Far-infrared radiation heating assisted pulsed vacuum drying (FIR-PVD) of wolfberry (Lycium barbarum L.): Effects on drying kinetics and quality attributes

Accepted Manuscript Title: Far-infrared radiation heating assisted pulsed vacuum drying (FIR-PVD) of Wolfberry (Lycium barbarum L.): effects on drying kinetics and quality attributes Authors: Long Xie, Arun S Mujumdar, Xiao-Ming Fang, Jun Wang, Jian-Wu Dai, Zhi-Long Du, Hong-Wei Xiao, Yanhong Liu, Zhen-Jiang Gao PII: DOI: Reference:

S0960-3085(17)30012-3 http://dx.doi.org/doi:10.1016/j.fbp.2017.01.012 FBP 835

To appear in:

Food and Bioproducts Processing

Received date: Revised date: Accepted date:

10-11-2016 30-12-2016 28-1-2017

Please cite this article as: Xie, Long, Mujumdar, Arun S, Fang, Xiao-Ming, Wang, Jun, Dai, Jian-Wu, Du, Zhi-Long, Xiao, Hong-Wei, Liu, Yanhong, Gao, Zhen-Jiang, Far-infrared radiation heating assisted pulsed vacuum drying (FIR-PVD) of Wolfberry (Lycium barbarum L.): effects on drying kinetics and quality attributes.Food and Bioproducts Processing http://dx.doi.org/10.1016/j.fbp.2017.01.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Far-infrared radiation heating assisted pulsed vacuum drying (FIR-PVD) of Wolfberry (Lyciumbarbarum L.): effects on drying kinetics and quality attributes

Running Title: FIR-PVD enhances drying kinetics and quality of wolfberry

Long Xiea, Arun S Mujumdarb, Xiao-Ming Fangc,Jun Wanga, Jian-Wu Daid, Zhi-Long Due, Hong-Wei Xiaoa*, Yanhong Liua, Zhen-Jiang Gaoa*

a

College of Engineering, China Agricultural University, 17 QinghuaDonglu, Beijing100083, China,

b

c

Department of Bioresource Engineering, McGill University, Ste. Anne de Bellevue, Quebec, Canada,

Institute of Bee Research, Chinese Academy of Agricultural Sciences, XiangshanBeigou, Beijing

100093, China, College of Mechanical and Electronic, Sichuan Agricultural University, Ya‘an, Sichuan625014, China,

d

e

Chinese Academy of Agricultural Mechanization Sciences, Beijing 100083, China

*

Corresponding authors. Tel/Fax.: +86 10 62736900; Fax: +86 10 62736978.

E-mail address: [email protected] (H.W. Xiao) [email protected](Z.J. Gao)

https://www.researchgate.net/profile/Hong_Wei_Xiao5

1

Graphical Abstract

(a) shade dried

(c)

(b) hot air dried at 65 °C

FIR-VPD dried at 65 °C

Highlights 

●FIR-PVD extensively enhances drying rate compared with hot air drying.



●The color of FIR-PVD samples was similar to that of the fresh ones.



●Weibull distribution model was used to calculate the moisture effective diffusivity.



●FIR-PVD generated porous and fissured microstructure, which enhance drying. 2

Abstract: The drying kinetics of wolfberry was investigated in a pulsed vacuum dryer using far-infrared radiation heating at different vacuum pressure durations (10, 15, and 20 min), ambient pressure durations (2, 4, and 6 min) and drying temperatures (60, 65, and 70 °C). The quality attributes such as colour, rehydration ratio, and microstructure of the dried products were also evaluated. Under appropriate conditions, FIR-PVD significantly lowers the drying time compared to hot air drying. The Weibull model was used to calculate the effective moisture diffusivity (Deff), which ranged from 3.72×10-10 to 7.31×10-10 m2/s. The activation energy was 54.30 kJ/mol determined by Arrhenius equation. The colour parameters of dried wolfberry dried by FIR-PVD were much similar to that of the fresh berries. The rehydration ratio of FIR-PVD dried wolfberry was 2.41 and 2.82 at drying temperatures of 70 and 60°C, respectively. Porous and fissured microstructure was observed on the surface of dried wolfberry, which can enhance drying kinetics as well as the rehydration process. By comprehensive considering the drying time and quality, drying temperature of 65 °C, vacuum pressure duration of 15 min and ambient pressure duration of 2 min were proposed as the most favourable drying conditions for wolfberry.

Keyword: Wolfberry; FIR-PVD; Weibull distribution model; Colour parameters; Microstructure

Nomenclature a*

Redness/greenness

PA

The ambient pressure

b*

Yellowness/blueness

PV

The vacuum pressure

C

Colour intensity

PDC

D0

Pre-exponential factor of Arrhenius equation (m2)

r

3

The pressure in drying chamber Volume equivalent radius (m)

d.b. Dry basis (kg water/kg dry matter)

R

Dcal Estimate moisture diffusivity (m2/s)

R2

D0

Constant diffusivity basis (m2/s)

Deff Effective diffusivity (m2/s) Ea

Universal gas constant (J/mol K)

Rg

Physical dimension constant (m2/s)

RR

Rehydration ratio

Activation energy (kJ/mol)

t

ΔE Total colour difference

td

ho

Hue angle

tAP

L*

Lightness

Drying time (s) Time to retort to the ambient pressure

ts

The treatment time at ambient pressure Time to reach the vacuum pressure

Me Equilibrium moisture content (kg water/kg dry matter) M0 Initial moisture content (kg water/kg dry matter) MR Moisture ratio (dimensionless) Mt

