Impact of different pretreatment methods on drying characteristics and microstructure of goji berry under electrohydrodynamic (EHD) drying process

Journal Pre-proof Impact of different pretreatment methods on drying characteristics and microstructure of goji berry under electrohydrodynamic (EHD) drying process

Jiabao Ni, Changjiang Ding, Yaming Zhang, Zhiqing Song PII:

S1466-8564(19)31189-0

DOI:

https://doi.org/10.1016/j.ifset.2020.102318

Reference:

INNFOO 102318

To appear in:

Innovative Food Science and Emerging Technologies

Received date:

18 September 2019

Revised date:

23 February 2020

Accepted date:

23 February 2020

Please cite this article as: J. Ni, C. Ding, Y. Zhang, et al., Impact of different pretreatment methods on drying characteristics and microstructure of goji berry under electrohydrodynamic (EHD) drying process, Innovative Food Science and Emerging Technologies(2018), https://doi.org/10.1016/j.ifset.2020.102318

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© 2018 Published by Elsevier.

Journal Pre-proof

Impact

of

different

characteristics

and

pretreatment

microstructure

methods of

goji

on

drying

berry

under

Electrohydrodynamic (EHD) drying process Jiabao Nia, Changjiang Dinga,*, Yaming Zhanga, Zhiqing Songa a

College of Science, Inner Mongolia University of Technology, Hohhot, China

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Abstract The aim is to find better pretreatment method to improve the drying speed of goji berry under

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electrohydrodynamic (EHD) drying process. The drying characteristics and microstructure of goji berry using different pretreatment methods under EHD drying process were investigated. The results showed that the drying rate after pretreatment was significantly higher than those of

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non-pretreated goji berry, and the different pretreatment methods had different effects on the drying characteristics of goji berry in an EHD drying system. The effects of different pretreatment

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methods on effective moisture diffusion coefficient were listed in descending order as follows: KOH > NaOH > Na2CO3 > ultrasonic > sucrose ester. The pretreatment has a great influence on

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the rehydration rate, specific energy consumption and microstructure of goji berry dried by EHD. Two index sequence analysis of infrared spectroscopy during goji berries drying process were

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established. The difference among five pretreatment methods were analyzed. Keywords: Pretreatment methods, Goji berry, EHD drying, Drying characteristics, Microstructure

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1. Introduction

Goji berry is a deciduous shrub that grows in many regions of China and has great medicinal and edible value (Amagase, Sun, & Borek, 2009; Zhao, Wei, Hao, Han, Ding, Yang, & Zhang, 2019). Goji berry has very high nutritional content, such as polysaccharides, flavonoids, a variety of amino acids and trace elements that are beneficial to the body, and has many medicinal values such as enhancing human immunity, lowering blood sugar, lowering blood fat, anti-tumor, and protecting liver (Chang & So, 2008; Donno, Mellano, Raimondo, Cerutti, Prgomet, & Beccaro, 2016; Jeszka-Skowron, Zgoła-Grześkowiak, Stanisz, & Waśkiewicz, 2017). However, fresh goji berry fruit is very perishable. The shelf life of fresh goji berry at room temperature is only about 3 days. Therefore, drying in time remains the nutrients of goji berry fruit and store for a long time. At present, the main drying techniques of goji berry include hot air drying (Fratianni, Niro, Alam, Cinquanta, Matteo, Adiletta, & Panfili, 2018), vacuum freeze drying (Donno, Mellano, Raimondo, Cerutti, Prgomet, & Beccaro, 2016), microwave drying (Liu, Wang, Zhu, Zhao, Ge, Zhao, & He, 2017), and far infrared *

Corresponding author. E-mail address: [email protected] (C. Ding).

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Journal Pre-proof drying (Xie, Mujumdar, Fang, Wang, Dai, Du, Xiao, Liu, & Gao, 2017). Although these drying techniques have many advantages, there are still many drawbacks. The hot air drying easily destroy the nutrients of goji berry. The microwave drying easily causes uneven drying of materials. The vacuum freeze drying and far infrared drying equipment are expensive and costly, and often limited in industrial application. Therefore, it is necessary to develop new drying technologies for goji berries. Electrohydrodynamic (EHD) drying technology is a new non-thermal drying technology of food, which has the advantages of very fast drying speed, no damage to the nutrients, sterilization, energy saving, and it is becoming a research hotspot (Dinani, Hamdami, Shahedi, & Havet, 2015; Defraeye & Martynenko, 2018; Dolati, Amanifard, & Deylami, 2018; Kudra & Martynenko, 2020). Li, Sun, & Tatsumi (2006) found that the drying time of okara cake using the

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electrohydrodynamic (EHD) drying was reduced by 15-40% compared with the control. Esehaghbeygi & Basiry (2011) found that the electrohydrodynamically dried tomato slices had

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higher shrinkage and better color than of non-treated and hot air dried samples. Bai, Qu, Luan, Li, & Yang (2013) found that the energy consumption of sea cucumber using electrohydrodynamic

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(EHD) drying was only 21.31% of that of the hot air drying and the protein content was higher than that of the hot air drying. Dinani, Havet, Hamdami, & Shahedi (2014) found that the effective moisture diffusion coefficient of the mushroom using the electrohydrodynamic (EHD) drying

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combined with the hot air drying was 2.3 times higher than that of the single hot air drying. The waxy layer of goji berry fruit is a part of the cuticle. The waxy composition of goji berry

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peel is complex, mainly composed of a mixture of organic substances, mainly consisting of long-chain fatty acid aldehydes and alkanes. During the drying process, the waxy layer will block the moisture removal from the material, thus affecting the drying effect. In Inner Mongolia, China,

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the traditional drying of goji berries is the Na2CO3 solution pretreatment combined with solar drying. But there are some disadvantages, such as long drying time, destruction of nutrients, etc.

