Pulsed vacuum drying enhances drying kinetics and quality of lemon slices

Journal of Food Engineering 224 (2018) 129e138

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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng

Pulsed vacuum drying enhances drying kinetics and quality of lemon slices Jun Wang a, Chung-Lim Law b, Prabhat K. Nema c, Jin-Hong Zhao d, Zi-Liang Liu a, Li-Zhen Deng a, Zhen-Jiang Gao a, Hong-Wei Xiao a, * a

College of Engineering, China Agricultural University, PO Box 194, 17 Qinghua Donglu, Beijing 100083, China Department of Chemical and Environmental Engineering, University of Nottingham, Malaysia Campus, Selangor, Malaysia Department of Food Engineering, National Institute of Food Technology Entrepreneurship and Management (NIFTEM), Kundli, Sonepat, 131028 Haryana, India d Institute of Food Science and Technology, Chinese Academy of Agricultural Sciences, Beijing 100193, China b c

a r t i c l e i n f o

a b s t r a c t

Article history:

The effect of drying temperature (60, 65, 70, and 75
C) on drying characteristics, shrinkage, rehydration kinetics, microstructure and color profiles of lemon slices were investigated using a far-infrared radiation heating assisted pulsed vacuum dryer. Results showed that the drying time of lemon slices was reduced from 10.5 to 5.5 h when the drying temperature was increased from 60 to 75
C. Weibull model could precisely describe the drying of samples under different drying temperatures (R2 > 0.99). Moisture effective diffusivity (Deff), which was determined by taking shrinkage effect into consideration, varied with moisture content. At the initial drying stages, the volume shrinkage of samples followed a linear relationship with decreasing moisture content as the volume shrinkage is approximately equal to the volume of evaporated water. Microstructure observation illustrated that the “skeleton” was fixed when moisture content decreased to approximate 60% w.b. Page model could well model the rehydration kinetics (R2 > 0.98). In terms of color evaluation, temperature of 75
C significantly caused color deterioration and it recorded the highest color change (DE) of 14.23, Browning Index (BI) of 27.14 and lowest Hue Angle (H0) of 79.46. The findings indicate that FIR-pulsed vacuum drying is a promising alternative method for lemon slices as it can enhance drying process as well as preserve the quality attributes of lemon slices. © 2018 Elsevier Ltd. All rights reserved.

Chemical compounds studied in this article: Ascorbic acid (PubChem CID: 54670067) Citric acid (PubChem CID: 311) Nicotinic acid (PubChem CID: 938) Malic acid (PubChem CID: 525) Flavone (PubChem CID: 10680) Flavonol (PubChem CID: 11349) Aurantiamine (PubChem CID: 11358551) Naringin (PubChem CID: 442428) Hesperetin (PubChem CID: 72281) Rutacultin (PubChem CID: 182049) Keywords: Lemon slice Pulsed vacuum drying Shrinkage Rehydration kinetics Color profiles

1. Introduction Lemon (Citrus limon (L.) Burm. f), is a popular fruits with attractive color, aroma and rich in nutrients such as ascorbic acid,
lez-Molina et al., citric acid, flavonoids and minerals etc (Gonza 2009; Lorente et al., 2014). Drying is one of the most frequently used methods for lemon preservation as a reduced moisture content can hinder growth and reproduction of microorganisms, and minimize many moisture-mediated deteriorative reactions (Xiao et al., 2012). The quality attributes of products in terms of color, rehydration capacity and appearance are affected by pretreatment,

* Corresponding author. E-mail address: [email protected] (H.-W. Xiao). URL: https://www.researchgate.net/profile/Hong_Wei_Xiao5 https://doi.org/10.1016/j.jfoodeng.2018.01.002 0260-8774/© 2018 Elsevier Ltd. All rights reserved.

drying technology as well processing conditions employed (Deng et al., 2017). Open sun-drying is still practised for lemon dehydration in almost all lemon-growing countries, especially for small scale production, which results in yellowish-brown, nonuniform shrinkage and contamination of products which are undesirable (Sadeghi et al., 2013). In addition, the open sun-drying method has several disadvantages, such as long drying time, rotting or rewetting caused by poor weather, dust and insects pollution (Xiao et al., 2015). Hot-air drying method is widely applied in industrialized production of agricultural materials due to the simple equipment, diversified form of energy utilization, mass production etc. (Chen et al., 2005; Sadeghi et al., 2013). However, hot-air drying can cause some undesirable quality changes in products including browning, oxidation, and case hardening as well as degradation of