Correlation coefficient (dimensionless)

tVP

The duration at vacuum pressure

α Scale parameter of Weibull model (min)

β Shape parameter of Weibull model (dimensionless)

Moisture content at time t (kg water/kg dry matter)

T

Drying temperature (°C)

1. Introduction

Wolfberry (Lyciumbarbarum L.) is one type of herbal plant belonging to the Lyciumgenus of the Solanaceae family and is widely planted around the world. Wolfberry contains a variety of bioactive components, such as polysaccharides (Zhao et al., 2015a), flavonoids (Wu et al., 2015a), and carotenoids (Wang et al., 2010). It is one of the most popular herbs in China and is frequently consumed as a functional food to promote glucose and lipid metabolism (Luo et al., 2004; Zhao et al., 2015b), stimulate the immune system (Gan et al., 2004), increase mental efficiency (Zhao et al., 2015c), or even to prevent neuro degeneration (Ho et al., 2015). Like other berries, the fresh wolfberry fruit has high water content and its tender tissue is very 4

susceptible to mechanical damage and microbial infection. Drying is one of the most common processing methods for wolfberry preservation by reducing water activity to minimize the potential microbiological spoilage and deteriorative chemical reactions. Wolfberry has a specific epidermal structure covered by a thin layer of wax, which consists mainly of nonacosane and iodooctadecane (Yang et al., 2011). The epidermal wax inhibits moisture movement from the inside to the outside. For this reason, it is difficult to dry wolfberry. There is increasing interest in usage of chemical pre-treatment, such as alkaline emulsion (mainly containing Na2CO3 and Na2SO3) dips for several minutes to dissolve the wax on the epidermis of wolfberry to enhance water diffusion through the epidermal wax layer (Wu et al., 2015b; Li et al., 2014). However, the chemical additive residue in the wolfberry triggers food safety risk and dealing with the corrosive waste chemical solutions is a challenge involving additional cost. Conventional drying methods such as natural sun drying and hot air drying have been widely adopted for drying of wolfberry as they present advantages of low capital investment and simple operation. However, natural sun drying is susceptible to bad weather and microbial exposure to the open environment for a long time may cause insect infestation, dust contamination and spoilage of the product. Furthermore, it requires large open areas, high labour costs and long drying time usually up to 4-5 days which is undesirable economically (Wu et al., 2010). Disadvantages associated with hot air drying include: large shrinkage and darkening, nutrition and flavour’s degradation, and collapsed microstructure (Aprajeeta et al., 2015; Bai-Ngew et al., 2015; Mujumdar, 2004). Therefore, a safer, efficient and automated drying technique needs to be developed and applied to enhance the drying process and quality attributes of wolfberry on a commercial scale. Far-infrared radiation heating assisted pulsed vacuum drying (FIR-PVD) is a novel drying 5

technology which applies far-infrared radiation to heat materials under pulsed vacuum condition. Far-infrared radiation (FIR) is a heat transfer alternative for the purpose of energy savings, and better uniformity of temperature distribution as it generates energy that is absorbed directly by the material without loss to the environment (Pan et al., 2008a; Pan et al. 2008b). Fig. 1 presents a schematic representation of the FIR-PVD operation. The FIR-PVD processing involves alternating change of pressure in the drying chamber during the whole drying operation. In FIR-PVD, materials are placed below the far-infrared heating panel in the drying chamber. Air is expelled from the drying chamber to maintain a constant pressure level (Fig. 1a) and maintained for a predetermined time (Fig. 1b). And then the pressure is returned to ambient pressure (Fig. 1c) and held there for a certain time (Fig. 1d). This procedure varies according to the properties of the raw material and the operating conditions (e.g., drying temperature, vacuum amplitude, duration of vacuum pulse etc.). The PA and PV indicate the ambient and vacuum pressure in drying chamber. tAP and tVP are the duration of the ambient and the vacuum pressure pulses. ts and td are the time required for the alternation of PV and PA, respectively. FIR-PVD can enhance the drying rate as well as the dried products quality compared to hot air drying and natural open sun drying. The pulsed operations at the ambient and vacuum pressures increase the drying rate by enhancing the circulation of drying medium (air) in the chamber. When the air in the chamber is kept at a specific pressure for a period of time, the air layer directly contacting with the product surface may become saturated (Loncin et al., 1998). The successive change of pressure in the drying chamber disturbs this air layer leading to more effective removal of surface moisture. In FIR-PVD operations, materials are heated under reduced pressure (7-10 kPa) causing a reduction in the boiling points of the moisture in the foods. Less energy is required for the removal of moisture from the product surface due to lower phase-change temperature (Chua and Chou, 2004) 6

during the FIR-PVD process. Meanwhile, vacuum environment (an oxygen deficient processing environment) also prevents adverse biochemical reactions (e.g., oxidation deterioration, browning reactions) (Wu et al., 2007). Therefore, FIR-PVD improves the retention of natural colour and nutrients in bioproducts. The objectives of this experiment are: (1) to evaluate the effects of ambient pressure duration (2, 4, and 6 min), vacuum pressure duration (10, 15, and 20 min) and drying temperature (60, 65, and 70 °C) on FIR-PVD characteristics of wolfberry by comparing the drying kinetics; (2) to evaluate the quality attributes of dried wolfberry in terms of the colour parameters (L*, a*, b*, ΔE, C, ho), rehydration ratio, and microstructure under different drying conditions.

