Am Dhm uj@B 100% ln Dr E ua 8:1 0:2628 0:9 ¼ ln 100% ¼ 80% 6:96 0:075
ð16Þ
CDr Ev ;eff ¼
Am Dhm pv s ðT m þ DT m Þ ln 100% pv s ðT a Þ Dr E 8:1 0:2628 16509 ln 100% ¼ 3% ¼¼ 6:96 15002
ð17Þ
CDr Es;eff ¼
ð18Þ
On the basis of the above calculations one can state that the ‘‘vibration effect” has contributed the most to an increase in drying efficiency by convective drying enhanced with ultrasound. These results correspond to those reported by Kowalski and Pawłowski [20], where the ‘‘heating effect” and the ‘‘vibration effect” in the case of apple drying amounted to about 17% and 80%, respectively. 5. Conclusion Based on the above presented results it can be concluded that application of differentiated energy sources in drying of green pepper affects positively the drying kinetics and the product quality. The reduction in drying time reached maximum in convectivemicrowave drying and amounted about 88%, but by the ultrasound enhancement it was about 39%, as compared to convective drying. Furthermore, the measurement of energy consumption in different drying schedules showed that convective-microwave drying of green pepper consumes much less energy (maximum about 80%)
´ ska et al. / Ultrasonics Sonochemistry 34 (2017) 531–539 J. Szadzin
than other hybrid drying processes. Also, convective-microwaveultrasound drying reduces energy consumption to the similar level. The analysis of the total colour change proved that microwave and ultrasound application in convective drying decreases discoloration of green pepper. However, the best dried biological materials from the colour point of view were obtained after the use of convection, microwaves and ultrasound simultaneously. It is also important to learn that hybrid drying of green pepper revealed a low water activity and better rehydration ability. In the case of the nutritional aspect, it was demonstrated that convectiveultrasound drying allows retaining most of the vitamin C, i.e. up to 70%. The applied in this work mathematical model of drying kinetics describes satisfactory the experimental data obtained by convective drying enhance with ultrasound, so the numerical and experimental curves are very well adjusted. As demonstrated, the use of ultrasound in drying processes contributes to drying effectiveness through the ‘‘vibration effect” and ‘‘heating effect”, and in some cases also the ‘‘synergistic effect” occurs as well. However, the ‘‘vibration effect” is the most dominant factor in the increase of moisture removal during convective-ultrasound drying. It decides the most about the effectiveness of drying processes assisted with ultrasound. However, convective drying with microwave radiation reveals at most with ‘‘heating effect” and not ‘‘vibration effect” Finally, it can be concluded that combined convective-microwave-ultrasound drying provides good quality of dried products in a relatively short drying time. Acknowledgement This study was conducted as a part of research project no. 2012/05/B/ST8/01773 sponsored by the National Science Center in Poland. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ultsonch.2016.06. 030.
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20] [21] [22] [23] [24] [25]
[26] [27]
[28]
[29] [30] [31]
References
[32]
[1] C.L. Hii, C.L. Law, Product quality evolution during drying of foods, vegetables and fruits, in: S.V. Jangam, C.L. Law, A.S. Mujumdar (Eds.), Drying of Foods, Vegetables and Fruits, vol. 1, TPR Group, Singapore, 2010, pp. 31–58. [2] E. Maltini, D. Torreggiani, E. Venir, G. Bertolo, Water activity and the preservation of plant foods, Food Chem. 82 (2003) 79–86. [3] C. Ratti, Hot air and freeze-drying of high-value foods: a review, J. Food Eng. 49 (2001) 311–319. [4] P.S. Schmidt, T.L. Bergman, J.A. Peace, P. Chen, Heat and mass transfer considerations in dielectrically-enhanced drying, in: A.S. Mujumdar (Ed.), Proceedings of the 8th International Drying Symposium Drying 92, Montreal, Canada, Elsevier, New York, 1992, pp. 137–160. [5] S.J. Kowalski, D. Mierzwa, Convective drying in combination with microwave and IR drying for biological materials, Dry. Technol. 27 (2009) 1292–1301. [6] D.G. Prabhanjan, H.S. Ramaswamy, G.S. Vijaya Raghavan, Microwave-assisted convective air drying of thin layer carrots, J. Food Eng. 25 (1995) 283–293. [7] T.S. Workneh, V. Raghavan, Y. Gariepy, Microwave assisted hot air ventilation drying of tomato slices, in: International Conference on Food Engineering and Biotechnology, ICFEB, 2011, pp. 150–161. [8] S.J. Kowalski, D. Mierzwa, Hybrid drying of red bell pepper: energy and quality issues, Dry. Technol. 29 (2011) 1195–1203. [9] I. Alibas, Determination of drying parameters, ascorbic acid contents and color characteristics of nettle leaves during microwave-, air- and combined microwave-air drying, J. Food Process Eng. 33 (2010) 213–233. [10] S.J. Kowalski, D. Mierzwa, US-assisted convective drying of biological materials, Dry. Technol. 33 (2015) 1601–1613. [11] J.A. Carcel, J. Benedito, C. Rossello, A. Mulet, Influence of ultrasound intensity on mass transfer in apple immersed in a sucrose solution, J. Food Eng. 78 (2) (2007) 472–479. [12] E. Riera-Franco de Sarabia, J.A. Gallego-Juárez, G. Rodríguez-Corral, V.M. Acosta-Aparicio, E. Andrés-Gallegos, Application of high-power ultrasound for
[33]
[34]
[35]
[36]
[37] [38]
[39] [40]
[41]
[42] [43]
539
drying vegetables, in: Forum Acusticum Sevilla 2002, Ultrasonics, ULT-05-004IP, 2002. Ó. Rodríguez, J.V. Santacatalina, S. Simal, J.V. Garcia-Perez, A. Femenia, C. Rosselló, Influence of power ultrasound application on drying kinetics of apple and its antioxidant and microstructural properties, J. Food Eng. 129 (2014) 21– 29. J.A. Gallego-Juárez, G. Rodriguez-Corral, J.C.G. Moraleda, T.S. Yang, A new highintensity ultrasonic technology for food dehydration, Dry. Technol. 17 (3) (1999) 597–608. Y. Deng, Y. Zhao, Effect of pulsed vacuum and ultrasound osmo-pretreatments on glass transition temperature, texture, microstructure and calcium penetration of dried apples (Fuji), LWT-Food Sci. Technol. 41 (2008) 1575–1585. M. Nowacka, M. S´ledz´, A. Wiktor, D. Witrowa-Rajchert, Physical and chemical properties of microwave dried food products, Food Sci. Technol. Qual. 6 (85) (2012) 5–20 (in Polish). T.S. Awad, H.A. Moharram, O.E. Shaltout, D. Asker, M.M. Youssef, Applications of ultrasound in analysis, processing and quality control of food: a review, Food Res. Int. 48 (2012) 410–427. J.V. García-Pérez, C. Ortuno, A. Puig, J.A. Cárcel, I. Perez-Munuera, Enhancement of water transport and microstructural changes induced by high-intensity ultrasound application on orange peel drying, Food Bioprocess Technol. 