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nutrition and flavour's due to prolonged exposure to high air temperature or long processing time which leads to product degradation. Therefore, in order to improve drying process to minimize color and nutrients loss, the traditional open sun-drying and hot air drying methods may be replaced by a more efficient and advanced drying technologies. In fact, many investigations have been carried out to explore alternative drying technologies, kinetic modelling and quality attributes during lemon slices drying. The closed-type dryer associated with a photovoltaic system, microwave, combined microwaveconvective and air-ventilated oven dryer have been employed in lemon drying (Chen et al., 2005; Sadeghi et al., 2013; TorkiHarchegani et al., 2016; Torki-Harchegani et al., 2015). These technologies can shorten drying time and improve quality of lemon slices compared with the traditional open sun drying method. However, how to protect the product's color is still a challenge as browning reactions often occur during drying process due to oxygen. The Dincer and Dost model could well predict mass transfer for all treatments of lemon slices during convective, microwave and combined microwave-convective drying (Sadeghi et al., 2013), while the Midilli and Kucuk model fitted the experimental data well during air-ventilated (Torki-Harchegani et al., 2016). As the main by-product, drying of lemon peel is crucial for its functional component content and sensory quality. There are many researches focused on the pretreatment methods to enhance the drying rate and the quality of dehydrated lemon peel, such as ultrasound
rez et al., 2009; Garcia-Perez et al., 2007). However, thus (García-Pe far hot air drying is still the main drying method employed in the lemon processing industry in producing dehydrated lemon peel. Pulsed vacuum drying (PVD) is a novel drying technology developed in recent years, which uses successive change of vacuum pressure in the drying chamber to enhance moisture transfer during drying process (Xie et al., 2017a). During PVD processing, the pulsed vacuum environment creates an oxygen deficient environment, which can reduce adverse biochemical reactions, such as oxidation deterioration, browning reactions and thereby improve the quality attributes of dried products (Bai, 2014; Xie et al., 2017a). Additionally, pressure pulsation results in a tunneling effect to enlarge and interconnect the micropores in the products (Moreno et al., 2016) and periodic pressure change can generate porous and fissured structures in the peel, that enhances the mass transfer through them (Mounir et al., 2014). With so many advantages, PVD has been employed in the drying of wolfberry (Xie et al., 2017a), rhizoma dioscoreae (Xie et al., 2017b) and seedless grape (Bai, 2014). Shrinkage, color and rehydration capacity are frequently used to evaluate the quality of dried products. During drying volume of sample decreases and due to moisture loss, surface area also simultaneously shrink, which significantly influence the drying process and the products quality attributes such as rehydration capability and texture (Koua et al., 2017; Ramallo and Mascheroni, 2012). Therefore, the relationship between shrinkage and moisture changes deserves more attention. Color is one of the most important quality attributes as the first quality judgment made by a consumer is by the products color and it influences consumer's food choices, perceptions, and purchase behaviour (Nourian et al., 2003). Color is also an indicator of thermal processing severity and it can be used to predict the corresponding quality deterioration caused by heat exposure (Pathare et al., 2013). Dried lemon slices are usually consumed as lemon tea, which is prepared or produced via a rehydration process. It is a complex process composed of two simultaneous processes: the absorption of water by dried product and the diffusion of soluble (Cunningham et al., 2008). Rehydration capability is one the important quality attributes for dried products as it could indicated the physico-chemical

changes such as cellular structure and water holding capacity (Zielinska and Markowski, 2016). Rapid and complete rehydration is a desired property of dried products. However, to the best of our knowledge, no reports have been found detailing the effect of pulsed vacuum drying on drying kinetics, shrinkage, color and rehydration capacity of lemon slice. Therefore, the objectives of this study are: 1) to explore the pulsed vacuum drying characteristics of lemon slices under different drying temperature via drying curves, further, study the Weibull and first-order model, and determine moisture effective diffusivity, 2) to analyze the shrinkage ratio versus moisture content during pulsed vacuum drying processing, as well as the microstructure observation, 3) to measure the rehydration kinetics and establish the mathematical model of shrinkage using Page model, 4) to evaluate the product color attributes in terms of a*, b*, L*, total color difference (DE), browning index (BI) and hue angle (H0). 2. Materials and methods 2.1. Materials The fresh ripened lemons (Citrus limon (L.) Burm. f, Sichuan Anyue variety) were purchased from Qinghe vegetable market, Beijing. All samples were stored in a refrigerator at 4
C and 90% relative humidity prior to experiments. To ensure uniformity of the physical characteristics of samples, lemons with the same size (average diameter, length and weight were 63 ± 3 mm, 86 ± 3 mm, and 145.8 ± 3.8 g, respectively) were selected. The initial moisture content (85.29 ± 1.61%, w.b.) of samples were determined by vacuum drying at 70
C for 24 h. 2.2. Pulsed vacuum dryer Pulsed vacuum dryer installed in the College of Engineering of China Agricultural University, Beijing, China was used in the present study. A schematic diagram of the pulsed vacuum drying equipment is shown in Fig. 1. The PVD equipment mainly consists of heating, cooling, control and vacuum systems, which have been described in detail by Xie et al. (2017a). A vacuum pump (2BV42060, Bo-Tong, Shanghai, China) is used to regulate the pressure within the drying chamber. The pressure is measured with CYYB110 Pressure Transmitter (Shi Da Chuang Ye, Beijing, China) in drying chamber. The thermocouple signals are collected by SHT75 sensors (Sensirion, Shenzhen, China). A Proportional-IntegralDerivative (PID) controller (Omron, model E5CN, Tokyo, Japan) with accuracy of ±0.1
C is used to control the temperature of water tank and pressure in the drying chamber. An electromagnetic valve isolates the pressure chamber from the ambient air and adjusts the air flow rate from the ambient back into the chamber. Both pressure and thermocouple signals are shown in real time in the touch screen (Weinview, Shenzhen, China). The minimum pressure level that the system can produce is 8.0 kPa (0.08 bar) and the time taken for the system to reach this minimum pressure from atmospheric pressure is approximately 40 s. The drying chamber pressure change kinetics during PVD is shown in Fig. 2. During drying, air is expelled from the drying chamber to a pre-set vacuum state and maintained for a predetermined time. This step is followed by a pressure recovery step where it is held for specified duration. This procedure varies according to the properties of the treated products and the operating conditions (e.g., drying temperature, vacuum amplitude, intermittent time). PA and PV indicate the highest and lowest pressure in drying chamber. The tAP and tVP are the duration at the highest and the lowest pressure, respectively. The ts and td are the time required