2. Materials and methods 2.1. Raw Material The fresh wolfberry samples were collected from Bairuiyuan wolfberry plantation, Yinchuan city, Ningxia Hui Autonomous Region in China (June 2015). The samples were checked carefully to remove defective fruit in to prevent contamination of the remainder by bacteria or fungi. To ensure uniformity of the physical characteristics of samples, wolfberries with the same size (average radius, length and weight were 4.5 + 0.5 mm, 18 + 2.0 mm, and 0.9 + 0.1 g, respectively) were selected. The initial moisture content of the fresh samples was 4.31-4.62 kg/kg (d.b.) or (81.68+0.53)% in wet basis, which was determined by vacuum drying at 70 °C for 24 h following the Association of Official Analytical Chemists (AOAC) method no.934.06 (AOAC, 1990). The total soluble solids content of wolfberry was 21 + 2 oBrix as measured with an Atago 0-32 °Brix temperature compensating refractometer (Atago Corporaion, Tokyo, Japan) following the AOAC method no. 932.12 (AOAC, 1980). The samples were packed in sealed plastic bags in a refrigerator maintained at 4 + 1 °C and 90% relative humidity for 7

about one day for equilibration of moisture prior to the experiments. Fresh wolfberry was difficult to store even under refrigerated conditions, so several batches of wolfberry were picked for the whole experimental design. To keep the uniform of relative test parameter effect on drying characteristic and avoid the water content and chemical component of various batches, each batch of wolfberry only was used for three experiments. 2.2. Experimental apparatus of FIR-PVD A schematic diagram of the pressure regulatory drying system with the typical pressure profile in the drying chamber is shownin Fig. 2. Three main units constitute the PVD equipment: (A) the vacuum system which mainly consists of a electromagnetic valve (8), vacuum pump (9), vacuum pipe (10), and vacuum chamber (12). (B) The heating system is composed of three far-infrared radiation heating panels (3). (C) The electronic control system that regulates the vacuum chamber pressure and temperature automatically according to the real time operational parameters such as the pressure, chamber temperature and relative humidity of the drying chamber. The single far-infrared radiation heating panel is rated at 250 W with a surface area of 50 × 50 mm2, which is used to supply thermal radiation to the drying sample and the drying medium. A vacuum pump (2BV4-2060, Bo Tong, Shanghai, China) is used to adjust the pressure within the chamber. The pressure in drying chamber was measured with CYYB-110 Pressure Transmitter (Shi Da Chuang Ye, Beijing, China). The SHT75 sensor (Sensirion, Shenzhen, China) was adopted to collect the thermocouple signals. A Proportional-Integral-Derivative (PID) controller (Omron, model E5CN, Tokyo, Japan) with accuracy of + 0.1 °C was used to control the temperature of heating panel and pressure in drying chamber. An electromagnetic valve isolates the pressure chamber from the ambient air and adjusts the airflow rate from the surrounding back into the chamber. The air flowing into the drying chamber through 8

electromagnetic valve is at temperature of 30-35 °C and relative humidity of 20-30% with the flow rate of 0-65 m/s. Touch screen (Weinview, Shenzhen, China) timely shows the pressure and thermocouple signals. The minimum pressure level that the system is capable of producing is 3.0 kPa or 0.03 bar and the time taken for the system to reach this minimum pressure from atmospheric pressure is approximately 60 s. 2.3. Experimental set-up and procedure Fresh wolfberry samples were taken from refrigerator and brought up to the room temperature (25 °C). The samples were spread in a single layer on a silica gel tray (300 mm × 200 mm × 10 mm) in the drying chamber. The mass of raw wolfberries was kept constant 150 + 5 g for all runs. The effects of three independent parameters, namely, ambient pressure duration (A), vacuum pressure duration (B) and drying temperature (C) on the drying characteristics and quality of wolfberry were investigated using the single factor experiment method as presented in Table 1. After the far-infrared radiation heating panel had reached steady state for the points, the samples were placed below the far-infrared heating panel in the drying chamber at ambient pressure. The distance between the wolfberry and the far-infrared radiation heating panel was 25 mm. Then the FIR-PVD operation was turned on and adjusted through the PID controller. The time of ambient pressure duration and vacuum pressure duration could be adjusted by the Proportional-Integral-Derivative (PID) controller according to different drying requirement. The weight loss was measured by an electronic balance (SP402, Ohaus Co., New Jersey, USA) with an accuracy of + 0.01 g at twice single cycle time of FIR-PVD intervals. Single cycle time (which contains one vacuum pressure duration and one ambient pressure duration) of FIR-PVD need be considered to determine the detail time of weight loss measurement. The drying experiments were 9

performed until the final moisture content of the samples decreased to 0.15 kg/kg (d.b.) from the initial value of 4.45 kg/kg (d.b.). The dried wolfberry samples were cooled to room temperature by exposure to ambient and then packed in heat-sealed low-density polyethylene bags. All experiments were performed in triplicate and the average values are used for analysis requested in this paper. The wolfberry dried by hot air at 65 °C and 15% relative humidity under ambient pressure with airflow rate of 1.8 m/s.The hot air dried wolfberry was used to compare the drying time, colour, rehydration ratio and surface microstructures with the FIR-PVD samples. The raw wolfberry was dried in shade at 20-25 °C with relative humidity of 25-30%. The surface microstructures of wolfberry dried in shade was compared with the FIR-PVD samples. 2.4. Drying kinetics and data analysis The moisture ratio (MR) of wolfberry during the thin-layer drying experiments is calculated using the following equation (Xiao et al., 2010b; Soysal et al., 2006; Doymaz, 2004):

MR=

Mt - Me M 0 - Me

(1)

Where Mt is moisture content at t of drying (kg/kg (d.b.)), M0 is the initial moisture content; Me is the equilibrium moisture content (kg/kg (d.b.)). The value of the equilibrium moisture content (Me) is relatively small compared to Mt or M0. Thus, Eq. (1) can be simplified as (Ju et al., 2016a; Xiao et al., 2015; Mundada et al., 2010)

MR 

Mt M0

(2)

Drying curves were fitted to the Weibull model (Eq. (3)) (Bantle et al., 2011; Bai et al., 2013b; Dai et al., 2015).