5 (2012) 2256–2265. B.I.O. Ade-Omowaye, N.K. Rastogi, A. Angersbach, D. Knorr, Osmotic dehydration of bell peppers: influence of high intensity electric field pulses and elevated temperature treatment, J. Food Eng. 54 (2002) 35–43. S.J. Kowalski, A. Pawłowski, Intensification of apple drying due to ultrasound enhancement, J. Food Eng. 156 (2015) 1–9. Cz. Strumiłło, Foundations of the Drying Theory and Technology, WNT, Warsaw, 1983 (in Polish). F. Nsonzi, H.S. Ramaswamy, Quality evaluation of osmo-convective dried blueberries, Dry. Technol. 16 (1998) 705–723. J. Tillmans, P. Hirsch, J.Z. Jackisch, Lebensmittel, Ztschr. Unters. 63 (1932) 267– 275. D. Witrowa-Rajchert, P.P. Lewicki, Rehydration properties of dried plant tissues, Int. J. Food Sci. Technol. 41 (2006) 1040–1046. J. Gamboa-Santos, A. Montilla, J.A. Cárcel, M. Villamiel, J.V. García-Pérez, Airborne ultrasound application in the convective drying of strawberry, J. Food Eng. 128 (2014) 132–139. M. Maskan, Drying, shrinkage and rehydration characteristics of kiwifruits during hot air and microwave drying, J. Food Eng. 48 (2001) 177–182. J. Łechtan´ska, J. Szadzin´ska, S.J. Kowalski, Microwave- and infrared-assisted convective drying of green pepper: quality and energy considerations, Chem Eng. Process. 98 (2015) 155–164. J.A. Gallego-Juárez, E. Riera, S.F. Blanco, G. Rodríguez-Corral, V.M. AcostaAparicio, A. Blanco, Application of high-power ultrasound for dehydration of vegetables: processes and devices, Dry. Technol. 25 (11) (2007) 1893–1901. J. Varith, P. Dijkanarukkul, A. Achariyaviriya, S. Achariyaviriya, Combined microwave-hot air drying of peeled longan, J. Food Eng. 81 (2007) 459–468. A. Motevali, S. Minaei, M.H. Khoshtagaza, Evaluation of energy consumption in different drying methods, Energy Convers. Manage. 52 (2011) 1192–1199. C. Contreras, M.E. Martín-Esparza, A. Chiralt, N. Martínez-Navarrete, Influence of microwave application on convective drying: effects on drying kinetics, and optical and mechanical properties of apple and strawberry, J. Food Eng. 88 (1) (2008) 55–64. Z.E. Sikorski, Food Chemistry, Vol. 1 Food Ingredients, WNT, Warsaw, 2007 (in Polish). M. Carvajal, J.L. Gimenez, F. Riquelme, C.F. Alcaraz, Antioxidant content and colour level in different varieties of red pepper (Capsicum annuum L.) affected by plant-leaf Ti4+ spray and processing, Acta Aliment. Hung. 27 (4) (1998) 365–375. J. Frias, E. Peñas, M. Ullate, C. Vidal-Valverde, Influence of drying by convective air dryer or power ultrasound on the vitamin C and b-carotene content of carrots, J. Agr. Food Chem. 58 (2010) 10539–10544. T. Labuza, S. Tannenbaum, M. Karel, Water content and stability of low moisture and intermediate-moisture foods, Food Technol. (Chicago) 24 (5) (1970). 543–544, 546–548, 550. J. Kumirska, M. Gołe˛biowski, M. Paszkiewicz, A. Bychowska, The Script of Environmental Protection, Food Analysis, Wydawnictwo Uniwersytetu Gdan´skiego, Gdan´sk, 2010 (in Polish). H. Feng, J. Tang, Microwave finish drying of diced apples in a spouted bed, J. Food Sci. 63 (1998) 679–683. K. Schössler, H. Jäger, D. Knorr, Novel contact ultrasound system for the accelerated freeze-drying of vegetables, Innovative Food Sci. Emerg. 16 (2012) 113–120. S.J. Kowalski, A. Pawłowski, Modeling of kinetics in stationary and intermittent drying, Dry. Technol. 28 (8) (2010) 1023–1031. Y. Yao, W. Zhang, K. Yang, S. Liu, B. He, Theoretical model on the heat and mass transfer in silica gel packed beds during the regeneration assisted by highintensity ultrasound, Int. J. Heat Mass Transfer 55 (23–24) (2012) 7133–7143. N. Suwannapum, P. Rattanadecho, S. Vongpradubchai, Analysis of heat-mass transport and pressure buildup induced inside unsaturated porous media subjected to microwave energy using a single (TE10) mode cavity, Dry. Technol. 29 (9) (2011) 1010–1024. S.J. Kowalski, G. Musielak, J. Banaszak, Heat and mass transfer during microwave–convective drying, AIChE J. 56 (1) (2010) 24–35. A. Krupowicz, Numerical Methods of Initial Value Problems of Ordinary Differential Equations, PWN, Warsaw, 1986 (in Polish).
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
The effect of high power airborne ultrasound and microwaves on convective drying effectiveness and quality of green pepper Justyna Szadzin´ska, Joanna Łechtan´ska, Stefan Jan Kowalski ⇑, Marcin Stasiak Poznan University of Technology, Institute of Technology and Chemical Engineering, Department of Process Engineering, Berdychowo 4, 60-965 Poznan, Poland
a r t i c l e
i n f o
Article history: Received 4 February 2016 Received in revised form 11 May 2016 Accepted 22 June 2016 Available online 22 June 2016 Keywords: Hybrid drying Ultrasound Microwave Energy Quality Modelling Green pepper
a b s t r a c t The effectiveness of hybrid drying based on convective drying with application of ultrasound and microwave enhancement is the main subject of the studies. The drying kinetics, energy consumption as well as the quality aspect of green pepper is analysed. It was shown that hybrid drying methods shorten significantly the drying time, reduce the energy consumption and affect positively the quality factors. Each of the analysed aspects depend on combination of the convective-ultrasound-microwave drying programs. Besides, based on the drying model elaborated earlier by one of the authors, the effects of ultrasound on convective drying assessed by such phenomena as ‘‘heating effect”, ‘‘vibration effect” and ‘‘synergistic effect” are presented. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Evaporation of water from fruits and vegetables is a highly complex process associated with many negative consequences, such as changes in the internal structure and sensory properties (colour, flavour, aroma), chemical composition and changes in the content of bioactive components. In drying of foods, it is important to reduce the water content (water activity) while preserving the product quality [1,2]. The quality of dried biomaterial is one of the fundamental indicators in the assessment of the drying process effectiveness. The conventional drying techniques, especially hot air drying is still extensively employed as a preservation technique, however, it affects the final quality of dried product adversely. Convective drying is considered rather as highly destructive due to, e.g. shrinkage, discoloration and loss of nutrients, particularly for thermally sensitive materials like fruits and vegetables. Moreover, hot air drying is usually undesirable long-lasting and energy consuming process of food preservation [3]. One of the recommended ways to minimize these adverse features of convective drying is application of hybrid methods, where the energy is provided alternatively by combination of different energy sources, e.g. convection with ultrasound or microwave ⇑ Corresponding author. E-mail addresses: [email protected] (J. Szadzin´ska), Joanna.L. [email protected] (J. Łechtan´ska), [email protected] (S.J. Kowalski), [email protected] (M. Stasiak). http://dx.doi.org/10.1016/j.ultsonch.2016.06.030 1350-4177/Ó 2016 Elsevier B.V. All rights reserved.