J. Wang et al. / Journal of Food Engineering 224 (2018) 129e138

131

1

12

2

11

3

4 10

5 6

9

7

8

Fig. 1. Schematic diagram equipment used for pulsed vacuum drying. 1. Touch screen control panel, 2. Material temperature sensor, 3. Pressure sensor, 4. Vacuum valve, 5. Condenser, 6. Vacuum pump, 7. Air solenoid valve, 8. Drain solenoid valve, 9. Sample, 10. Far-infrared radiation heating element, 11. Drying chamber, 12. Infrared-board temperature sensor.

Fig. 2. Schematic diagram of drying chamber pressure change kinetics during pulsed vacuum drying. PA-the highest pressure in drying chamber, PV- the lowest pressure in drying chamber, tAP- the duration at the highest pressure, tVP- the duration at the lowest pressure, ts-the time required during alternation of PV, and td e the time required during alternation of PA.

during alternation of PV and PA, respectively. 2.3. Experimental design According to preliminary experiments conducted earlier, 10 min, 5 min and 3 mm were selected for the vacuum duration time, normal pressure duration time and material thickness, respectively. For each sub-sample, 10 lemon slices (the weight was range of 130e150 g) were selected randomly from the diameter between 50 and 60 mm. The radiation temperature of 60, 65, 70, and 75
C were selected as the parameters of experiment. The distance from radiation heating element to material tray was 15 mm. A lemon slicer (JS-603, Jiu Si Trading Company Ltd) was used for cutting sample. About 80 g of lemon slices were spread on the stainless steel tray (15 cm  20 cm) with a thin coat of abherent. During the drying process, the sample tray was taken out at 30 min interval, and was rapidly weighted on a digital balance with accuracy of ±0.01 g (SP402, Ohaus Co., New Jersey, USA) and then was put back into the

chamber. The weight for each group of samples was finished in 30 s. Drying was continued until the weight change of samples within 0.1 g at two successive weight checking. Each treatment was performed in triplicate.

2.4. Drying characteristics 2.4.1. Drying rate curves The moisture ratio (MR) of lemon slices was calculated according to Eq. (1) (Bai et al., 2013):

MR ¼

Mt  Me M0  Me

(1)

where, Mt is the moisture content at t time of drying, kg$kg1; M0 is the initial moisture content, kg$kg1; Me is the equilibrium moisture content, kg$kg1. All moisture content was expressed on dry basis.

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The drying rate of lemon slices under various drying conditions was calculated according to Eq. (2):

DR ¼

Deff ¼

Mt1  Mt2 t2  t1

(2)

where, t1 and t2 are the drying times in hours at different times during drying, respectively; Mt1 and Mt2 are the moisture contents of lemon slices at time t1 and t2, respectively. Mt1 and Mt2 expressed on dry basis. 2.4.2. Weibull model Weibull model is widely used for describing moisture changes of food materials under different drying conditions (Wang et al., 2017a; Xie et al., 2017b; Dai et al., 2015). The Weibull distribution function was calculated as shown in Eq. (3):


 a  t MR ¼ exp 

(3)

b

where t is the drying time, a is the scale parameter of the Weibull model, and b is the shape parameter of the model. 2.4.3. First-order model First-order mathematical model has also been proposed to describe moisture changes of agricultural materials during drying (Wang et al., 2017b), as shown as Eq. (4): MR ¼ M0exp(-k0t)