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In the early stage, we studied the drying characteristics of electrohydrodynamically dried goji berry and found that electrohydrodynamic (EHD) drying improved the drying rate and quality of goji berry, and achieved satisfactory results (Ni, Ding, Zhang, Song, Hu, & Hao, 2019). Due to the influence of the wax layer on the surface of the goji berry, the effect of the electrohydrodynamic (EHD) drying technology is greatly reduced. Some researchers pretreated the goji berry fruit before drying to destroy the wax layer and improve the drying speed, and achieved good results (Adiletta, Alam, Cinquanta, Russo, Albanese, & Matteo, 2015; Dermesonlouoglou, Chalkia, & Taoukis, 2018; Fratianni, Niro, Alam, Cinquanta, Matteo, Adiletta, & Panfili, 2018). In this paper, to study the effect of these methods on the drying characteristics of electrohydrodynamic (EHD) drying goji berry, several methods such as sucrose ester, ultrasonic, Na2CO3 solution, NaOH solution and KOH solution were used to pretreat goji berry fruits. The drying rate, moisture ratio, effective moisture diffusion coefficient and unit energy consumption of goji berry during EHD drying process were measured. The effects of different pretreatment methods on the microstructure of goji berry during EHD drying process were studied by means of scanning electron microscopy and infrared spectroscopy. This work provides a facile and effective strategy for experimentally and theoretically determining the drying properties of goji berry, and, as a

Journal Pre-proof result, it provides deeper insight into the drying mechanism of pretreatment methods combined with EHD drying technology.

2. Materials and methods 2.1 Experimental equipment The ultrasonic pretreatment system and the electrohydrodynamic (EHD) drying experiment device are shown in Fig. 1. The electrohydrodynamic (EHD) drying experimental device is mainly composed of a high voltage power supply (YD (JZ)-1.5/50, Wuhan), a controller (KZX-1.5KVA, Wuhan) and a multi-needle-to-plate electrode system. The needle electrode is connected to a high voltage power supply. The length and diameter of each needle are 60mm and 1mm, respectively,

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and the distance between the neighbouring needles is 4cm. The ground electrode is a 100cm×45cm stainless steel plate. The distance between the needlepoint and the ground electrode

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is 10cm. In order to facilitate the setting of parameters required for electrohydrodynamic (EHD) drying, the high voltage power supply is connected to a controller, with an adjustable voltage

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ranging from 0–50 kV for alternating current (AC). The microammeter is connected between the ground electrode and the ground wire to monitor the current change during the drying process. The ultrasonic pretreatment system is mainly composed of an ultrasonic generator, a processing

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chamber and a control system. The power range and the water temperature control range of ultrasonic generator is 0W–300W and 20°C–80°C, respectively. Samples are put into water for

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pretreatment. So, pretreatment temperature of the samples is the water temperature.

(a)

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9 15

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(b)

Fig. 1 Schematic diagram of EHD drying and ultrasonic pretreatment system

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(a) Schematic diagram of EHD drying; (b) Ultrasonic pretreatment system. 1. Hygrometer 2. Thermometer 3. Sample 4. Ground electrode 5. Needle electrode 6. High voltage power supply 7. Control system 8. Microammeter 9. Purified water 10. Ultrasonic

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generator 11. Power regulator 12. Time regulator 13. Water temperature regulator 14. Sample

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15. Ultrasonic. 2.2 Determination of initial moisture content

Goji berries (Lycium barbarum L.) were purchased from local growers in the county of

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Tuoketuo, Hohhot, Inner Mongolia, and immediately placed in the 4°C refrigerator for experimental needs. The initial moisture content of fresh goji berry was measured by the moisture

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rapid tester (Sh10A, Precision Science Instrument Co., Ltd., Shanghai, China). After three independent measurements, the average value of initial water content was obtained. The initial water content of fresh goji berry fruits was 78 ± 1%. 2.3 Experimental method

Fresh goji berry fruits of uniform size and ripeness were selected from the refrigerator and placed in the ultrasonic processor, the sucrose ester solution, the Na2CO3 solution, the NaOH solution, and the KOH solution, respectively, for pretreatment. The traditional pretreatment of goji berry is the Na2CO3 solution. The experiment was independently repeated three times and averaged. Experimental conditions of the pretreatment group can be seen in Table 1. Table 1 The pretreatment types and pretreatment methods of goji berry Pretreatment types

Pretreatment methods

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Ultrasonic

The ultrasonic power, treatment time and water temperature are 300W, 30min and 35°C, respectively. The alcohol and sucrose ester (SE) were formulated into a solution at a weight ratio of 80% to 20%. Then, the above

Sucrose ester solution

solution was mixed with sodium ascorbate and distilled water in a weight ratio of 5%, 1% and 94%. The fresh goji berry fruits were placed in the mixture for 10 minutes at 38 °C. The 5% Na2CO3 solution was prepared. Then Na2CO3 solution

Na2CO3 solution

was mixed with sodium ascorbate and distilled water in a weight ratio of 90%, 1%, 9%. The goji berry fruits were soaked in the

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mixture for 10 min at 38 °C.