10

  t   MR  exp -     a   

(3)

Where MR is moisture ratio of wolfberry; t is the drying time; α is the shape parameter of Weibull model; β is the scale parameter of Weibull model. The value of α and β can be obtained by fitting the Weibull model to drying curves. 2.5. Determination of effective moisture diffusivity Effective moisture diffusivity (Deff) can be calculated with Weibull model as follows (Bai et al., 2013b; Dai et al., 2015):

Dcal r2 Deff =  Rg  Rg

(4)

Where Dcal is the estimate moisture diffusivity (m2/s); r is the volume equivalent diameters of wolfberry, with 9.0 × 10-3 m as its value; α is the scale parameter of Weibull model; Rg is the physical dimension constant. For agriculture products with a shape of flat or globular, Rg is usually in the rage of 13.1-18.6 m2/s (Marabi et al., 2003; Zhang et al., 2015). The value of Rg is 13.1 m2/s, which is used to calculate the value of Deff in this study. 2.6. Calculation of activation energy The effective moisture diffusivity can be related with temperature by Arrhenius-type relationship (Xiao et al., 2012a; Ju et al., 2016b; Mundada et al., 2010; Pathare & Sharma, 2006):

Deff  D0 exp[

Ea ] R(T  273.15)

In( Deff )  In( D0 ) 

(5)

Ea 1 R T  273.15

(6)

Where D0 is the effective moisture diffusivity at 273.15K, m2/s, the value of D0 is 0.1232 m2/s in current work; Ea is the active energy, kJ/mol; R is the universal gas constant with 8.314 J/mol·K as its 11

value; T is the drying temperature,°C. By taking the logarithm of both sides of Eq. (5), we can get Eq. (6). The activation energy (Ea) was determined from the slope of the Arrhenius plot about In (Deff) versus the reciprocal of the temperature 1/ (T + 273.15). 2.7. Colour measurements The colour attributes of fresh (as the reference value) and dried wolfberry were measured using a spectral photometer (SMY-2000ST, Shengmingyang Co., Beijing, China) in terms of the CIE LAB parameters L* (lightness), a*(redness/greenness) and b* (yellowness/blueness) following the methodology described by Xiao et al. (2012a) with slight modifications. The colour of wolfberry sample dried by hot air at 65 °C was compared with the FIR-PVD samples. The total colour difference (ΔE), chroma (C) and hue angle (ho) were calculated according to Eqs. (7) - (9) (Bai et al., 2013a; Dai et al., 2015; Wang et al., 2015b; Artnaseaw et al., 2010; Alibas, 2007). Each sample was taken in triplicates after crushing and the mean values were calculated.



  2

  2

E= L  L0  a  a0  b  b0

 a   b  2

C=



2

(7)

2

(8)

 b  ho =tan 1    a 

(9)

Where ΔE is the difference between the colours of fresh and dried wolfberry; L0*, a0*, and b0* were used as the reference values for fresh wolfberry; L*, a*, and b* represents the lightness, the redness/greenness, and the yellowness/blueness of the dried samples, respectively. 2.8. Measurement of rehydration ratio 12

The ability of dried wolfberry to recover to its original state when immersed in hot water is described by the rehydration ratio (RR). The rehydration tests were performed following the methodology described by Doymaz et al. (2014) and Artnaseaw et al. (2010) with some modifications. Five gram dried wolfberry sample was immersed in hot water at 75 °C for 30 minutes. Then the rehydrated samples were taken out from hot water and blotted with tissue paper to eliminate excess water on the surface. The rehydrated samples were weighed on an electric balance (SP402, Ohaus Co., New Jersey, USA) having the accuracy of + 0.01 g. All experiments were carried out in triplicate and the rehydration ratios (RR) was calculated with the equation as below (Xiao et al., 2010a). The rehydration ratio of wolfberry sample dried by hot air at 65 °C was compared with the FIR-PVD samples.

RR=

Weight of sample after rehydration (g) Weight of dry sample (g)

(10)

2.9 Determination of microstructure of dried samples using scanning electron microscopy (SEM) The surface microstructure of FIR-PVD samples was observed using scanning electron microscopy (SEM) (Borém et al., 2008) and compared with that of hot air dried ones. Prior to SEM, the samples were mounted on aluminium stubs with conductive adhesive. They were then coated with a thin gold film using an ion coater Eiko IB-3 (Eiko, Ibaragi) order to stabilize the structure. The samples were observed using SEM (FEI QUANTA 200, FEI, Netherlands), with an accelerating voltage of 15 kV. Duplicate specimens were observed at various magnifications, and the images of representative areas were saved for further analysis. 2.10. Statistical analysis Analysis of variance and nonlinear regression analysis were performed to determine the significance of the effect of drying temperature, ambient pressure duration and vacuum pressure duration on drying characteristic, colour parameters using SPSS statistical software (version 21.0, SPSS Inc., Chicago, IL, 13

USA). The least significant difference (LSD) test was used to analyse differences between the means. Statistical significance for differences was tested at 5% probability level (p< 0.05).