(4)

where, M0 is the initial moisture ratio of lemon slices; k0 is the change rate constant and t is drying time.



dMR dt exp   L2 dMR dFo th

(7)

where, Deff is the effective moisture diffusivity (m2/s); Fo is the Fourier number for diffusion, Fo ¼ Dt/L2. Due to the thickness of lemon slices changed during drying, L in Eq. (7) varied with time. 2.5. Shrinkage The sectional area was calculated using a caliper (SATA, 91511, USA) following the method described by Wang et al. (2016) with slight modification. Diameter of each sample was also measured using the caliper (SATA, 91511, USA) with measuring range 0e150 mm. The sectional area was calculated based on dimensional measurements. To avoid nonuniform shrinkage and bending of samples, the thickness and diameter determinations with physical caliper were repeated at least three times and at different positions. The shape of lemon slice was regarded as a cylinder and the shrinkage of volume calculated using Eq. (8).

 p r2t 2 Lt rt2 Lt Vt VR ¼ ¼  ¼ 2 V0 p r0 2 L r0 L0 0 2

(8)

where Vt is the sample volume at elapsed drying time t, V0 is the initial lemon slice volume; rt and r0 is the mean geometric diameters of the sample section at elapsed time and initially, respectively; Lt and L0 is the mean geometric thickness of lemon slices at elapsed time and initially, respectively. 2.6. Rehydration kinetics

2.4.4. Moisture effective diffusivity and activation energy Fick's second law (Eq. (5)) is widely employed to describe the moisture effective diffusivity (Deff) during drying of various agricultural materials (Hamdami et al., 2004; Aral and Bes¸e, 2016).

vM ¼ Deff V2 M vt

(5)

Eq. (5) can be solved using the analytical solution for infinite slab as Eq. (6) with the assumption that neglecting shrinkage, constant temperature and diffusion coefficients as well as uniform initial moisture distribution (Thuwapanichayanan et al., 2008):

MR ¼

∞ Mt  Me 8 X 1 ¼ 2 exp M0  Me p n¼0 ð2n þ 1Þ2



ð2n þ 1Þ2 p2 Dt 4L2

!

Rehydration ratio (RR) determination was carried out in a water bath at 40 and 90
C. Three slices of dried lemon as a subsample, were kept in 120 mL distilled water in a 150 mL glass beaker. Samples were weighing at 10 min intervals and the adhering water was carefully absorbed with filter paper. After weighting, samples immediately returned to the same soaking baker. This procedure was repeated until constant weight was obtained in two consecutive weighing. All the experiments were performed in triplicate. RR of each subsample was calculated according to Eq. (9) (Aral and Bes¸e, 2016):

RR ¼ (6)

where, D is the moisture diffusivity (m2/s); t is the drying time (s); n is a positive integer; It was assumed that the moisture transfer is one-dimensional diffusion in the upward direction from the bottom of the lemon slices toward the top surface, hence, L in Eq. (6) is the thickness of the lemon slice. Shrinkage due to drying of lemon slices results in changes of moisture diffusion path (L), as well as the resistance to mass transfer at the surface of the sample as compared to inside, which should be taken into consideration (Hamdami et al., 2004). In such case, the effective moisture diffusivity (Deff) at various moisture content can be estimated using the method of slope (Thuwapanichayanan et al., 2008). The slopes of the experimental drying curve (dMR/dt)exp and the theoretical curve (dMR/dFo)th are estimated at a given value of MR. The Deff at a given MR is estimated from Eq. (7) (Hamdami et al., 2004; Sakin et al., 2007):

Wt W0

(9)

where, Wt and W0 is the weight of rehydrated sample at time, t and weight of dried sample, respectively. Page mathematical model is widely employed to describe the kinetics of rehydration ratio of dried samples (Ramallo and Mascheroni, 2012). The Weibull distribution function was calculated as Eq. (10), which followed the assumption of constant water temperature and the same initial moisture content of dried lemon samples. RRmodel ¼ exp(-k1tnR)

(10)

where, k1 is the various rate constant and tR is rehydration time. 2.7. Scanning electron microscopy (SEM) observations Fresh and rehydrated lemon samples were observed using scanning electron microscopy. Samples (5 mm  5 mm) were cut