radiation, etc. By microwave radiation heat is provided to the entire material volume in a relatively short time, and not only to the surface as in convective drying. The increase of temperature in the material interior involves thermo-diffusion and pressure gradient that cause ‘‘pumping” of the moisture towards material surface. Thus, the moisture transport is more effective and results in shortening of drying time [4]. Kowalski and Mierzwa [5] showed that convective-microwave drying of beetroot increased the drying rate nearly four times compared with pure convective drying. Convective-microwave drying can be also favourable in terms of product quality. Prabhanjan et al. [6] proved that in microwaveassisted convective drying of carrot cubes the product revealed better reconstitution properties by rehydration than in pure convectively dried material. Moreover, Workneh et al. [7] showed that low temperature air ventilation with microwave assisted drying could be considered as an alternative drying method for tomato slices, as it maintains a superior quality in terms of colour. As presented Kowalski and Mierzwa [8], the convective-microwave drying of red bell pepper caused smaller shrinkage and deformation of this product, and reduced energy consumption by about 62%. Application of microwaves reduces also the degree of vitamin C degradation, i.e. by combined microwave–air–drying the ascorbic acid content was retained up to 98% [9]. Application of ultrasound in drying of foods is a relatively new item and one of the emerging technologies. High power ultrasound generates acoustic cavitation, and the absorption of the acoustic energy causes the so-called ‘‘heating effect” and micro-‘‘vibration
´ ska et al. / Ultrasonics Sonochemistry 34 (2017) 531–539 J. Szadzin
532
Nomenclature A A0 ADrE Am AT
a aUD aw a⁄ b b⁄ CDrEs, eff CDrET, eff CDrEv, eff CV CVMW CVUD CV cl cs Dr Dr E EC hm hT L⁄ l MW
surface [m2] initial surface of sample [m2] ratio of drying rate enhancement [%] surface of mass exchange [m2] surface of heat exchange [m2] microwave energy absorbed by the skeleton [W] absorption coefficient of ultrasonic wave [] water activity [] colour parameter from red to green [] microwave energy absorbed by the moisture [W] colour parameter from yellow to blue [] contribution ratio of ‘‘vibration effect” [%] contribution ratio of ‘‘heating effect” [%] contribution ratio of ‘‘vibration effect” [%] convection convective-microwave drying convective-ultrasound drying convective drying specific heat for liquid [J/kgK] specific heat for liquid [J/kgK] drying rate [g/h] drying rate enhancement [g/h] energy consumption [kWh] mass transfer coefficient [kg/m2 h] heat transfer coefficient [W/m2K] lightness [] latent heat of evaporation [J/kg] microwave
effect” in the material surface layer [10]. Therefore, ultrasound action changes the physical, mechanical or chemical/biochemical properties of biomaterials [11]. Riera-Franco de Sarabia et al. [12] showed that material vibrations induced by ultrasound accelerate notably the drying process and reduces the duration of the process three times. As presented by Rodríguez et al. [13], the ultrasound application in convective drying involved lower total polyphenol and flavonoid content losses in comparison to air dried apples. Gallego-Juárez et al. [14] showed that by dehydration of carrot with application of ultrasound the energy consumption was lower, and the sample rehydration was higher by about 70% than in processing without ultrasound. Deng and Zhao [15] observed that the water activity after the drying of apples with ultrasonic pretreatment is lower as compared to pulsed vacuum pre-treatment. Ultrasound assisted drying can also leads to energy savings. One can state a number of advantages while combining various drying techniques such as convection, microwave and ultrasound in an appropriate way. By combination of several energy sources in one drying process a significant improvement of drying efficiency can be achieved. Then, hybrid drying can be a very attractive and promising solution from the drying kinetics, energy consumption and as well as the product quality point of view. A number of reports in the literature allow to state that use of microwaves and ultrasound to enhance convective drying usually positively affects many properties of dried fruits and vegetables. Among some benefits of such an approach one can mention, e.g. elimination of shrinkage (higher porosity), retention of colour and aromas as well as bioactive components, while shortening processing time and reducing energy consumption [16,17]. As reported García-Pérez et al. [18], high-intensity ultrasound application in convective drying of orange peel reveals energy saving ranging from 12% to 20% in comparison with pure convective hot air drying.
mRt ms mt m0 PUD pvs t te Ta Tm UD
va
(xn) X Xeq Xi Xcr
aV
DE DQ DQMW DQUD dMW ua uj@B
vUD
sample mass during rehydration at time t [kg] mass of dry sample [kg] mass of sample at time t [kg] mass of sample before rehydration [kg] power of ultrasonic generator [W] vapor partial pressure for the saturated state at given temperature [Pa] time [min] drying time at which the moisture content reaches equilibrium [min] air temperature [°C] material temperature [°C] ultrasound air flow velocity [m/s] distance of wave propagation [m] moisture content (dry basis) [kg/kg] equilibrium moisture content (dry basis) [kg/kg] initial moisture content (dry basis) [kg/kg] critical moisture content (dry basis) [kg/kg] volumetric shrinkage coefficient [] total colour change [] heat source [W] microwave heat source [W] ultrasound heat source [W] decay factor of microwave energy [] relative air humidity [%] relative air humidity near the material surface [%] working efficiency of the ultrasonic transducer []
Dried pepper has a lot of application. It is a rich source of valuable nutrients such as ascorbic acid, carotenoids and flavonoids. It is mainly used as a compound for soups, sauces and dry salad mixes [19]. Therefore, the main purpose of this study was to examine the effect of hybrid drying based on convection with ultrasound action and microwave radiation on drying kinetics, energy consumption and final quality of green pepper, i.e. total colour change, water activity, vitamin C retention and the ability to rehydration. In addition, the mathematical modelling is applied to assess the electiveness of ultrasound-convective drying, and to determine some positive effects followed from ultrasound action in drying processes. 2. Material and methods 2.1. Material and apparatus Fresh green pepper (Capsicum annuum L.) imported from Spain were purchased at a local market. The biological material with an average initial water content of 13.19 kg/kg db was washed in tap water, drained with blotting paper and cut into 6 slices (45 mm length, 30 cm width, 2 mm thick), each about 7.5 g. The samples were placed on the pan in the form of ring, skin-side down to allow the most effective application of ultrasound in the focusing area. Then, the green pepper samples were dried to a final moisture content of 0.03 kg/kg db, on average. Seven different drying tests were carried out including convective drying (CV) as a reference, as presented in Table 1. For this purpose an innovative laboratory hybrid dryer (Promis–Tech, Poland), equipped with a microwave and airborne ultrasound generator (Pusonics, Spain) was used. The scheme and photo of the experimental set-up is presented in Figs. 1a and 1b.
´ ska et al. / Ultrasonics Sonochemistry 34 (2017) 531–539 J. Szadzin Table 1 Programs of drying. Drying program
Total energy consumption EC kWh
CV CVUD100 CVUD200 CVMW100 CVMW10010min CVMW100UD200 CVMW10010minUD200
7.97 ± 0.19 7.28 ± 0.20 7.12 ± 0.16 1.26 ± 0.20 3.52 ± 0.12 1.50 ± 0.13 4.73 ± 0.07
*
CV – convection, MW – microwave, UD – ultrasound.