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The NaOH solution (50% NaOH + 50% distilled water), alcohol and vegetable oil are arranged into a saponification solution at a weight ratio of 10%, 20%, 70%, and placed in the environment for 4 minutes at 20 °C. Then, the saponification solution was

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NaOH solution

mixed with K2CO3, sodium ascorbate and distilled water in a

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weight ratio of 2%, 4%, 1% and 93%. The goji berry fruits were immersed in the mixture for 10 minutes at 38 °C.

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The KOH solution (50% KOH + 50% distilled water), alcohol and vegetable oil are arranged into a saponification solution at a KOH solution

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weight ratio of 10%, 20%, 70%, and placed in the environment for 4 minutes at 20 °C. Then, the saponification solution was

mixed with K2CO3, sodium ascorbate and distilled water in a

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weight ratio of 2%, 4%, 1% and 93%. The goji berry fruits were

The control

immersed in the mixture for 10 minutes at 38 °C. The goji berry without the pretreatment under

electrohydrodynamic (EHD) drying process was used as the control.

After pretreatment, the goji berry fresh fruits were placed in an electrohydrodynamic (EHD) drying system for drying experiments. The experimental conditions of electrohydrodynamic (EHD) drying were as follows: the electrode distance, the needle spacing and voltage are 10cm, 4cm and 30kV, respectively. The drying temperature, relative humidity and ambient wind speed were 25±2°C, 30±2%, and 0m/s, respectively. During the electrohydrodynamic (EHD) drying experiment process, the electronic balance (BS124S, Sartorius Scientific Instrument Co., Ltd., Beijing, China) was used to record the mass of goji berry every 2 hours. The drying rate and water content of the goji berry at different times were calculated according to the formula. Each experiment was repeated 3 times independently, and the results were expressed as mean ± standard deviation (SD). 2.4 Determination of moisture content

Journal Pre-proof The moisture content and moisture ratio of goji berry fruits during drying are defined as (Yu, Bai, Yang, & Wang, 2018) : 𝑀𝑖 =

𝑚𝑖 −𝑚𝑔 𝑚𝑔

× 100%

(1)

𝑀 −𝑀

MR = 𝑀 𝑖 −𝑀𝑒 0

(2)

𝑒

where 𝑚𝑔 is the dry mass of goji berry fruits; 𝑚𝑖 is the mass of goji fruits at time i; 𝑀0 is the moisture content of the goji berry at time 0 hours; 𝑀𝑖 is the moisture content of goji berry fruits at time i; 𝑀𝑒 is the equilibrium moisture content of the goji berry fruit; MR is the moisture ratio of goji berry fruits.

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2.5 Determination of drying rate The formula for the drying rate of goji berry fruits is (Yu, Bai, Yang, & Wang, 2018) : 𝑀𝑡 −𝑀𝑡+𝛥𝑡

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DR =

𝛥𝑡

(3)

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where DR is the drying rate, 𝑀𝑡 is the moisture content of goji berry fruits at time t, 𝑀𝑡+𝛥𝑡 is the moisture content of goji fruits at time t+Δt.

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2.6 Determination of rehydration rate

Measurement method of rehydration rate is described by Esehaghbeygi, Pirnazari, & Sadeghi

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(2014) with some modifications. The dried goji berry products were immersed in a constant temperature water bath at 37°C for 7 hours, then the goji berry fruits were removed and the surface moisture of the goji berry was completely blotted with filter paper. The mass of the goji

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berry before and after rehydration was measured with a Sartorius BS124S electronic balance. The formula for calculating the rehydration rate of goji berry fruits is:

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RR =

𝑚𝑎

(4)

𝑚𝑏

where RR is the rehydration rate of goji berry fruit, 𝑚𝑎 is the mass of the goji berry after rehydration, 𝑚𝑏 is the mass of the goji berry before rehydration. 2.7 Determination of shrinkage rate The shrinkage rate of goji berry was measured by using toluene method described by Elmizadeh, Shahedi, & Hamdami (2018). The formula for calculating the shrinkage rate is as follows: SR =

𝑉0 −𝑉𝑓 𝑉0

× 100%

(5)

where SR is the shrinkage rate of goji berry fruits, 𝑉0 is the volume of fresh goji berry fruits, 𝑉𝑓 is the volume of dried goji berry fruits. 2.8 Determination of the effective moisture diffusion coefficient The effective moisture diffusion coefficient during the drying process of fresh goji berry fruit is calculated by Fick's second law. The expression is (Ding, Lu, & Song, 2015) :

Journal Pre-proof 𝑑𝑀 𝑑𝑡

= 𝐷𝑒𝑓𝑓

𝑑2 𝑀

(6)

𝑑𝑟 2

for a long drying process, MR < 0.6, the equation can be expressed as: 8

MR = 𝜋2 𝑒𝑥𝑝( −

𝜋2 𝐷𝑒𝑓𝑓 𝑡 4𝐿2

)

(7)

where Deff is the effective moisture diffusion coefficient of the goji berry fruits and L is the thickness of the goji berry fruits. 2.9 Determination of the specific energy consumption The specific energy consumption (SEC) is the energy required to evaporate 1 kg of water 𝑉×𝐼

SEC𝐸𝐻𝐷 = 𝑚

0 −𝑚𝑖

× ∆𝑡

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inside the goji berry fruit. The expression is as follows (Martynenko & Zheng, 2016) : (8)

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where SEC𝐸𝐻𝐷 is the energy required to evaporate 1 kg of water inside the goji berry fruit by EHD drying. 𝑉 is the voltage and 𝐼 is the current. 𝑚0 is the initial mass of goji berry fruit, 𝑚𝑖

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is the mass when the moisture content of goji berry achieves 10%. ∆𝑡 is the drying time of goji berry fruits in the EHD system.