3. Results and discussion 3.1. Drying curves To explore the effects of different ambient pressure duration (2, 4, and 6 min), vacuum pressure duration (10, 15, and 20 min) and drying temperature (60, 65, and 70 °C) on the drying kinetics of wolfberry, the drying kinetic curves of wolfberry under different FIR-PVD conditions are presented in Fig. 3. The drying temperature, ambient pressure duration and vacuum pressure duration had a significant effect on the moisture content of the wolfberry as expected. Fig. 3a shows that the times taken to decrease the moisture content of wolfberry from an initial value of 4.45 kg/kg (d.b.) to final value of 0.15 kg/kg (d.b.) were 714, 398, and 514 min for the vacuum pressure durations of 10, 15, and 20 min, respectively. The shortest drying time was obtained at 15 min of vacuum pressure duration, which illustrates more vacuum pressure duration is not desirable for shortening the drying time. FIR-PVD is a various of intermittent drying to benefit from the period of tempering (Golmohammadi et al., 2012). Tempering, which allows time for moisture migration from the inside to the surface of the berry, may be obstructed by the long duration of vacuum pressure. Due to long duration of vacuum pressure is not benefit to break the balance of water vapour pressure, therefore, the drying time is longer cost by 20 min than 15 min for vacuum pressure duration. Fig. 3b showed that the drying time increased from 376 min to 724 min when the ambient pressure duration was increased from 2 min to 6 min with 65 °C and 15 min as the constant drying temperature and vacuum pressure duration, respectively. This indicates that decrease of ambient pressure duration can reduce drying time. This phenomenon can be attributed to the fact that the short ambient pressure 14

duration increases the frequency of pressure pulsations over the drying period, thus enhancing the change of water vapour pressure continuously. From Fig. 3c, it can be observed that the drying time increases when the drying temperature was lowered. The drying times are 386, 494, and 616 min for drying at temperatures of 70, 65, and 60 °C, respectively. The results confirmed that as increasing the drying temperature speeds up the drying kinetics. Similar results have been reported by Xiao et al. (2010b) for Monukka seedless grape and Moon et al. (2015) for potato. Traditional drying with coal-fired drying room (70 °C, 25% relative humidity) of wolfberry takes about 2040 min from the initial moisture content to the final 0.12 kg/kg (d.b.) (Wang et al., 2015c). The drying time cost by FIR-PVD is dramatically reduced by 81.08% compared to the traditional drying methods at 70 °C. 3.2. Effective moisture diffusivity The effective moisture diffusivity values for different FIR-PVD runs, calculated by Weibull distribution, Eq. (5) are shown in Table 2, Deff values varied from 3.72 × 10-10 to 7.31 × 10-10 m2/s under different FIR-PVD process conditions. The values of the Weibull scale parameter (α) ranged from 141.07 to 276.70 min and values of the shape parameter (β) varied from 1.12 to 1.44. This result shows that Deff values of wolfberry are significantly affected by vacuum pressure duration, ambient pressure duration and drying temperature. Table 2 shows that the Deff values firstly increased from 4.12 × 10-10 to 7.31 × 10-10 m2/s when the vacuum pressure duration was increased from 10 to 15 min and then it dropped from the highest point to 5.95 × 10-10 m2/s as the vacuum pressure duration was increased from 15 to 20 min with 4 min and 65 °C as the constant ambient pressure duration and drying temperature, respectively. With increase of the vacuum pressure duration, duration of the low boiling point zone in wolfberry is increased. This helps improve the moisture diffusivity at low boiling point of the moisture 15

at the same operating temperature. It can be also observed that with increase of the ambient pressure duration from 2 to 6 min, Deff values of decrease from 6.51×10-10 to 3.82×10-10 m2/s. The reduction of ambient pressure duration increases the single cycle number of FIR-PVD in the same time, which enhances the imbalance of vapour partial pressure between the surface and inside of wolfberry. Furthermore, Table 2 indicated that the Deff values increase with the drying temperature. The values of effective moisture diffusivity under different FIR-PVD process conditions and for other agricultural products were summarized in Table 3. Deff values obtained from current work varied in the range from 10-11 to 10-9 m2/s for drying of agriculture materials (Xiao et al., 2010b). These results confirm the fact that drying temperature has a positive effect on Deff with higher temperature yielding higher Deff. At higher temperature, activity of the water molecules is increased leading to higher moisture diffusivity. This is in agreement with the earlier research on the drying of various agriculture products such as blueberries (Shi et al., 2008), Monukka Seedless grapes (Xiao et al., 2010b), olive-waste cake (Uribe et al., 2014) and apple (Santacatalina et al., 2014). The higher Deff of FIR-PVD samples maybe due to the larger imbalance of vapour partial pressure generated by the pulsing pressure. Additionally, vacuum environment facilitates moisture diffusion since the reduced pressure decreases the boiling point of the liquid. The scale parameter (α) and shape parameter (β) are observed to be strongly dependent on vacuum pressure duration, ambient pressure duration and drying temperature effect. All values of the shape parameter (β) > 1 show that a lag existed at the beginning of drying process (Bai et al., 2013b). The results indicate that values of β decreased with increase of temperature. Similar observations were also reported by Zhang et al. (2015), who observed that the scale parameter (α) for poria cocos dried by different methods decreased with increasing the temperature. 16