Fig. 1a. Scheme of the hybrid dryer: 1. Fan, 2. Airborne Ultrasound System (AUS), 3. Ultrasound feeder, 4. Electric heater, 5. Air outlet, 6. Ultrasound transducer AUS, 7. Pyrometer, 8. Rotating sample pan, 9. Drive sample pan, 10. Balance, 11. Microwave generator, 12. Control cupboard [20].
Fig. 1b. Photo of the experimental set-up.
This hybrid dryer enables convective drying with microwave and ultrasound enhancement separately as well as in different combinations. High power ultrasound is generated by the Airborne Ultrasound System – AUS, that enables to achieve the sound pressure about 176 dB. The control cupboard of the dryer allows to measure all of the process parameters online such as inlet and outlet air temperature and humidity, air flow rate, material mass and its surface temperature, microwave and ultrasound power, as well as total energy consumption.
533
green pepper was determined on the basis of fresh material dried for 24 h at 60 °C in the chamber dryer, model SN75 (Memmert, Germany). The moisture content X kg/kg db was calculated using the following formula (1):
X¼
mt ms ms
ð1Þ
where mt kg denotes the sample mass measured at time t, and ms kg is the mass of the dry sample [21]. Each drying test was performed in triplicate. Energy consumption EC (in kW h) is defined as the total electric energy consumed in drying process by both the dryer and the control equipment (computer, control cupboard etc.). Moreover, in order to evaluate the effect of hybrid drying on quality of green pepper, several quality factors were assessed such as total colour change, water activity, vitamin C retention and ability to rehydration. Total colour change DE between fresh and dried product was measured 25 times in 5 different points on the sample surface using a colorimeter, model CR-400 (Konica Minolta, Japan), and indicated by CIELab colour space. The value of total colour change was calculated using the following formula (2):
DE ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 ðDL Þ þ ðDa Þ2 þ ðDb Þ
ð2Þ
where L⁄ is lightness, a⁄ is the colour parameter from red to green and b⁄ denotes the colour parameter from yellow to blue [22]. Water activity aw in fresh and dried material was measured with the temperature and humidity converter having the function of water activity measurement, model 650/0628.0024 (Testo, Germany). In this measurement, the randomly selected dried samples were placed in a converter chamber and kept there until equilibrium was reached. Since the moisture profile in the sample has to be aligned after processing, the measurements of aw in dried product started after three hours from the end of the drying process. The DE and aw measurements were carried out in triplicate and the standard deviations were calculated. The content of vitamin C in green pepper was determined using Tillmans method based on titration of the analysed sample with 2,6-dichlorophenolindophenol standard solution in an acid environment [23]. To determine the content of vitamin C, three portions of raw material (in the form of slurry) of 10 g and dried material of 0.5 g were prepared. Then the samples were placed in beakers covered with 3% oxalic acid (50 mL) and held in dark and cool place for 15 min. Next, the solution was filtered with blotting paper and 5 ml of the filtrate was titrated with solution of 2,6-dichlorophenolindophenol until the light pink colour occurred, which was sustained for 30 s. By rehydration tests the dried pepper slices were immersed in 50 cm3 of a distilled water at room temperature of 20 ± 0.2 °C. The kinetics of rehydration process was analysed in time intervals of 10, 20, 30, 60, 90, 120, 150, 180, 210, 240, 270, and 300 min. The experiments were carried out in triplicate. The relative increase in mass of dried green pepper sample was calculated as the ratio of mRt to m0, where mRt is the sample mass during rehydration at time t and m0 is the sample mas before rehydration [24]. 3. Results and discussion
2.2. Methodology
3.1. Drying kinetics
The process parameters used in these studies were as follows: average air temperature of 54 °C, air flow velocity va = 2 m/s, microwave power of 100 W, ultrasound power of 100 and 200 W, respectively. The drying kinetics as well as the air temperature Ta and material temperature Tm plots, and the total energy consumption EC were determined. The initial moisture content Xi of the
First, the convective drying test CV (Fig. 2a) as a reference was carried out at constant air temperature of 54 °C. In this case, the material temperature was maintained below the air temperature for the period of drying time. The final moisture content was reached after te = 759 min, on average. Due to long lasting hot air drying the dried biomaterial was strongly deformed and shrunken.
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´ ska et al. / Ultrasonics Sonochemistry 34 (2017) 531–539 J. Szadzin
Fig. 2. The drying curves and the temperature profiles: a) convective drying (CV), b) convective-ultrasound drying with power of 100 W (CVUD100), c) convective-ultrasound drying with power of 200 W (CVUD200).
Furthermore, as it follows from Fig. 2a the convective drying was found to be the longest process. The second test was convective drying with ultrasonic assistance with UD power of 100 and 200 W. The total drying time of green pepper samples in this case was te = 514 min (100 W) and te = 475 min (200 W) on average, thus, the UD application reduced the overall drying time by about 32% and 37% in CVUD100 and CVUD200 test with respect to CV. Similar observations have been noticed by Gamboa-Santos et al. [25], where the applied acoustic power in drying of strawberries caused a significant reduction of drying time (13–44%). In hybrid drying, an additional energy in the form of ultrasound caused a slight increase in the material temperature, i.e. the ‘‘heating effect” (Fig. 2b and c). However, the heating phenomenon was different in the CVUD200 drying. Due to higher ultrasound power the material
temperature raised already after about 180 min and rose above the air temperature by 3 °C (Fig. 2c). In these two cases the dried green pepper samples were less deformed as compared to pure convective drying. Thus, modification of classical convective drying method allows to eliminate or limit its defects. Another tests concerned the convective drying with microwave assistance (Fig. 3a and b). In the convective-microwave drying (CVMW100), the microwave power of 100 W was applied through the whole process. This value is a minimal stable microwave power generated by the magnetron and also has been selected to provide a suitable radiation to the sample volume. In the CVMW100 drying, by the action of microwave, the heat produced inside the biological material caused a rapid temperature increase, high above the air temperature (Fig. 3a).
Fig. 3. The drying curves and the temperature profiles: a) convective-microwave drying with power of 100 W (CVMW100), b) convective drying assisted with microwave with power of 100 W for the first 10 min (CVMW10010min).