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2.10 Determination of infrared spectrum and two index sequence analysis The dried goji berry fruit product was pulverized and mixed with potassium bromide, after

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sieving. It was placed in a tableting machine (HY-12, Tianguang Spectrometer Co., Ltd., Tianjin, China) to form a tablet. The sample was scanned with a Fourier transform infrared spectrometer (Nicolet iS10, Thermo Nicolet Corporation, New York, New York State, USA) to remove the

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interference of water and carbon dioxide, thereby obtaining a scanning spectrum. According to the infrared spectrum, the common peak ratio and the variant peak ratio of the

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infrared fingerprint can be analyzed (Zhou, Zhang, Luo, Li, Song, & Zhang, 2014). For a group of absorption peaks, if the maximum difference of wave number of absorption peaks in a group is significantly smaller than the average difference of wave number of adjacent absorption peaks, it is determined that the group of peaks is a set of common peaks. The formula for calculating the common peak ratio is as follows: 𝑃 = (𝑁𝑔 / 𝑁𝑑 ) × 100% 𝑁𝑑 = 𝑁𝑔 + 𝑛𝑎 + 𝑛𝑏

(9) (10)

where P is the common peak rate and 𝑁𝑔 is the number of absorption peaks that appear in both infrared spectrum figures being compared, 𝑁𝑑 is the number of independent peaks that appear in the two infrared spectrum figures that are compared with each other, 𝑛𝑎 is the number of non-common peaks corresponding to the common peak in the fingerprint a, called the variation of a, 𝑛𝑏 is the number of non-common peaks corresponding to the common peak in the fingerprint b, called the variation of b. The formula for calculating the variant peak ratio is as follows: 𝑃𝑣𝑎 = (𝑛𝑎 / 𝑁𝑔 ) × 100%

(11)

𝑃𝑣𝑏 = (𝑛𝑏 / 𝑁𝑔 ) × 100%

(12)

Journal Pre-proof 𝑁𝑎 = 𝑁𝑔 + 𝑛𝑎

(13)

𝑁𝑏 = 𝑁𝑔 + 𝑛𝑏

(14)

where 𝑃𝑣𝑎 is the variant peak ratio of the fingerprint a, 𝑃𝑣𝑏 is the variant peak ratio of the fingerprint map b, 𝑁𝑎 is the peak number of fingerprint a, 𝑁𝑏 is the peak number of fingerprint b. Based on the calculation formula of the common peak rate and the variation peak rate of the fingerprint, and using each sample as a reference, we can calculate the common peak ratio and the variation peak ratio of the infrared fingerprints of other samples. Then a sequence is formed according to the values of the common peak ratio (including the values of the common peak ratio and the variation peak ratio), which is called the double-index sequence of the common peak ratio

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and the variation peak ratio. Through this sequence, the difference between different pretreatment methods can be accurately analyzed.

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2.11 Scanning electron microscopy

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To observe the influence and change of each treatment on the surface microstructure of goji berry, the dried goji berry products were sprayed with gold, scanned and photographed by

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scanning electron microscope (S3400, Hitachi corporation, Tokyo, Japan), then compared in turn. 2.12 Statistical analysis

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One-way analysis of variance and significance analysis were performed using relevant data analysis software. One-way ANOVA calculates the difference in data such as drying rate, moisture ratio, rehydration rate, shrinkage rate, unit energy consumption, and the effective moisture significant differences).

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3. Results and discussion

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diffusion coefficient. Significant differences were expressed as p values (p<0.05 indicates

3.1 Effect of different pretreatment methods on moisture ratio of goji berry Fig. 2 shows the changes of moisture ratio of goji berry treated with different pretreatment methods combined with EHD drying. It can be seen from Fig. 2 that different pretreatment methods have different effects on the decline rate of the moisture content of goji berry during electrohydrodynamic drying process. The effects of different pretreatment methods were listed in descending order by the decline of moisture ratio as follows: KOH > NaOH > Na2CO3 > ultrasonic > sucrose ester > the control. One-way analysis of variance showed significant differences between the pretreatment and the control (p<0.05). It also indicated from the experimental results that the pretreatment of the strong alkali solution is the most serious damage to the wax layer on the surface of goji berry.

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1.1 Control Sucrose ester solution Ultrasonic Na2CO3 solution NaOH solution KOH solution

1.0 0.9 0.8 0.7

MR

0.6 0.5 0.4 0.3 0.2 0.1 0

5

10

15

20

25

30

35

40

45

50

55

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0.0

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Time(h)

Fig. 2 Changes of moisture ratio under different pretreatment methods

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3.2 Effect of different pretreatment methods on drying rate of goji berry Fig. 3 shows the changes of drying rate of goji berry using different pretreatment methods

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during EHD drying process. As can be seen from Fig. 3, the pretreatment method had a major effect on the enhancement of the drying rate, increasing by 3.3683, 3.2280, 2.9062, 2.2151 and

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1.7915 times, respectively, under KOH solution, NaOH solution, Na2CO3 solution, ultrasonic and sucrose ester solution pretreatment compared to that of the control in the first 10 hours. So, the effects of different pretreatment methods were listed in descending order by the drying rate as

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follows: KOH > NaOH > Na2CO3 > ultrasonic > sucrose ester > the control. It can also be seen from Fig. 3 that when the moisture ratio of goji berry is higher, the pretreatment method has a

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greater influence on the drying rate of goji berry. Zhao et al. found that the total average drying rate of goji berry using the sodium carbonate solution pretreatment was nearly double that of the control during the hot air drying process at 50 °C (Zhao, Wei, Hao, Han, Ding, Yang, & Zhang, 2019). Our experimental results also showed that the total average drying rate of goji berry using the sodium carbonate solution pretreatment is about 2.5 times higher than that of the control during electrohydrodynamic (EHD) drying process. So, these conclusions are similar to the results of our previous studies.