J. Wang et al. / Journal of Food Engineering 224 (2018) 129e138

with new razor blade and then they were immediately fixed using 0.5% glutaraldehyde solution overnight to stabilize the structure and the composition of the biological system. The fixed samples were transferred to supercooled nitrogen (210
C) for dehydration. For 120 s gold coated, samples were viewed in a scanning electron microscope (SU8010, Hitachi, Japan). 2.8. Color evaluation In present work, the CIE Lab color parameters (L*, a*, b*) were used to describe the quantitatively the surface color change of samples with PVD at temperature of 60, 65, 70, and 75
C. L* represents light-dark spectrum with a range from 0 (black) to 100 (white), while a* is the red-green spectrum with a range from 60 (green) to þ60 (red), and b* indicates yellow-blue spectrum with a ~ a, 2012; range from 60 (blue) to þ60 (yellow) (Fante and Noren Xiao et al., 2010). The color parameters of fresh and dried samples under different conditions were measured using a colorimeter (CIE Lab Scan XE, s/n: LX 18423, USA) following the method described by Wang et al. (2017b) with some modifications. Briefly, the colorimeter was calibrated by placing the tip of measuring head flat against the surface of the white calibration place. After standardization, L* (lightness), a* (redness/greenness), and b* (yellowness/blueness) values were measured on the surface of fresh and dried lemon slices. Total color difference (DE), browning index (BI) and hue angle (H0) was calculated as Eq. (11)e(14) (Pathare et al., 2013; Bai et al., 2013):

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  2  2 2  L*  L*0 þ a*  a*0 þ b*  b*0 DE ¼  BI ¼ 100 

 X  0:31 0:17

(11)

(12)

where

X¼

 * a þ 1:75L* ð6:645L* þ a*  3:012b* Þ

H0 ¼ tan1

 * b a*

(13)

(14)

where L*0, a*0, and b*0 are the color parameters of fresh samples; L*, a*, b* are the color parameters of dried lemon slice samples. For each sample at least six measurements were made at different positions of the sample and the measured values were compared with those of the fresh sample. 2.9. Data analysis The data are presented as the mean of three determinations±standard deviation. The data were analyzed by ANOVA and Duncan's multiple-range test using SPSS statistics software (Version 21.0, SPSS Inc., Chicago, IL, USA). Statistical significance for differences was tested at 5% probability level (P < .05). The statistical analysis of drying experiments for model fitting was performed with Matlab software (Version 7.0, MathWorks, USA). 3. Results and discussion 3.1. Drying characteristic and mathematical modelling Drying temperature is key factor to drying kinetics and quality attributes (Onwude et al., 2016). With the advantage of energy

133

savings, uniform temperature distribution and rapid heating-up, far-infrared radiation was selected as the heating source of pulsed vacuum drying in the present work (Xie et al., 2017a). Fig. 3 illustrates the drying curves which represent the change of moisture ratio with drying time for lemon slices under temperature of 60, 65, 70, and 75
C at slices thickness of 3 mm and pulsed vacuum ratio of 10 min: 5 min, respectively. As expected, the drying time decreased with increasing drying temperature, and the total drying time from the initial moisture to the final 3% were 10.5, 9.0, 7.5 and 5.5 h for samples dried at 60, 65, 70 and 75
C, respectively. Sadeghi et al. (2013) observed similar phenomenon during hot air drying of lemon slices. In addition, Chen et al. (2005) also investigated the influence of temperature on water activity (aw) of lemon slices using a closed-type solar dryer, the result showed an obvious decrease of aw with increasing of drying temperature. Weibull and first-order mathematical model were fitted to the drying experimental data of lemon slices (Fig. 3). Obviously, all the coefficients of determination (R2) were greater than 0.998 for Weibull model, which demonstrated goodness of the fitting. As it describes drying kinetics well, Weibull model has been employed earlier also to describe the drying behavior of apricot halves under temperature and humidity integration control strategies (R2 > 0.996) (Dai et al., 2015), rhizoma dioscoreae slices drying in pulsed vacuum drying conditions (R2 > 0.996) (Xie et al., 2017b), wolfberry drying in far-infrared radiation heating assisted pulsed vacuum dryer (Xie et al., 2017a) and Aloe vera gel drying in convective hot air dryer (R2 > 0.97) (Miranda et al., 2010). Fig. 4a shows the drying rate versus moisture content of lemon slices under different temperatures and 10 min: 5 min as pulsed vacuum ratio. It can be observed, for all four drying temperatures, there were typical three distinct drying periods, a short warm-up period in the beginning, followed by a constant period and towards the end a long falling drying rate period, which in accordance with drying of most materials (Dai et al., 2015; Ju et al., 2016). However, Torki-Harchegani et al. (2016) found that a falling rate period during the whole drying processing, this difference maybe due to different drying method employed. From Fig. 4a, it was found that there was a high coefficient of variation especially at the short warm-up period and constant period. This may be due to the nonuniform of the material temperature and the relative humidity variation of drying medium during pulsed vacuum drying process. Materials' temperature increased slowly at the early stage, but it was heated up during every atmospheric pressure retention period and dropped rapidly when the atmospheric pressure converted to vacuum condition (Xie et al., 2017b). Additionally, the alternative vacuum and atmospheric pressure caused great variations of the relative humidity of drying medium. Furthermore, the drying medium was not uniform in the air inlet and outlet and led to differences in moisture diffusion of materials in different position. The effective moisture diffusivity (Deff) values of lemon slice samples considering the changes of moisture content and shrinkage phenomenon during drying were presented in a nonlinear relationship (Fig. 4b). For the whole drying processing, the higher of drying temperature the larger Deff values were obtained. At the initial phase of drying, Deff enhanced with moisture losing, causing the rapid rise of samples temperature (Thuwapanichayanan et al., 2008). As drying progressed, a decreasing trend was observed of Deff with decreasing moisture content, this may closely related to the shrinkage of lemon slices, which caused the changes in the structure of sample (Bai et al., 2013), the collapse of cytoskeleton and surface hardening of materials (Rizvi, 1986) and increased the resistance in mass transfer. From Fig. 4b, the minimum and maximum Deff value was 1.66  1011 and 1.90  1010 m2/s at drying temperature of 60 and 75
C, respectively. Generally, the most of food materials are in the