´ ska et al. / Ultrasonics Sonochemistry 34 (2017) 531–539 J. Szadzin
Due to more effective internal heat and moisture transport the drying material reached the final moisture content after te = 87 min, on average. Because, the dried product was characterized by some dark spots, in the next test it was decided to apply microwave at the beginning of the drying process, but only for the first 10 min. The overall drying time in CVMW10010min was te = 300 min, on average, and was shorter by about 60% in comparison with CV. A similar finding was made by Maskan [26], in reference to hot air-microwave combination drying of kiwi fruit. In those studies, the drying time was reduced to about 40%, as compared to hot air drying. In this case, when the microwave energy was applied the material temperature raised up to about Tm = 65 °C, but after 10-min period it decreased and then remained at the level close to air temperature (Fig. 3b). Although, a longer drying time was gained as compared to CVMW100 program, however, after CVMW10010min drying a better quality product was obtained. Next series of experiments concerned convective-microwaveultrasound drying. In the combination of CVMW100UD200 (Fig. 4a) the total drying time was about te = 80 min, on average. In this case the shortest total drying time was obtained. It can be explained by the collision of water molecules and acoustic cavitation, which caused the ‘‘heating effect” and ‘‘vibration effect”. For this reason, the material temperature increased rapidly and much above the air temperature, i.e. up to about Tm = 70 °C (Fig. 4a). In drying of green pepper Łechtan´ska et al. [27] demonstrated that the fastest drying technique was by hybrid drying. In that case the convection was combined with microwave and infrared radiation, and the drying time reduction amounted to about 70%, as compared to convective drying. It proved that hybrid methods consisted of several different processes of moisture evaporation enhances the drying efficiency. The last drying test carried out in these studies was CVMW10010minUD200 (Fig. 4b). In this drying program the green pepper samples reached the final moisture content after te = 263 min, on average. Similarly to CVMW10010min program, when the microwave application was shortened to only 10 min, the total drying time increased considerably. However, the dried material avoided an excessive heating and revealed a better visual appearance, as compared to CVMW100UD200 drying. 3.2. Total energy consumption Comparison of total electric energy consumed in different drying programs was shown in Table 1. The most energy consuming was CV process, as the electric energy usage equalled to 7.97 kW h, on average. It was found that
535
the application of microwave or ultrasound in convective drying as additional energy sources can significantly reduce the total electric energy consumption. Such a low value of energy consumed in CVMW100UD200 is due to simultaneous application of MW and UD. Depending on the applied ultrasound power, the energy usage amounted to about 7.28 and 7.12 kW h for CVUD100 and CVUD200, respectively. As demonstrated Gallego-Juárez et al. [28], by direct contact of ultrasonic vibration the dehydration of carrot slices is quicker, less energy consuming and more powerful than the convective forced-air drying. However, the lowest EC value was observed after convective-microwave drying (CVMW100), i.e. 1.26 kW h. Varith et al. [29] showed that combined microwave-hot air drying greatly reduced specific energy consumption up to 48% as compared to conventional hot-air treatment. A quite similar energy consumption was obtained for CVMW10010minUD200 program. In the case of short microwave application followed by convective drying (CVMW10010min), the EC decreased over two times in comparison with CV. Experimental results of Motevali et al. [30] have also revealed that the use of microwave pre-treatment (for the first 20 min/100 W) in drying pomegranate arils decreased energy consumption maximum up to about 2 times in comparison with pure convective drying.
3.3. Quality Table 2 shows the results of the total colour difference DE between fresh green pepper samples and those after drying, and also the vitamin C retention. Data obtained from the pure convective drying process (CV) served as a reference. The highest value of DE was observed for the green pepper samples dried solely convectively. It proves that a long-lasting hot air drying causes a meaningful colour alteration. Natural dyes present in vegetables and fruits readily undergo destruction. The colour of natural pigment is unstable and susceptible to various factors such as light, temperature, oxidation, pH, metal ions etc. Application of microwave and ultrasound shortened effectively the drying time, thus the green pepper dried in CVMW100UD200 program revealed the lowest colour change. Satisfactory results were also obtained in the case of convective-ultrasound drying, whereas the samples dried with microwave application (CVMW100) were characterized by the highest DE value among all the hybrid drying programs. As noted Contreras et al. [31], shorter process time associated with higher air temperature or microwave application results in lesser colour changes in the dried samples. It is necessary to point out that despite of lower values in total colour change after MW and UD application, the obtained values are still high, since above
Fig. 4. The drying curves and the temperature profiles: a) convective-microwave-ultrasound drying with microwave power of 100 W and ultrasound power of 200 W (CVMW100UD200), b) convective-ultrasound drying with power of 200 W assisted with microwave with power of 100 W for the first 10 min (CVMW10010minUD200).
´ ska et al. / Ultrasonics Sonochemistry 34 (2017) 531–539 J. Szadzin
536 Table 2 Total colour change and vitamin C retention. Drying process
CV
CVUD100
CVUD200
CVMW100
CVMW10010min
CVMW100 UD200
CVMW10010min UD200
Total colour change DE [] Vitamin C retention [%]
15.12 ± 0.40 44 ± 2
11.32 ± 0.67 62 ± 1
11.51 ± 0.16 69 ± 1
12.34 ± 0.29 51 ± 2
11.94 ± 0.59 63 ± 3
10.88 ± 0.22 61 ± 1
12.16 ± 0.64 67 ± 2
DE = 5 the observer may have an impression of two different colours. Vitamin C is the most heat labile of all vitamins. Therefore, drying processes should be carried out in a way ensuring the preservation of the highest amount of vitamins contained in fresh biomaterials [32]. Convective drying contributed to the lowest vitamin C retention from all the analysed drying programs. However, as reported Carvajal et al. [33], the variety of pepper determines the retention of the vitamin C after drying. In most studied varietals, the retention was 24–28%, while in one of them it was comparable with that in Table 2, i.e. about 42%. In turn, the lowest vitamin C retention in hybrid drying was observed after CVMW100 drying. For other programs, a quite similar vitamin C retention was observed, i.e. about 62%, except CVUD200 and CVMW10010minUD200. In those programs, a rather mild conditions were used, so the green pepper retained the highest content of vitamin C. Frias et al. [34] reported about 82–92% retention of vitamin C after ultrasound drying of blanched carrots and less than 50% for air dried carrots. It proves
Fig. 5. Water activity change after different drying programs.
that high power ultrasound drying allows to preserve high amount of vitamin C. However, when UD is combined with MW, the content of vitamin C in dried product is lower. The second important indicator in the quality assessment was water activity (aw). The results of aw for raw and dried product are presented in Fig. 5. Water activity influences the microbial growth fundamentally including: bacteria, yeast and moulds. However, it is worth noting that above the value of 0.4 the relative speed of lipid oxidation, Maillard, hydrolysis, enzymatic and microbiological reactions increases considerably [35]. Thus, aw is a critical parameter that allows to determine the product stability and safety [36]. The fresh material was characterized by water activity of about 0.970, on average. All samples dried in hybrid programs, except the samples dried only convectively, were characterized by water activity less than 0.4. Thus, bacteria, yeast and mould growth was inhibited. Water activity of all green pepper samples decreased significantly after drying, but the lowest value of about aw = 0.208 was reached after convective drying with MW and UD application, i.e. CVMW100UD200. In turn, the samples dried by convectivemicrowave drying (CVMW100) had a visibly lower value of aw, in comparison with the samples dried in CVUD100-200 programs. The last quality parameter evaluated in this work was the degree of rehydration, which determines the dried product ability to irrigation. Fig. 6 shows the rehydration curves of dried green pepper samples as the relative increase in mass of dried material during rehydration in a distilled water. After 300 min of rehydration the relative increase in mass of samples dried only convectively (CV) reached the smallest value. It proves that, hot air drying has a negative influence on structure properties. In turn, the highest mass gain was observed in the case of samples dried in CVMW100 program. It means that hybrid drying consisting of convection and microwave assistance affects the structure of plant tissue to a lesser extent. Feng and Tang [37] found that spouted-bed drying combined with microwave of
Fig. 6. Rehydration curves.