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0.35 EHD drying Sucrose ester solution Ultrasonic Na2CO3 solution NaOH solution KOH solution

Drying rate(g water/DM*H)

0.30 0.25 0.20 0.15 0.10 0.05 0.00

1.0

1.5

2.0

2.5

3.5

4.0

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Moisture content

3.0

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0.5

Fig. 3 Changes of drying rate under different pretreatment methods

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3.3 Effect of different pretreatment methods on rehydration rate of goji berry The rehydration rate is an indicator for detecting the ability of dry products to absorb

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moisture and then restore the original appearance, reflecting the effect of pretreatment methods on quality. The higher the rehydration rate is, the smaller the effect of the pretreatment method on the

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internal structure and quality of the goji berry is. The rehydration rates of goji berry in the sucrose ester solution pretreatment, the ultrasonic pretreatment, the Na2CO3 solution pretreatment, the NaOH solution pretreatment, the KOH solution pretreatment and the control were 1.9536, 2.1231,

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2.2829, 2.4289, 2.5661, 2.0718, respectively. It can be seen that, except for the sucrose ester pretreatment, the rehydration rate of each pretreatment group was higher than that of the control. It

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can be seen from Fig. 2, 3 and 4 that the change of rehydration rate is not only related to the pretreatment method, but also mainly related to the drying rate and the decline rate of moisture ratio. The faster the rate of moisture reduction and the drying rate are, the higher the rehydration rate is, which is similar to the results of our previous studies (Ni, Ding, Zhang, Song, Hu, & Hao,

2.8 2.6 2.4 2.2

Rehydration ratio

2019).

2.0

bc

ab

a

cd

d a

1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 Co nt

H

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OH

KO

Na

te

ic

es

on

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CO 3 Na 2

e

as

os

tr

cr

Ul

Su

r

Journal Pre-proof Fig. 4 Changes of rehydration rate under different pretreatment methods. Data are shown as the mean ± SD. For each response, means with different lower case letters are significantly different (p < 0.05). 3.4 Effect of different pretreatment methods on shrinkage rate of goji berry Fig. 5 shows the effect of different pretreatment methods on goji berry shrinkage rate. As can be seen from Fig. 5, the shrinkage rate of goji berry using the sucrose ester solution pretreatment, the ultrasonic pretreatment, the Na2CO3 solution pretreatment, the NaOH solution pretreatment, the KOH solution pretreatment and the control was 0.7372, 0.7692, 0.7567, 0.7431, 0.7477 and 0.7464, respectively. It indicated that different pretreatment methods have little effect on the shrinkage rate of electrohydrodynamically dried goji berry. The shrinkage rate of goji berry is only

a

a

a

0.7

a

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0.6

a

a

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0.8

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related to water content and is less affected by other factors.

0.5

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0.4 0.3

0.1 0.0

nt

H

ro

on

te

ic

es

l

na

Co

KO

OH

e

as

os

tr

CO 3 Na 2

cr

Ul

Su

Na

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0.2

r

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Fig. 5 Changes of shrinkage rate under different pretreatment methods. Data are shown as the mean ± SD. For each response, means with different lower case letters are significantly different (p < 0.05).

3.5 Effect of different pretreatment methods on the effective moisture diffusion coefficient of goji berry

Table 2 shows the effective moisture diffusion coefficient of goji berry under different pretreatment methods. Table 2 shows that the effective moisture diffusion coefficient of goji berry pretreated with KOH solution is the highest, which is 1.010×10-9m2/s. The effective water diffusivity of goji berry in the control group was the smallest, which was 0.421×10-9m2/s. Therefore, each pretreatment group significantly improved the effective moisture diffusion coefficient of goji berry, and had a significant difference compared with the control (p<0.05). The KOH solution pretreatment had the best effect among the five pretreatment methods. Zhao, Wei, Hao, Han, Ding, Yang, Zhang, & Shahedi (2019) reported that the effective moisture diffusion coefficient of the wolfberry using the sodium carbonate solution pretreatment was significantly higher than that of untreated samples under hot air drying process. Fernandes, Oliveira, & Rodrigues (2008) found that the effective diffusion coefficient of papaya pretreated by ultrasound

Journal Pre-proof during hot air drying process was increased by more than three times compared with the control. It can be seen that the effective moisture diffusion coefficient of fruits after pretreatment significantly increased during drying process, which is similar to our experimental results. Higher voltage or electric field strength destroyed cell membranes and alter permeability. The ion wind continuously and impinges on the moist material and disturbs the saturated air layer, thereby obviously increasing the moisture evaporation inside the material (Alam, Lyng, Frontuto, Marra, & Cinquanta, 2018; Amami, Khezami, Vorobiev, & Kechaou, 2008; Won, Min, & Lee, 2015). The pretreatment causes some damage to the wax layer of the goji berries, which improves the effect of electrohydrodynamic (EHD) drying and plays a multiplier role with half the effort. Therefore, the effective moisture diffusion coefficient during EHD drying process further

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improved, thereby increasing the drying speed. Table 2

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The effective moisture diffusion coefficient of goji berry under different pretreatment methods.