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1.2 60 65

1.0

70

Moisture Ratio

75

0.8

0.6

0.4

60

Weibull Model R²=0.99897

65

Weibull Model R²=0.99840

70

Weibull Model R²=0.99932

75

Weibull Model R²=0.99833

60

First-Order R²=0.98941

65

First-Order R²=0.99214

70

First-Order R²=0.98736

75

First-Order R²=0.96852

0.2

0.0 0.0

2.0

4.0

6.0

8.0

10.0

Drying Time (h) Fig. 3. Drying kinetics and modelling of lemon slices under different temperatures using Weibull distribution model and first-order model.

2.00E-10

2.5

a

2.0

65

b

1.60E-10

70

-1

75

D eff (m /s)

Drying Rate (g/g h )

60

1.20E-10

2

1.5

1.0

8.00E-11 60 65

4.00E-11

0.5

70 75 0.00E+00

0.0 0

1

2

3

4

5 -1

Moisture Content (dry basis/g g )

6

0

1

2

3

4

5

Moisture content (kg/kg db)

Fig. 4. Drying rate versus moisture content (a) and moisture diffusivity (b) of lemon slices under different temperatures.

range of 1011 -109 m2/s (Madamba et al., 1996).

3.2. Shrinkage of lemon slices under different temperatures Shrinkage during drying occurs simultaneously with moisture removal and thus can reduce the drying rate as the moisture can be trapped in the dense cells and its movement toward the outer surface for subsequent removal is hindered (Zenoozian and Devahastin, 2009). In addition, shrinkage affects the physical properties of dried products, such as rehydration capability, texture, apparent density, appearance shape, mechanical and elastic properties (Khraisheh et al., 2004; Gulati and Datta, 2015). Therefore, exploration of the shrinkage phenomenon is essential for a better understanding of the drying process and to control the product quality.

The relationships between shrinkage and moisture content of lemon slices during PVD process under different drying temperature are shown in Fig. 5. It was found that at the initial drying stages, the volume shrinkage of dried lemon slices follows a linear relationship with moisture content as the volume shrinkage is approximately equal to the volume of evaporated water. This is in agreement with the results reported by Moreira et al. (2005), who found that the chestnuts shrank during hot air drying followed a linear relationship with normalized moisture content. When the moisture content of lemon slices decreased to about 60%, the shrinkage remains almost constant. So, at this state the shrinkage can be assumed negligible to facilitate solving heat and mass transfer equations. In addition, it was found that all lemon slices shrank during different temperatures drying up to approximately 60% w.b and then samples volume leveled off at about 78% of the

J. Wang et al. / Journal of Food Engineering 224 (2018) 129e138

1.10

1.10

60

1.00

0.90

Shrinkage

Shrinkage

65

1.00

0.90

0.80

0.70

0.80

0.70

y = 0.0155x - 0.3124

0.60

y = 0.0150x - 0.283

0.60

2

R = 0.9494

0.50

2

R = 0.9413

0.50 0

20

40

60

80

100

0

20

40

60

80

100

Moisture Content (% wb)

Moisture Content (% wb) 1.10

1.10

70

1.00

75

1.00

0.90

0.90

Shrinkage

Shrinkage

135

0.80

0.70

0.80

0.70

y = 0.0171x - 0.5149

0.60

2

R = 0.8904

y = 0.0163x - 0.386

0.60

2

R = 0.9617

0.50

0.50 0

20

40

60

80

100

Moisture Content (% wb)

0

20

40

60

80

100

Moisture Content (% wb)

Fig. 5. Dependence of shrinkage of lemon slices on moisture content under different temperatures.