3.3. Activation energy The value of activation energy (kJ/mol) for wolfberry drying was found to be 54.30kJ/mol. The activation energy of wolfberry was within the range 12.70-110.00 kJ/mol which is reported for most agricultural materials (Zogzas et al., 1996). In drying, activation energy is the threshold energy, or the energy barrier must be overcome to initiate mass diffusion from the wet material. Hence, a material with lower Ea value, indicates that moisture diffusion coefficient is more susceptible to temperature effect during drying. To compare the activation energy of wolfberry with other agricultural products, activation energy of various agricultural materials are presented in Table 4. Table 4 shows that wolfberry has higher activation energy than that for faba bean and squash seeds, but lower than that of blueberry, Monukka seedless grapes, Murta (Ugni molinae Turcz) berries and barberry fruit. This reflects the fact that activation energy is significantly influenced by chemical composition, characteristic surface area and tissue structures of the material (Shi et al., 2008; Chayjan et al., 2014; Wang et al., 2015a). Generally, agriculture products which have high content of carbohydrate, small evaporation area, and compact tissue structures display higher activation energy than the ones which have low content of carbohydrate, large evaporation area and loose tissue structure. Wolfberries have higher content of carbohydrate and specific wax structure on the surface peel than the faba bean and squash seeds, which results in higher activation energy than other agriculture products. To compare with blueberries and Monukka seedless grapes which belong to the berries mentioned in Table 4, wolfberry have lower activation energy. 3.4. Colour evaluation Colour is one of the key quality attributes for visually estimating the quality of dried wolfberry as colour can indicate even the contents of bioactive and thermal sensitive components (Garcia-Pascual et

17

al., 2006; Ban et al., 2015). Colour parameter values of the samples in terms of CIELAB parameters L*, a* and b*were measured and the results are displayed in Table 5. It can be observed that, compared to the fresh samples, the lightness (L*), redness/greenness (a*), and yellowness/blueness (b*) of all dried wolfberry are decreased significantly (p <0.05). The value of L*, a* and b* own by FIR-PVD wolfberry samples were higher than these of the samples dried by hot air. Colour values obtained for FIR-PVD were close to those of the fresh samples according to the less ΔE value. This is a result of vacuum operation and short drying time, which prevent the colour degradation caused by oxidation deterioration and loss of heat sensitive components. In addition, increasing the ambient pressure duration, drying temperature or decreasing the vacuum pressure duration resulted in a decrease in the L*, a* and b* values. This result might be related to the Maillard reaction and carotenoids degradation reactions, which produces dark substances (e.g., melanoidin), and deteriorates the colour at higher drying temperatures in aerobic environment. Similar results were reported by Ahmed et al. (2002), who observed that carotenoids degradation reaction of papaya puree increased when increasing the thermal treatment temperature from 70 to 105 °C. Regarding the effect of ambient pressure duration and vacuum pressure duration, it is noted that long vacuum pressure duration and short ambient pressure duration have significant positive effect on the changes of L*, a* and b* values in all cases. This phenomenon could be attributed to the fact that materials drying under vacuum condition can decrease oxidative degradation of pigment (e.g., astaxanthin, anthocyanin and carotenoids) in agriculture materials, which is positively correlated to the colour value (Cui et al., 2004; Wojdyło et al., 2014). Additionally, the effect of drying temperature and pressure condition in drying chamber on the change of L* is more predominant than a*and b*. 3.5. Rehydration ratio and microstructure 18

The rehydration ratio (RR) for dried wolfberry under different drying conditions is shown in Fig.4. Generally, the samples dried by FIR-PVD have higher rehydration ratio and shorter drying time compare with hot air dried sample at 65 °C. Meanwhile, the drying time of samples dried by FIR-PVD is significantly shorter than that for samples dried by hot air drying at 65 °C. As for the effect of vacuum pressure duration, Fig.4 (a) shows that RR of dried samples increased significantly from 2.53 to 2.82 with increasing vacuum pressure duration from 10 to 20 min (p<0.05). This result may beascribed to the vacuum condition which can contribute to the inducement of greater internal stresses and creation of more micro-fissures pores during drying (Kiranoudis, Tsami, & Maroulis, 1997). Fig. 4 (b) illustrates that RR of dried samples decreased from about 2.66 to 2.58 when ambient pressure duration was increased from 2 to 6 min, though the affect is not significant (p>0.05). The phenomenon might be caused by the ambient pressure duration (2 to 6 min) which is a very short time compared with vacuum pressure duration (10 to 20 min) during single cycle of FIR-PVD, this affect inappreciably on the change of microstructure. From Fig. 4 (c), it is noted that drying temperatures has significant effects on RR of dried wolfberry; RR decreased from 2.76 to 2.41 when the drying temperature was increased from 60 to 70 °C. Rehydration ability indicates microstructure damage which occurred during drying e.g., internal collapse (Al-Khuseibi et al., 2005). High drying temperatures can cause case hardening on the surface or the internal collapsed structure, which can hinder water penetration during the rehydration process since the microstructure of the product determines its macroscopic properties (Xiao & Gao, 2012). This phenomenon is in agreement with that reported by Xiao et al. (2015), who observed that rehydration ability of ginseng slices dried at 35 °C was significantly higher than that dried at 65 °C. The surface microstructures of wolfberry dried in shade, under FIR-PVD and hot air were observed 19