´ ska et al. / Ultrasonics Sonochemistry 34 (2017) 531–539 J. Szadzin
apples increases the rehydration rates compared to that of conventional spouted-bed drying. Satisfactory results were also obtained after rehydration of samples dried with CVMW100UD200 program. On the other hand, hybrid drying with ultrasound enhancement, i.e. CVUD100 and CVUD200, improved the rehydration properties insignificantly, as compared to CV drying. Similarly, Schössler et al. [38] found no differences in rehydration characteristics in studies on freeze-dried and ultrasound assisted freezedried bell pepper. 4. Modelling The effect of ultrasound on drying process can be determined using a mathematical model describing the kinetics of convective drying assisted with ultrasound. For this purpose, the coupled ordinary differential equations by Kowalski and Pawłowski [39] and Kowalski and Pawłowski [20] were used. The proposed global mathematical model of drying kinetics allows the numerical calculation of drying curves and material temperature plots, which should be reflected in the experiments. The mass and heat balance equations provide the basis for the drying kinetics determination. The final form of these equations is as follows:
ms
dX uj p ðT m Þ ¼ Am hm ln @B v s dt ua pv s ðT a Þ
ms
d uj p ðT m Þ ½ðcs þ cl XÞT m ¼ AT hT ðT a T m Þ Am lhm ln @B v s þ DQ dt ua pv s ðT a Þ
ð3Þ
ð4Þ where ms is the mass of dry sample, X is the moisture content (dry basis), Am is the surface of mass exchange, AT is the surface of heat exchange, hm is the mass transfer coefficient, hT is the heat transfer coefficient, uj@B denotes the relative air humidity near the material surface, ua is the relative air humidity, pms is the vapor partial pressure for the saturated state at given temperature, Tm denotes the material temperature, Ta denotes the air temperature, cs and cl denote the specific heat for solid and liquid, l is the latent heat of evaporation, and DQ = {DQUD; DQMW} is the heat source expressing an additional heat supply (e.g. ultrasound UD or microwave MW), and is an auxiliary relation that has to be determined independently. The ultrasound heat source is expressed as DQUD = aUDvUDPUD, where aUD [] denotes the dimensionless absorption coefficient of the ultrasonic wave, vUD [] is the dimensionless working efficiency of the ultrasonic transducer, and PUD [W] is the power of the ultrasonic generator [40]. The microwave heat source DQMW is possible to determine on the basis of Maxwell electromagnetic equations [41]. In practical applications the heat source in microwave drying can be written as DQMW = (a + bX) exp [2dMW(xn)], where a and b express the amount of microwave energy absorbed by the skeleton and moisture, (xn) denotes the distance of microwave propagation in n-direction, and dMW expresses the decay of microwave energy due to its absorption with distance, respectively. When the dried samples are small-sized, then the decay of microwave energy due to absorption can be considered as a small one, and therefore dMW(xn) 0 [42]. It was assumed that the heat and mass exchange occurs on the whole sample surface, and also that the dried material undergoing linear volumetric shrinkage. Hence, the change of surface dimension is a function of moisture content described by the following formula:
Am ¼ AT ¼ AðXÞ ¼ ½1 aV ðX i XÞ2=3 A0
ð5Þ
where aV and Xi denotes the volumetric shrinkage coefficient and initial moisture content, respectively. In the present simulations
537
an averaged geometry for all samples was assumed. It was also assumed that the air relative humidity close to the material surface uj@B is a function of the sample moisture content X:
(
uj@B ¼
1 for
X P X cr
X 1 ð1 ua Þ XXcrcrX eq
for
ð6Þ
X cr P X P X eq
where Xcr and Xeq are the critical and equilibrium values of the sample moisture content. Based on data given in [21], one can stated that the temperature dependent saturated vapor pressure pms can be approximated using the following function:
pms ðTÞ ¼ 9:61966 104 T 4 1:08405264T 3 þ 4:61325529 102 T 2 2:77803513 104 T þ 6:29588464 106
ð7Þ
In order to make use of the above presented drying model for complex hybrid drying of biological materials, the physical heat (hT) and mass (hm) transfer coefficients as well as the heat sources DQ have to be determined first. For this purpose an optimization methodology of assessment of the heat and mass transfer coefficients and the heat source DQ, based on adjusting the drying kinetics plots determined numerically to the experimentally established data, was used. The initial value problem based on the set of kinetic Eqs. (3) and (4) was solved by the Adams–Bashforth non-selfstarting multistep method. Selection of this method was motivated by good convergence and stability in long-term simulations [43]. The physical heat (hT) and mass (hm) transfer coefficients existing in this model were assessed on the basis of drying kinetics obtained from experimental studies. Usually, the convective heat and mass transfer coefficients depend on both the moisture content and the temperature. Below, an example of modelling of ultrasound assisted connective drying is presented. A good fit of experimental curves to the theoretical one (Fig. 7) allows to determine these coefficients. As it follows form Fig. 7, a good adjustment of the numerical drying curves as well as the material temperature plots to the experimental one was achieved, although the calculations were confined to about 500 min of drying time. Drying rate Dr [g/h] expresses the rate of moisture decrease in the drying material as a function of time:
Dr ðtÞ ¼ ms
dX uj p ðT m Þ ¼ Am hm ln @B v s ua pv s ðT a Þ dt
ð8Þ
The average drying rate Dr,ave for a given drying process is determined as:
Dr;av e ¼
1 te
Z
te
Dr ðtÞdt
ð9Þ
0
where te is the drying time at which the moisture content reaches equilibrium Xeq. The average drying time for convective drying of (Fig. 2a) and for green pepper equals t e ¼ tCV e ¼ 759 min convective-ultrasound drying is t e ¼ t UD ¼ 475 min (Fig. 2c). e Drying rate enhancement DrE and the ratio of drying rate enhancement ADrE are used to determine the effectiveness of convective drying assisted with UD, that is: CV Dr E ¼ DUD r;av e Dr;av e
¼ Am ðhm þ Dhm Þ ln
uj@B pv s ðT m þ DT m Þ uj p ðT m Þ Am hm ln @B v s ua pv s ðT a Þ ua pv s ðT a Þ ð10Þ
As it was stated in Chapter 2.1 the average initial water content of the pepper sample was 13.19 kg/kg db, and the single sample slice (45 mm length, 30 cm width, 2 mm thick) about 7.5 g. Then, the green pepper samples were dried to a final moisture content
´ ska et al. / Ultrasonics Sonochemistry 34 (2017) 531–539 J. Szadzin
538
Fig. 7. Drying curves and material temperature plots determined experimentally (exp) and numerically (num) for: (a) convective drying (CV), (b) and convective-ultrasound drying with power of 200 W (CVUD200).