original volume. This phenomenon may be caused by the formation of fixed “skeletal structure” of samples. However, the shrinkage ratio of lemon slices varied with the increase in drying temperature. Obviously, high temperature enhanced moisture diffusion rate which is in favour of the immobilization of texture and microstructure of the samples. From the microstructure observation of fresh and dried sample with moisture content of about 60% w.b, the microstructure of samples were showing similar shrinkage, it formed fixed vascular bundle and matrix structure, which may stabilize the changes of volume when moisture content is about 60%. This is in agreement with the previous report given by Yan et al. (2008), who revealed similar trend of shrinkage in banana, pineapple and mango slices during air-drying process. 3.3. Rehydration kinetic and microstructure The rehydration ratio (RR) of PVD dried lemon slices under different temperatures is illustrated in Fig. 6. For different rehydration temperature, a higher rehydration ratio was found for samples rehydrated at 90
C than the samples rehydrated at 40
C. This phenomenon may be due to the fact that high temperature increases the molecular diffusion rate, hence enhancing the imbibe capacity of samples. In addition, high rehydration temperature contributes to the expansion of sample tissue and space density, which also improved the expansion of capillaries with enhanced hydrophilic properties (Krokida and Maroulis, 2001). From Fig. 6, it

is also observed that the rehydration ratio of dried lemon slices increases with drying temperature. Similar result was also reported
lvez et al. (2008), who found that the rehydration ratio by Vega-Ga of red bell pepper increased with drying temperature. This could be due to the fact that a higher drying temperature result in the tissue collapse and cell damage, creating larger spaces in dried samples and thus enhance the rehydration capability of the dried products. To study the influences of temperature on microstructure, the SEM micrographs of fresh and rehydrated lemon samples were captured and are shown in Fig. 7. From micrographs, it can be observed that the epidermis of fresh lemon samples consisted of distinct bulges and smooth surface structure. Evident epidermis structure changes were observed among different dried samples, it was found that the surface bulges faded and micro-cracks increased with the increasing of drying temperature, which confirm the hypothesis mentioned above, that higher drying temperature causes the tissue to collapse and cell damage. Thus create larger spaces in dried samples which in turn enhance the rehydration capability of the dried products. The Page model was employed to fit to the rehydration process at temperature of 40 and 90
C. As seen in Fig. 6, all R2 values are higher than 0.98, indicating that Page model could well describe the variation of moisture uptake in lemon slices during rehydration. Similar results were reported by Ramallo and Mascheroni (2012) in the research of pineapple rehydration kinetics.

136

J. Wang et al. / Journal of Food Engineering 224 (2018) 129e138

4.5

4.5

Rehydration Ratio (RR)

4.0

2

0.20851

) R =0.99090

Rehydration Ratio (RR)

0.25029

RC90 = exp(0.39635t

3.5 3.0 2.5

60

2.0 1.5

0.26838

RC40 = exp(0.34380t

RC90 = exp(0.47948t

4.0 3.5 3.0 2.5

65

2.0 1.5

2

) R =0.98769

0.24486

RC40 = exp(0.38919t

2

) R =0.99486

1.0

1.0 0

20

40

60

80

100

120

0

140

20

Rehydration Time (min)

40

60

80

100

120

140

Rehydration Time (min)

4.5

4.5

RC90 = exp(0.47124t

4.0

2

) R =0.98830

0.20882

Rehydration Ratio (RR)

0.22172

Rehydration Ratio (RR)

2

) R =0.98302

3.5 3.0 2.5

70

2.0 1.5

0.31013

RC40 = exp(0.29468t

RC90 = exp(0.52462t

4.0 3.5 3.0 2.5

75

2.0 1.5

2

) R =0.99506

2

) R =0.99733

0.24254

RC40 = exp(0.42328t

2

) R =0.99689

1.0

1.0 0

20

40

60

80

100

120

140

0

20

40

60

80

100

120

140

Rehydration Time (min)

Rehydration Time (min)

Fig. 6. Rehydration ratio of lemon slices under different drying temperatures.