using a scanning electron microscope (SEM), as shown in Fig. 5. From Fig. 5a, it can be found that the epidermis of wolfberry dried in shade consisted of distinct cell structure and some fragments of wax layer. From Fig. 5b, a smooth epidermis with wax layer was found in the samples dried in hot air at 65 °C. In the case of FIR-PVD samples (dried at 65°C, with vacuum pressure duration of 15 min and ambient pressure duration of 2 min) a more fissured microstructure was observed, as shown in Fig. 5c. Meanwhile, from Fig. 5d, a more porous microstructure was shown. Generally, both FIR-PVD and hot air drying resulted in damage of the original cell structure. The numbers of pores in epidermis of samples dried by FIR-PVD were obviously greater than these for samples dried by hot air drying and in shade. In general, the microstructure of the biomaterial determines its macroscopic properties, such as drying characteristics and rehydration capacity (Xiao et al. 2012b). Therefore, the wolfberry dried by FIR-PVD with more fissured and porous microstructure (Fig. 5c-5d) facilitated drying and yielded higher rehydration ratio (Fig. 4) than the samples dried by hot air dying. This result is consistent with the previous reports of Albitar et al. (2011), who dried onion using the instant controlled pressure drop technology and Mounir (2015) for chicken breast meat drying intensification with instant controlled pressure drop technology. 4. Conclusion The effects of vacuum pressure duration, ambient pressure duration and drying temperature on the drying kinetics and quality of wolfberry were examined in this investigation in thin-layer FIR-PVD.The drying curves indicate that vacuum pressure duration, ambient pressure duration and drying temperature have significant effects on the drying time. The Deff values for wolfberry dried by FIR-PVD during various drying conditions ranged from 3.72×10-10 to 7.31×10-10 m2/s calculated using the Weibull model. The Ea of wolfberry was determined as 54.30 kJ/mol. The colour parameters of 20

FIR-PVD samples were close to that of the raw material. The rehydration ratio of dried wolfberry showed a decreasing trend as the drying temperature increased and vacuum pressure decreased, but no significant relationship was found between ambient pressure duration and rehydration ratio.The surface microstructures of dried wolfberry showed that the FIR-PVD could form porous and fissured structures on the surface of wolfberry, which can facilitate moisture transfer during drying or rehydration process. By consideration of considering the drying time and quality, drying temperature of 65 °C, vacuum pressure duration of 15 min and ambient pressure duration of 2 min were proposed as the most favourable drying conditions for wolfberry. The findings in current workindicate that FIR-PVD is a very promising drying technology for wolfberry drying without chemical pretreatment, which could not only enhance the drying process but also improve the quality of dried products compared with hot air drying.

Acknowledgements

This work was supported by the Project in the National Science & Technology Pillar Program during the Twelfth Five-year Plan Period (2015BAD19B010201), the National Natural Science Foundation of China (No.31501548), and the Chinese Transformation Fund of Agricultural Scientific and Technological Achievements (No. 2014GB2G410112). Moreover, Dr. Long Xie thanks China Scholarship Council (File No. 201506350206) for supporting his overseas research in Washington State University. References Albitar, N., Mounir, S., Besombes, C., Allaf, K., 2011. Improving the drying of onion using the instant controlled pressure drop technology. Drying Technology 29(9), 993-1001. Artnaseaw, A., Theerakulpisut, S.,Benjapiyaporn, C., 2010. Development of a vacuum heat pump dryer 21

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29

Figure captions

Fig.1. Schematic diagram of the FIR-PVD: air pressure (Pa) vs. time (minutes). Fig.2. Schematic diagram equipment used for FIR-PVD. Fig.3. Drying kinetics of wolfberry dried under different vacuum pressure duration at same ambient pressure duration of 4 min and temperature of 65 °C (a), different ambient pressure duration, at same vacuum pressure duration of 15 min and temperature of 65 °C (b) and different drying temperature at same vacuum pressure duration of 15 min and ambient pressure duration of 2 min (c). Fig.4. Rehydration ratio of wolfberry dried under different vacuum pressure duration at same ambient pressure duration of 4 min and temperature of 65 °C (a), different ambient pressure duration, at same vacuum pressure duration of 15 min and temperature of 65 °C (b) and different drying temperature at same vacuum pressure duration of 15 min and ambient pressure duration of 2 min (c). Fig.5. The surface microstructure of wolfberry dried in shade (a), under hot air at 65 °C (b) and under FIR-PVD with 15 min of vacuum pressure duration, 2 min of ambient pressure duration and 65 °C of drying temperature (c-d).

30

Fig.1. Schematic diagram of the FIR-PVD: air pressure (Pa) vs. time (minutes).

31

Fig.2. Schematic diagram equipment used for FIR-PVD. 1. touch screen 2. detecting sensors of drying chamber 3.far-infrared radiation heating panel 4. far-infrared radiation temperature sensor 5. drying tray 6. materials 7. support frame 8. electromagnetic valve 9. water ring vacuum pump 10. vacuum pipe 11. materials’ temperature sensor 12. drying chamber 13.date and electric wire 14.adjusting switch 15. indicator light

32

(a)

(b)

(c) Fig.3. Drying kinetics of wolfberry dried under different vacuum pressure duration at same ambient pressure duration of 4 min and temperature of 65 °C (a), different ambient pressure duration, at same vacuum pressure duration of 15 min and temperature of 65 °C (b) and different drying temperature at same vacuum pressure duration of 15 min and ambient pressure duration of 2 min (c). 33

(a)

(b)

(c) Fig.4. Rehydration ratio of wolfberry dried under different vacuum pressure duration at same ambient pressure duration of 4 min and temperature of 65 °C (a), different ambient pressure duration, at same vacuum pressure duration of 15 min and temperature of 65 °C (b), and different drying temperature at same vacuum pressure duration of 15 min and ambient pressure duration of 2 min (c). 34

(a)

(b)

(c)

(d)

Fig.5.The surface microstructure of wolfberry dried in shade (a), under hot air at 65 °C (b) and under FIR-PVD with 15 min of vacuum pressure duration, 2 min of ambient pressure duration and 65 °C of drying temperature (c-d).