of 0.03 kg/kg db, on average. The total mass of 6 sample slices was about 45 g, and total mass of moisture removed from 6 samples was about 25.33 g. Thus, the rate of single sample drying amounted about 2.02 g/h for pure convective drying, and about 3.18 g/h for convective drying enhanced with ultrasound. Thus, the drying rate enhancement DrE and the ratio of drying rate enhancement ADrE of 6 samples amount to: CV Dr E ¼ DUD r;av e Dr;av e ¼ 19:09 12:13 ¼ 6:96 g=h
ADr E ¼
CV DUD r;av e Dr;av e
DCV r;av e
100% ¼
6:96 100% ¼ 57% 12:13
ð11Þ
ð12Þ
A high frequency vibration of air with a certain sound pressure near the body surface causes ‘‘vibration effect”, which contributes to the increase of mass transfer. At the same time a part of the ultrasonic energy is absorbed by the material under drying and converted to heat (‘‘heating effect”). This effect causes material temperature increase, which contributes to the vapor pressure increase. Due to ‘‘vibration effect” and ‘‘heating effect” in CVUD process the heat and mass transfer coefficients are greater than in pure CV drying. An interesting phenomenon is the creation of the third effect that is associated with both the ‘‘heating effect” and the ‘‘vibration effect”, i.e. the ‘‘synergistic effect”. The ‘‘synergistic effect” occurs when the ‘‘heating effect” is significant, that is, when Tm + DTm > Ta, and contributes to drying performance improvement. To describe the ‘‘heating effect” (T), ‘‘vibration effect” (v) and the ‘‘synergistic effect” (s) quantitatively, the equation of drying rate enhancement (10), together with the equation of the evaporation [20], is reformulated as follows:
CDr ET;eff ¼
Am hm l 1 1 100% Dr E R T m T m þ DT m
ð13Þ
CDr Ev ;eff ¼
Am hm uj@B 100% ln Dr E ua
ð14Þ
CDr Es;eff ¼
A m Dh m l 1 1 D r E R T a T m þ DT m
100%
ð15Þ
The average air temperature was Ta = 54 °C, the average material temperature during CV drying was Tm = 53 °C, and in CVUD200 drying Tm = 56 °C. Knowing the temperatures of drying medium and dried body, the vapor partial pressures pms for saturated state can be read directly from tables [21]:
pms = 15,002 Pa for air temperature Ta = 54 °C pms = 14,293 Pa for material temperature Tm = 53 °C in CV drying pms = 16,509 Pa for material temperature Tm + DTm = 56 °C in CVUD200 drying The average surface area of 6 green pepper samples equals Am = 0.0081 m2, the average relative air humidity ua = 0.075, the hm coefficients for CV and CVUD200 processes are 1.0067 and 1.2695 kg/m2 h, so that Dhm = 0.2628 kg/m2 h. Therefore, after substituting the above data in formulas (13)–(15), one can obtain the component values of ultrasound action (C) for the ‘‘heating effect” (T), ‘‘vibration effect” (v) and the ‘‘synergistic effect” (s):
Am hm pv s ðT m þ DT m Þ ln 100% pv s ðT m Þ Dr E 8:1 1:0067 16509 ln 100% ¼ 17% ¼¼ 6:96 14293
CDr ET;eff ¼
Am Dhm uj@B 100% ln Dr E ua 8:1 0:2628 0:9 ¼ ln 100% ¼ 80% 6:96 0:075
ð16Þ
CDr Ev ;eff ¼
Am Dhm pv s ðT m þ DT m Þ ln 100% pv s ðT a Þ Dr E 8:1 0:2628 16509 ln 100% ¼ 3% ¼¼ 6:96 15002
ð17Þ
CDr Es;eff ¼
ð18Þ
On the basis of the above calculations one can state that the ‘‘vibration effect” has contributed the most to an increase in drying efficiency by convective drying enhanced with ultrasound. These results correspond to those reported by Kowalski and Pawłowski [20], where the ‘‘heating effect” and the ‘‘vibration effect” in the case of apple drying amounted to about 17% and 80%, respectively. 5. Conclusion Based on the above presented results it can be concluded that application of differentiated energy sources in drying of green pepper affects positively the drying kinetics and the product quality. The reduction in drying time reached maximum in convectivemicrowave drying and amounted about 88%, but by the ultrasound enhancement it was about 39%, as compared to convective drying. Furthermore, the measurement of energy consumption in different drying schedules showed that convective-microwave drying of green pepper consumes much less energy (maximum about 80%)
´ ska et al. / Ultrasonics Sonochemistry 34 (2017) 531–539 J. Szadzin
than other hybrid drying processes. Also, convective-microwaveultrasound drying reduces energy consumption to the similar level. The analysis of the total colour change proved that microwave and ultrasound application in convective drying decreases discoloration of green pepper. However, the best dried biological materials from the colour point of view were obtained after the use of convection, microwaves and ultrasound simultaneously. It is also important to learn that hybrid drying of green pepper revealed a low water activity and better rehydration ability. In the case of the nutritional aspect, it was demonstrated that convectiveultrasound drying allows retaining most of the vitamin C, i.e. up to 70%. The applied in this work mathematical model of drying kinetics describes satisfactory the experimental data obtained by convective drying enhance with ultrasound, so the numerical and experimental curves are very well adjusted. As demonstrated, the use of ultrasound in drying processes contributes to drying effectiveness through the ‘‘vibration effect” and ‘‘heating effect”, and in some cases also the ‘‘synergistic effect” occurs as well. However, the ‘‘vibration effect” is the most dominant factor in the increase of moisture removal during convective-ultrasound drying. It decides the most about the effectiveness of drying processes assisted with ultrasound. However, convective drying with microwave radiation reveals at most with ‘‘heating effect” and not ‘‘vibration effect” Finally, it can be concluded that combined convective-microwave-ultrasound drying provides good quality of dried products in a relatively short drying time. Acknowledgement This study was conducted as a part of research project no. 2012/05/B/ST8/01773 sponsored by the National Science Center in Poland. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ultsonch.2016.06. 030.