Deff(10-9m2/s)

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Pretreatment types

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Control

Na2CO3 solution NaOH solution KOH solution

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Ultrasonic

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Sucrose ester solution

0.421±0.025a 0.633±0.042b 0.675±0.056b 0.844±0.035c 0.965±0.043d 1.010±0.038d

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Note: Data are shown as the mean ± SD. For each response, means with different lower case letters are significantly different (p < 0.05). 3.6 Analysis of unit energy consumption of different pretreatment methods Fig. 6 shows the change law of unit energy consumption when the moisture content of electrohydrodynamic dried goji berry reaches a final moisture content of 10% after different pretreatment methods. It can be seen from Fig. 6 that the unit energy consumption of the control is up to 412 kJ/kg water, and the unit energy consumption of the KOH solution pretreatment is only 183 kJ/kg water. From the experimental results, we can see that the unit energy consumption is mainly related to the total drying time. The longer the drying time is, the higher the unit energy consumption is. We can also see from the experimental results and some literature studies that electrohydrodynamic (EHD) drying has better energy saving characteristics than any other drying technology(Bai, Qu, Luan, Li, & Yang, 2013; Elmizadeh, Shahedi, & Hamdami, 2017). Through pretreatment, it increases the drying rate and saves energy.

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c

450 400 350 300

a

a b

250

b

200

b

150 100 50 0

Co nt ro l

r

ro

H

OH

KO

Na

CO 3 Na 2

te

ic

es

on

e

as

os

tr

cr

Ul

Su

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Specific energy consumption(kJ/㎏ water)

500

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Fig. 6 Different pretreatment methods for drying goji berry unit energy consumption. Data are shown as the mean ± SD. For each response, means with different lower case letters are

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significantly different (p < 0.05).

3.7 Infrared spectrogram analysis and two index sequence analysis of goji berry

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Fig. 7 shows an infrared spectrum of dried goji berry under different pretreatment methods. From Fig. 7, the infrared spectra of goji berry under different pretreatment methods showed that the pretreatment groups and the control had the same peak intensity, peak position and chemical

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composition, but the position and intensity of the characteristic peaks of the control were lower than those in the pretreatment group. So, it indicates that pretreatment combined with EHD drying

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can significantly improve the structural integrity and nutritional preservation ability of goji berry. However, there are some differences between the pretreatment groups. For example, NaOH solution pretreatment had the largest characteristic peak strength near 3282 cm-1, while the control group had the smallest characteristic peak strength near 3282 cm-1. Compared with NaOH solution pretreatment, KOH solution pretreatment was more conducive to the preservation of polysaccharides, glycosides, amino acids, proteins and glycols. The characteristic peaks of Na2CO3 solution and KOH solution pretreatment near 1623 cm-1, 1399 cm-1 and 1250 cm-1 showed the greatest strength, indicating that the above two pretreatment methods may be more conducive to the preservation of unsaturated esters of amide-band alkaloids of amino acids and proteins. The ultrasonic pretreatment had the highest characteristic peak intensity near 1098 cm-1, indicating that the ultrasonic pretreatment was more conducive to the preservation of carbohydrates such as polysaccharides and glycosides. It is found from the experimental results that different pretreatment methods have certain effects on the internal structure and active components of goji berry, which is helpful for further research on the mechanism of different pretreatment methods combined with electrohydrodynamic drying technology.

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C-OH

Absorbance

Control Sucrose ester solution Ultrasonic Na2CO3 solution NaOH solution KOH solution

O-H、N-H C-O C=C

C=O

C=0-C

C-C C=C

3600

3200

2800

2400

2000

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C=O

1600

1200

800

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Wave number/cm-1

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Fig. 7 Infrared spectra of goji berry under different pretreatment methods With the common peak ratio and the variation peak ratio as two indicators, two fingerprints

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can be comprehensively described in terms of commonality and difference. The higher the common peak ratio is, the greater the commonality of the two fingerprints is. In the variation peak

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ratio index, the variation of the fingerprint spectrum can be well measured by the ratio of the number of mutated peaks and the number of common peaks in each fingerprint. The greater the difference of variation peak rate between the two fingerprints is, the greater the difference of the

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two pretreatment methods is. The variation peak rates of the two fingerprints are small, indicating that the two pretreatment methods have similar properties, that is, the variation is small. In this experiment, six pretreatment groups were used as reference points to establish six common peak

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ratio and variation peak ratio double index sequences, which formed a six dimensional sequence space. Adding the common indicator peak ratio and the variation peak ratio double index space, the similarities and differences between the various pretreatment groups can be investigated in the 2+n dimension (n: equal to the number of samples), which makes the method have strong discriminating ability.

According to the determination method of common peaks, multiple groups of common peaks can be found in Table 3. For example, for the two groups of peaks corresponding to the wave numbers of 1740 cm-1 and 1625 cm-1, the average wave number of a group corresponding to the wave number of 1740 cm-1 is 1740.83 cm-1. The maximum wave number difference within the group is 2. The difference between the wave number of a group corresponding to the wave number of 1740 cm-1. The peak number of the two groups before and after is 1112.67 cm-1 and 116.5 cm-1. The two values are significantly larger than 2. It can be confirmed that a group of peaks corresponding to 1740 cm-1 is a common peak. Similarly, the average wave number of a set of peaks corresponding to 1625 cm-1 is 1624.33 cm-1, this average wave number difference between the two groups of peaks before and after is 116.5 cm-1 and 29.83 cm-1, respectively, which is significantly larger than the maximum wave number difference 2 in the group. It can be judged that the group peak is also a common peak.