35

Table 1 Experiments were scheduled and analysed by single factor experiment design Experiments

Drying

Vacuum pressure

Ambient pressure

Drying

number

temperature (°C)

duration tVP(min)

duration tAP(min)

time(min)

1

65

10

4

714

2

65

15

4

494

3

65

20

4

386

4

65

15

2

376

5

65

15

4

535

6

65

15

6

724

7

60

15

2

616

8

65

15

2

494

9

70

15

2

386

Table 2 Different drying conditions on moisture effective diffusion coefficients of wolfberry Experiments

Parameters

Statistical tests

conditions

α(min)

β(dimensionless)

R2

Х2 (× 10-5)

RMSE

(× 10-10m2s-1)

10:4 65°C

249.88

1.2576

0.9996

4.640

0.0064

4.12

15:4 65°C

141.07

1.3051

0.9987

17.309

0.0119

7.31

20:4 65°C

173.10

1.3356

0.9991

12.614

0.0102

5.95

15:2 65°C

158.19

1.4366

0.9990

13.776

0.0106

6.51

15:4 65°C

186.91

1.3560

0.9985

18.458

0.0126

5.51

15:6 65°C

270.03

1.3328

0.9995

5.305

0.0068

3.82

15:2 60°C

276.71

1.2699

0.9983

17.765

0.0125

3.72

15:2 65°C

204.88

1.2222

0.9996

4.5654

0.0063

5.03

15:2 70°C

156.26

1.1180

0.9946

61.551

0.0224

6.60

Deff

Note: The experiments conditions of different FIR-PVD shown vacuum pressure: ambient pressure duration and drying temperature in

sequence.

36

Table 3 Moisture effective diffusivity values of wolfberry and other products. Products

Drying Temperature (°C)

Deff (× 10-10 m2 s-1)

References

Wolfberry

60-70

3.72-7.31

Present work

Blueberry

60-90

2.24-16.4

Shi et al. (2008)

Monukka seedless grapes

50-65

1.82-5.85

Xiao et al. (2010b)

Sweet cherry

60-75

5.68-15.44

Doymaz et al. (2011)

Squash seeds

50-80

0.5-1.6

Chayjan et al. (2013)

Olive-waste cake

40-90

19.70-60.50

Uribe et al. (2014)

Unpeeled figs

45-65

3.97-7.52

Xanthopoulos et al. (2010)

Peeled figs

45-65

5.54-7.80

Xanthopoulos et al. (2010)

Table 4 Activation energies of wolfberry and other related products Products

Ea (kJ mol-1)

References

Wolfberry

54.30

Present work

Blueberries(O’Neil, Vaccinium)

57.85

López et al. (2010)

Wild blueberries

60.40

Martynenko et al. (2015)

Monukka seedless grapes

67.29

Xiao et al. (2010b)

Murta (Ugni molinae Turcz) Berries

79.40-90.23

Puente-Díaz et al. (2013)

Barberry fruit

110.84-130.61

Aghbashlo et al. (2008)

Faba bean

21.68-27.42

Chayjan et al. (2014)

Squash seeds

31.94-34.49

Chayjan et al. (2013)

Table 5 Colour parameters L*, a*, b*, ΔE, C, and ho of fresh and wolfberry dried by FIR-PVD Run No.

L*

a*

b*

ΔE

C

ho

Fresh

50.32+0.02a

48.23+0.57a

48.1+0.21a

-

68.12+0.56a

44.92+0.21b

(1) 10:4 65°C

42.23+0.23d

36.43+0.71d

35.33+0.35d

19.18+0.26d

50.75+0.74e

44.12+0.26b

(2) 15:4 65°C

44.93+0.06c

45.61+0.11b

40.25+0.39c

9.88+0.72b

60.83 +0.34c

41.43+0.22c

(3) 20:4 65°C

47.18+0.21b

46.22+0.55b

43.13+0.31b

6.21+0.05a

63.22+0.61b

43.02+0.13b

(4) 15:2 65°C

42.45+0.43d

44.67+0.70b

42.25+0.65b

10.43+0.02b

61.49+0.06c

43.41+0.89b

(5) 15:4 65°C

39.39+0.63e

41.89+0.37c

45.32+0.08b

12.94+0.33c

61.71+0.20c

47.25+0.30a

(6) 15:6 65°C

34.76+0.25f

39.77+0.25c

39.96+0.16c

19.49+0.22d

56.38+0.06d

45.14+0.30b

b

44.00+0.71b

(7) 15:2 60°C

47.10+0.13b

45.79+0.11b

44.22+0.18b

5.60+0.29a

63.66+0.89

(8) 15:2 65°C

45.28+0.21c

44.93+0.23b

41.45+0.39c

8.97+0.10b

61.13+0.44c

42.69+0.19b

(9) 15:2 70°C

39.97+0.49e

39.48+0.05c

33.12+0.10e

20.20+0.73d

51.53+0.03e

39.99+0.32c

65 oC hot air

28.92+0.09g

37.31+0.28d

26.51+0.01f

32.30+0.39e

45.77+0.22f

35.40+0.22d

Note: the letters reveal significant differences (p<0.05) according to the Duncan test.

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