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20] [21] [22] [23] [24] [25]
[26] [27]
[28]
[29] [30] [31]
References
[32]
[1] C.L. Hii, C.L. Law, Product quality evolution during drying of foods, vegetables and fruits, in: S.V. Jangam, C.L. Law, A.S. Mujumdar (Eds.), Drying of Foods, Vegetables and Fruits, vol. 1, TPR Group, Singapore, 2010, pp. 31–58. [2] E. Maltini, D. Torreggiani, E. Venir, G. Bertolo, Water activity and the preservation of plant foods, Food Chem. 82 (2003) 79–86. [3] C. Ratti, Hot air and freeze-drying of high-value foods: a review, J. Food Eng. 49 (2001) 311–319. [4] P.S. Schmidt, T.L. Bergman, J.A. Peace, P. Chen, Heat and mass transfer considerations in dielectrically-enhanced drying, in: A.S. Mujumdar (Ed.), Proceedings of the 8th International Drying Symposium Drying 92, Montreal, Canada, Elsevier, New York, 1992, pp. 137–160. [5] S.J. Kowalski, D. Mierzwa, Convective drying in combination with microwave and IR drying for biological materials, Dry. Technol. 27 (2009) 1292–1301. [6] D.G. Prabhanjan, H.S. Ramaswamy, G.S. Vijaya Raghavan, Microwave-assisted convective air drying of thin layer carrots, J. Food Eng. 25 (1995) 283–293. [7] T.S. Workneh, V. Raghavan, Y. Gariepy, Microwave assisted hot air ventilation drying of tomato slices, in: International Conference on Food Engineering and Biotechnology, ICFEB, 2011, pp. 150–161. [8] S.J. Kowalski, D. Mierzwa, Hybrid drying of red bell pepper: energy and quality issues, Dry. Technol. 29 (2011) 1195–1203. [9] I. Alibas, Determination of drying parameters, ascorbic acid contents and color characteristics of nettle leaves during microwave-, air- and combined microwave-air drying, J. Food Process Eng. 33 (2010) 213–233. [10] S.J. Kowalski, D. Mierzwa, US-assisted convective drying of biological materials, Dry. Technol. 33 (2015) 1601–1613. [11] J.A. Carcel, J. Benedito, C. Rossello, A. Mulet, Influence of ultrasound intensity on mass transfer in apple immersed in a sucrose solution, J. Food Eng. 78 (2) (2007) 472–479. [12] E. Riera-Franco de Sarabia, J.A. Gallego-Juárez, G. Rodríguez-Corral, V.M. Acosta-Aparicio, E. Andrés-Gallegos, Application of high-power ultrasound for
[33]
[34]
[35]
[36]
[37] [38]
[39] [40]
[41]
[42] [43]
539
drying vegetables, in: Forum Acusticum Sevilla 2002, Ultrasonics, ULT-05-004IP, 2002. Ó. Rodríguez, J.V. Santacatalina, S. Simal, J.V. Garcia-Perez, A. Femenia, C. Rosselló, Influence of power ultrasound application on drying kinetics of apple and its antioxidant and microstructural properties, J. Food Eng. 129 (2014) 21– 29. J.A. Gallego-Juárez, G. Rodriguez-Corral, J.C.G. Moraleda, T.S. Yang, A new highintensity ultrasonic technology for food dehydration, Dry. Technol. 17 (3) (1999) 597–608. Y. Deng, Y. Zhao, Effect of pulsed vacuum and ultrasound osmo-pretreatments on glass transition temperature, texture, microstructure and calcium penetration of dried apples (Fuji), LWT-Food Sci. Technol. 41 (2008) 1575–1585. M. Nowacka, M. S´ledz´, A. Wiktor, D. Witrowa-Rajchert, Physical and chemical properties of microwave dried food products, Food Sci. Technol. Qual. 6 (85) (2012) 5–20 (in Polish). T.S. Awad, H.A. Moharram, O.E. Shaltout, D. Asker, M.M. Youssef, Applications of ultrasound in analysis, processing and quality control of food: a review, Food Res. Int. 48 (2012) 410–427. J.V. García-Pérez, C. Ortuno, A. Puig, J.A. Cárcel, I. Perez-Munuera, Enhancement of water transport and microstructural changes induced by high-intensity ultrasound application on orange peel drying, Food Bioprocess Technol. 5 (2012) 2256–2265. B.I.O. Ade-Omowaye, N.K. Rastogi, A. Angersbach, D. Knorr, Osmotic dehydration of bell peppers: influence of high intensity electric field pulses and elevated temperature treatment, J. Food Eng. 54 (2002) 35–43. S.J. Kowalski, A. Pawłowski, Intensification of apple drying due to ultrasound enhancement, J. Food Eng. 156 (2015) 1–9. Cz. Strumiłło, Foundations of the Drying Theory and Technology, WNT, Warsaw, 1983 (in Polish). F. Nsonzi, H.S. Ramaswamy, Quality evaluation of osmo-convective dried blueberries, Dry. Technol. 16 (1998) 705–723. J. Tillmans, P. Hirsch, J.Z. Jackisch, Lebensmittel, Ztschr. Unters. 63 (1932) 267– 275. D. Witrowa-Rajchert, P.P. Lewicki, Rehydration properties of dried plant tissues, Int. J. Food Sci. Technol. 41 (2006) 1040–1046. J. Gamboa-Santos, A. Montilla, J.A. Cárcel, M. Villamiel, J.V. García-Pérez, Airborne ultrasound application in the convective drying of strawberry, J. Food Eng. 128 (2014) 132–139. M. Maskan, Drying, shrinkage and rehydration characteristics of kiwifruits during hot air and microwave drying, J. Food Eng. 48 (2001) 177–182. J. Łechtan´ska, J. Szadzin´ska, S.J. Kowalski, Microwave- and infrared-assisted convective drying of green pepper: quality and energy considerations, Chem Eng. Process. 98 (2015) 155–164. J.A. Gallego-Juárez, E. Riera, S.F. Blanco, G. Rodríguez-Corral, V.M. AcostaAparicio, A. Blanco, Application of high-power ultrasound for dehydration of vegetables: processes and devices, Dry. Technol. 25 (11) (2007) 1893–1901. J. Varith, P. Dijkanarukkul, A. Achariyaviriya, S. Achariyaviriya, Combined microwave-hot air drying of peeled longan, J. Food Eng. 81 (2007) 459–468. A. Motevali, S. Minaei, M.H. Khoshtagaza, Evaluation of energy consumption in different drying methods, Energy Convers. Manage. 52 (2011) 1192–1199. C. Contreras, M.E. Martín-Esparza, A. Chiralt, N. Martínez-Navarrete, Influence of microwave application on convective drying: effects on drying kinetics, and optical and mechanical properties of apple and strawberry, J. Food Eng. 88 (1) (2008) 55–64. Z.E. Sikorski, Food Chemistry, Vol. 1 Food Ingredients, WNT, Warsaw, 2007 (in Polish). M. Carvajal, J.L. Gimenez, F. Riquelme, C.F. Alcaraz, Antioxidant content and colour level in different varieties of red pepper (Capsicum annuum L.) affected by plant-leaf Ti4+ spray and processing, Acta Aliment. Hung. 27 (4) (1998) 365–375. J. Frias, E. Peñas, M. Ullate, C. Vidal-Valverde, Influence of drying by convective air dryer or power ultrasound on the vitamin C and b-carotene content of carrots, J. Agr. Food Chem. 58 (2010) 10539–10544. T. Labuza, S. Tannenbaum, M. Karel, Water content and stability of low moisture and intermediate-moisture foods, Food Technol. (Chicago) 24 (5) (1970). 543–544, 546–548, 550. J. Kumirska, M. Gołe˛biowski, M. Paszkiewicz, A. Bychowska, The Script of Environmental Protection, Food Analysis, Wydawnictwo Uniwersytetu Gdan´skiego, Gdan´sk, 2010 (in Polish). H. Feng, J. Tang, Microwave finish drying of diced apples in a spouted bed, J. Food Sci. 63 (1998) 679–683. K. Schössler, H. Jäger, D. Knorr, Novel contact ultrasound system for the accelerated freeze-drying of vegetables, Innovative Food Sci. Emerg. 16 (2012) 113–120. S.J. Kowalski, A. Pawłowski, Modeling of kinetics in stationary and intermittent drying, Dry. Technol. 28 (8) (2010) 1023–1031. Y. Yao, W. Zhang, K. Yang, S. Liu, B. He, Theoretical model on the heat and mass transfer in silica gel packed beds during the regeneration assisted by highintensity ultrasound, Int. J. Heat Mass Transfer 55 (23–24) (2012) 7133–7143. N. Suwannapum, P. Rattanadecho, S. Vongpradubchai, Analysis of heat-mass transport and pressure buildup induced inside unsaturated porous media subjected to microwave energy using a single (TE10) mode cavity, Dry. Technol. 29 (9) (2011) 1010–1024. S.J. Kowalski, G. Musielak, J. Banaszak, Heat and mass transfer during microwave–convective drying, AIChE J. 56 (1) (2010) 24–35. A. Krupowicz, Numerical Methods of Initial Value Problems of Ordinary Differential Equations, PWN, Warsaw, 1986 (in Polish).