3.4. Color evaluation of dried lemon slices under different temperatures Color as one of the most important sensory evaluation indicators for dried lemon slices, a yellowish-brown color can severely restrict its acceptability and value. The color parameters L*, a*, b*, DE, BI and H0 of dried samples under different pulsed vacuum drying temperatures are summarized in Table 1. It was found that with increasing of drying temperature, L* decreased whereas a* and b* increased. L* value, namely whiteness or brightness, which is closely related to the browning level of samples and the lower of its value the more browning reaction occurred (Pathare et al., 2013; Bai et al., 2013). From Table 1, it was observed that samples dried at 75
C obtained the lowest L* value and had a significant difference (P < .05) compared to others samples, revealing high temperature accelerated the browning reaction of lemon slices during drying. a* value is defined as the red-green spectrum with a range from 60 (green) to þ60 (red) (Wang et al., 2017b), and the a* value of dried lemon samples was increased by 0.52, 0.41, 0.63 and 3.44 at temperature of 60, 65, 70 and 75
C, respectively, compared with the fresh samples. High temperature of 75
C significantly increased a* value of lemon slices, even so, this result still better than the best products (L* 41.74 and a* 3.48) reported Chen et al. (2005) who used a closed-type solar dryer. The total color different (DE), is often used to evaluate the magnitude of overall color difference between dried and fresh samples. Differences in perceivable color can be analytically classified as very distinct (DE>3), distinct (1.5
small difference (DE<1.5) (Adekunte et al., 2010). From Table 1, dried lemon slices show significant color difference if compared to fresh ones, indicating that temperature significantly affected the total color changes. The browning index (BI) was used to evaluate the purity of brown color which is caused by enzymatic and non-enzymatic reaction during processing (Mohapatra et al., 2010; Maskan, 2001). In the present study, the BI values increased from 3.88 to 27.14 with increase drying temperature, indicating that high temperature greatly increase non-enzymatic browning reaction. Even so, the color of dried lemon slices in the present study is better than the samples dried in the closed-type solar dryer (Chen et al., 2005). The phenomenon occurred probably due to the fact that the pulsed vacuum environment creates an oxygen deficient environment, which could reduce adverse biochemical reaction (Xie et al., 2017a). In terms of hue angle (H0), it is considered the most visual color parameters from others (Bai et al., 2013). From Table 1, the H0 values decreased from 88.58 to 79.46 with different drying temperatures, and the lowest H0 value was obtained at temperature of 75
C. An orange-red color be observed when H0 < 90
, and the lower H0 value the darker of the color (Sant'Anna et al., 2013; Maskan, 2001). The changes in H0 values at temperature 75
C were significant lower than others.

4. Conclusions Pulsed vacuum drying technology was employed to lemon slices

J. Wang et al. / Journal of Food Engineering 224 (2018) 129e138

65 oC

60 oC

Fresh

137

75 oC

70 oC

Fig. 7. Epidermis SEM micrographs of fresh and rehydrated lemon samples (rehydration temperature of 40
C) under different drying temperatures.

Table 1 The color parameters L*, a*, b*, DE, BI and H0 of dried lemon slices under different PVD temperatures.Note: In the same row, the different letter reveal significant differences (P < .05) according to the Duncan test.

L* a* b* DE BI H0

Fresh

60
C

65
C

70
C

75
C

48.75 ± 1.53a 0.82 ± 0.13c 14.53 ± 0.93c e e e

43.20 ± 0.94b 0.52 ± 0.16b 15.64 ± 0.10bc 5.88 ± 0.59c 3.88 ± 0.86c 87.94 ± 0.63a

42.05 ± 0.88b 0.41 ± 0.03b 16.54 ± 0.85b 7.13 ± 0.86c 6.91 ± 1.58bc 88.58 ± 0.12a

41.33 ± 1.12b 0.63 ± 0.09b 16.78 ± 0.23b 10.73 ± 2.16b 8.66 ± 1.13b 87.84 ± 0.32a

35.80 ± 1.42c 3.44 ± 0.39a 18.47 ± 0.60a 14.23 ± 0.92a 27.14 ± 4.41a 79.46 ± 1.11b

Note: In the same row, the different letter reveal significant differences (P < .05) according to the Duncan test.

drying, and the influences of drying temperature (60, 65, 70 and 75
C) on drying kinetics, shrinkage, rehydration ratio, microstructure and color parameters were investigated. Drying duration was shortened at about 5.0 h when the drying temperature was increased from 60 to 75
C. Drying temperatures also significantly affected the volume changes and quality attributes of lemon samples. Weibull model could well model the PVD drying kinetics of lemon slices. The effective moisture diffusivity (Deff) is presented in a non-linear trend when taking moisture content and shrinkage into consideration. The shrinkage ratio of samples increased with increasing drying temperature. All lemon slices obtained about the same volume shrinkage (about 78% of the original volume) for different temperatures at moisture content about 60% w.b, which closely related to the fixed microstructure. Page model adequately predicted the rehydration kinetics of dried lemon slices under the

rehydration temperature of 40 and 90
C. In terms of color attributes, color changed increased with increase of drying temperature. The current work indicated that pulsed vaccum drying is a promising technology for lemon slices and the findings contribute to better understanding of PVD drying process. Acknowledgements This research is supported by the National Key Research and Development Program of China (No. 2017YFD0400905); National Natural Science Foundation of China (No. 31772026, 31760471), the Project in the National Science & Technology Pillar Program during the Twelfth Five-year Plan Period (2015BAD19B010201), and the Chinese Transformation Fund of Agricultural Scientific and Technological Achievements (No. 2014GB2G410112).

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