Journal Pre-proof Table 3 Wavenumbers and common peaks of absorption peaks of goji berry infrared fingerprints with different pretreatment methods. the wave numbers of peaks in IR fingerprint spectra(cm-1)

sample G1

3282

3012

2921

2872

2852

1740

1624

G2

3284

3013

2920

2875

2852

1741

1625

1400

1364

G3

3283

2922

2854

1740

1625

1402

1362

G4

3281

2923

2854

1741

1624

1400

G5

3282

2924

2855

1741

1625

G6

3281

2854

1742

1623

1143

1098

1027

919

900

817

1025

918

901

817

1026

919

900

817

1026

919

900

817

1027

919

900

817

1027

919

900

817

2923

2874

1243

G2

1243

1195

1142

1097

1246

1194

1141

1098

1244

1197

1141

G3

1253

G4 G5

1253 1243

1185

1194

1141

1098

1142

1097

1046

1399

1401 1399

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G6

1194

1595

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G1

1518

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3014

1594

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Table 4

Sequence

(P；Pva，Pvb)

Sequence

Sequence

(P；Pva，Pvb)

G1:G2

(78.9％; 13.3, 13.2)

G2:G1

(78.9％; 13.2, 13.3)

G3:G1

(65.0％; 23.1, 30.8)

G1:G3

(65.0％; 30.8, 23.1)

G2:G3

(83.3％; 13.3, 6.0)

G3:G2

(83.3％; 13.6, 6.0)

G1:G4

(66.7％; 30.8, 8.0)

G2:G4

(82.3％; 30.8,

0)

G3:G4

(81.2％; 23.0,

G1:G5

(65.0％; 30.8, 23.1)

G2:G5

(65.0％; 30.8, 23.1)

G3:G5

(77.8％; 14.3, 14.3)

G1:G6

(78.9％; 13.3, 13.3)

G2:G6

(88.9％; 6.0, 6.0)

G3:G6

(73.7％; 14.3, 21.4)

G4:G1

(66.7％； 8.0, 30.8)

G5:G1

(65.0％; 23.1, 30.8)

G6:G1

(78.9％; 13.3, 13.3)

G4:G2

(82.3％；

0, 30.8)

G5:G2

(65.0％; 23.1, 30.8)

G6:G2

(88.9％; 6.0, 6.0)

G4:G3

(81.2％；

0, 23.0)

G5:G3

(77.8％; 14.3, 14.3)

G6:G3

(73.7％; 21.4, 14.3)

G4:G5

(70.5％； 8.0, 33.3)

G5:G4

(70.5％; 33.3, 8.0)

G6:G4

(76.5％; 30.8,

G4:G6

(76.5％；

G5:G6

(65.0％; 23.1, 30.8)

G6:G5

(65.0％; 30.8, 23.1)

0, 30.8)

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(P；Pva，Pvb)

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Double indicator sequence of goji berry infrared fingerprints with different pretreatment methods.

Note: G1: G2 (78.9%; 13.3, 13.2) indicates that the common peak rate and the variation peak rate of the fingerprints of other pretreatment groups are calculated using G1 as the standard. The sequence fragment indicated that the common peak ratio of G1 and G2 was 78.9%, the variation peak ratio of G1 was 13.3%, and the variation peak ratio of G2 was 17.2%. The results of double index sequence analysis of goji berry infrared fingerprint by different pretreatment methods are shown in Table 4. Group A: G2:G3(83.3％; 13.3, 6.0), G3:G2(83.3％; 6.0, 13.3). Group B: G1:G5(65.0％; 30.8, 23.1), G1:G4(66.7％; 30.8, G4:G1(66.7％；8.0, 30.8).

8.0), G5:G1(65.0％; 23.1, 30.8),

0)

0)

Journal Pre-proof In group A, G2 and G3 have relatively high common peak ratio and the relatively small variation peak ratio. The above experimental results showed that for the pretreatment group, the G2 and G3 solutions are the only ones that are not alkaline solution pretreatment. So, the relationship is the closest and thus the highest similarity. In group B, G1 is the unpretreated group, while G4 and G5 are pretreatment of Na2CO3 solution and NaOH solution, respectively. Both are alkaline solutions. They have a very low common peak ratio and a very high variant peak ratio. From the results, compared with other pretreatment methods, alkaline solution pretreatment may be more likely to erode the material, resulting in variation of the infrared spectrum peak. It can be seen from the above analysis that the treatment with similar pretreatment method has a higher common peak rate and higher similarity. Compared with the control, the strong

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alkaline solution pretreatment had a higher variation peak ratio, which led to a larger difference. These analyses correctly reflect the actual situation and are similar to our experimental results.

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3.8 Microstructure analysis of goji berry with different pretreatment methods

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Fig. 8 shows the effect of the control and pretreatment group on the surface microstructure of goji berry. Scanning electron microscopy showed that the texture of the control group was relatively regular but a few holes appeared, while the ultrasonic pretreatment group showed a large

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number of cavitation bubbles, and the surface of the material became extremely irregular. In the sucrose ester pretreatment group, micro surface lines of goji berry were eroded very seriously,

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while strong alkaline solution pretreatment group such as Na2CO3 solution, NaOH solution and KOH solution completely eroded the micro surface of goji berry, and many regular holes appeared due to the action of electrohydrodynamic (EHD) drying in the later stage. KOH solution

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pretreatment group had the most holes and the most serious damage. Na2CO3 solution pretreatment group was much inferior to KOH solution pretreatment. It produced some large,

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small holes and irregular lines.

Fig. 8 Microstructure of goji berry under different pretreatment conditions. (a) Control; (b) Sucrose ester solution; (c) Ultrasonic; (d) Na2CO3 solution; (e) NaOH solution; (f) KOH solution.

Journal Pre-proof The main principle of NaOH solution and KOH solution pretreatment is that a strong alkaline solution can convert vegetable oil into alkali metal fatty acid salt. As alkali metal fatty acid salt is composed of a long chain polar molecule, its structure is characterized by two functional groups (hydrophilic and lipophilic) with opposite functions on the chain, so it has good water-solubility. Due to the action of two opposite functional groups of polar molecules, when the goji berry fresh fruit is soaked with the sodium fatty acid solution or the fatty acid potassium solution, the immersion solution is filled in the wax gap of the epidermis, and the hydrophilic group is arranged towards the gap. With the transpiration of the moisture in the epidermis, the goji berry epidermis layer appears to have several small channels similar to the tube, leading to the destruction of the goji berry epidermis wax layer. From Fig. 8, two methods have the greater impact on the surface of goji berry, thereby increasing the drying speed of fresh goji berry and shortening the drying

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time during EHD drying process. The total average drying rate of goji berries pretreated by the NaOH solution and KOH solution increased by 1.3792 and 1.3768 times compared to the control,

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respectively.

Aqueous solution of Na2CO3 shows alkalinity, and has a certain dissolution effect on the

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waxy layer whose main component is a long-chain aliphatic hydrocarbon compound, so that the waxy layer of the epidermis is thinned or broken, and a water passage is formed, which would increase the drying speed of fresh goji berry and shorten the drying time. The total average drying

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rate of goji berries pretreated by the Na2CO3 solution increased by 1.1890 times compared to the control.

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Sucrose esters are composed of lipids of sucrose and edible fatty acids. The sucrose ester structure has a hydrophilic group of sucrose groups and hydrophobic fatty acid groups, so sucrose ester is a very strong nonionic surfactant with strong surface activity and can destroy the goji berry

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epidermis wax layer, which would increase the drying speed of fresh goji berry and shorten the drying time. Compared with other pretreatment methods, the sucrose ester pretreatment method

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had the least effect on the microstructure of goji berry. The main principle of the ultrasonic pretreatment mechanism is based on the action of mechanical waves, and the role of mechanical waves is mainly related to cavitation. When the ultrasonic wave propagates through a medium containing water, it causes a series of rapid alternating compressions and expansions. This phenomenon leads to the formation of microchannels in cells and from which cellular fluids leak into the surrounding environment (Mothibe, Zhang, Nsor-atindana, & Wang, 2011). In addition, when the ultrasonic wave propagates in the material medium containing water, the gas nucleus in the fluid forms cavitation bubbles (cavitation phenomenon). These cavitation bubbles distributed throughout the liquid will grow to a critical value in a very short period of time and then become unstable and cause severe collapse. As a result, very high shear energy waves and turbulence result in cell damage (Soria & Villamiel, 2010). Both of these effects can destroy the waxy layer of the goji berry epidermis, thereby increasing the drying speed of fresh goji berry and shortening the drying time. From the above analysis, the principle of destroying the wax layer by each pretreatment method is slightly different, which caused that there are great differences in the surface microstructure of goji berries. The microstructure analysis further verified that each pretreatment method had a great influence on the surface of goji berry from different aspects and different

Journal Pre-proof angles, and then affected the drying speed, quality and various indexes of goji berry macroscopically. 4. Conclusions Compared with the control group, each pretreatment method combined with EHD drying can accelerate the goji berry drying rate, reduce the drying time, save energy and maintain good quality. The infrared spectrum showed that the position and intensity of the functional group peaks of the dried goji berry products of each pretreatment group were significantly different from those of the control. The observation of goji berry surface by scanning electron microscopy revealed that each pretreatment method can significantly change the surface microstructure of goji berry. During EHD drying process the strongest alkaline solution pretreatment, such as NaOH and KOH,

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has the better drying effect for goji berry. So, the strongest alkaline solution pretreatment can be used as the optimal pretreatments for industrial application purposes of goji berries dried by EHD.

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Acknowledgements

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This research was funded by National Natural Science Foundations of China (nos. 51467015 and 51767020), Natural Science Foundation of Inner Mongolia Autonomous Region of China (no. 2017MS(LH)0507). The authors also would like to express their gratitude to the anonymous

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referees for their valuable comments and suggestions.

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Journal Pre-proof Conflicts of interest

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The authors declare no conflicts of interest.

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Author statement All of the authors contributed significantly to the research. Jiabao Ni and Changjiang Ding performed the majority of the experiments and wrote the manuscript; Jiabao Ni, Changjiang Ding, Yaming Zhang and Zhiqing Song contributed to data analysis; and Changjiang Ding designed and supervised the study and checked the

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final manuscript.

Journal Pre-proof Highlights Different pretreatments combined with EHD drying significantly improved the drying rate of goji berries. The two index sequence analysis of FTIR was investigated during goji berries drying process. The pretreatment significantly affects characteristics, quality and microstructure of goji berry dried by EHD.

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The difference among five pretreatment methods of goji berry was analyzed.