Effect of electrohydrodynamic partially combined with oven drying on Chinese wolfberry

Abstract

In order to study the drying characteristics of Chinese wolfberry using the electrohydrodynamic (EHD) partially combined with oven drying method, in this paper, the drying experiment of Chinese wolfberry was carried out by different combinations of EHD drying and oven drying. The drying parameters, active ingredients and microstructure of Chinese wolfberry under different drying processing were measured. The results showed that under the two-stage drying processing of EHD drying and oven drying, at the early stages the EHD drying has a greater effect on the drying of Chinese wolfberry, and in the later stage the oven drying has a greater effect. The drying rate of Chinese wolfberry using the EHD partially combined with oven drying was significantly higher than those of the oven drying and the control. The EHD partially combined with oven drying have great influence on drying rate, rehydration rate and polysaccharide content, and have little effect on shrinkage and flavonoid content. Compared with the EHD drying or oven drying, the EHD drying partially combined with the oven drying significantly increased the effective moisture diffusion coefficient of the Chinese wolfberry. The results also showed that the EHD drying partially combined with oven drying caused a significant change in the microstructure of Chinese wolfberry according to scanning electron microscopy and infrared spectroscopy. It provides a theoretical basis and practical guidance for understanding the parameter characteristics and drying mechanism of EHD drying partially combined with oven drying technology.

Keywords

EHD drying oven drying Chinese wolfberry active ingredients

1. Introduction

Chinese wolfberry fruits are increasingly favored by people due to their high polysaccharides, flavonoids, various beneficial amino acids and micronutrients absorbed by the human body, as well as their medicinal values of enhancing human immunity, lowering blood sugar, lowering blood lipid, anti-tumor and protecting liver [1–3]. The storage period of Chinese wolfberry fruits is very short. The Chinese wolfberry fruits are easy to mold and rot, and must be dried in real time before it can be stored for a long time. At present, Chinese wolfberry drying technology is mainly sun drying and hot air drying [4], but the two drying techniques have a great adverse effect on the color, quality and nutrient preservation of Chinese wolfberry fruits. Recently, some new drying technologies such as vacuum drying [5] and microwave drying [6] have been applied to the field of Chinese wolfberry fruits drying and achieved certain results. However, they also have some disadvantages, such as the microwave drying is easy to cause uneven drying of materials, the equipment of vacuum drying is expensive, and the energy consumption of vacuum drying is high. Consequently, it is very meaningful to develop new drying technology for the Chinese wolfberry.

EHD drying is a new type of non-thermal drying technology, and has many advantages such as faster drying speed, no damage to effective nutrients, energy conservation and no pollution to the environment. It is becoming a research hotspot [7–10]. Singh et al. found that the drying rate of wheat with EHD drying was much higher than that of the control, and it increased with the increase of the voltage [11]. Elmizadeh et al. used EHD drying technology to dry quince slices and found that the energy consumption treated with hot air drying was 47.66 times higher than that treated with EHD drying [12]. Martynenk et al. found that grapes treated with EHD drying have better sensory properties than that treated with hot air drying [13]. Esehaghbeygi et al. found that compared with microwave drying, bananas had higher rehydration capacity, appearance and quality using EHD drying [14]. However, EHD drying also has some disadvantages, such as when the moisture content of the material is low, the drying speed rapidly decreases. With the development of modern drying technology, the single drying technology is far from satisfying. To better utilize the advantages and overcome the shortcomings of each drying technology, the combination of various drying technologies has become current research trend.

Some researchers studied the combination of EHD drying with other drying technology and found that it can lead to improvements in the quality of the end product and energy savings [15–17]. Bai et al. studied found that the protein content preserved using EHD combined with vacuum freeze drying was 3.5% higher than that using single EHD drying [15]. Chen et al. found that compared with other drying, hydrodynamic pretreatment combined with EHD drying significantly reduced enzyme activity in dried blueberries [16]. Dinani et al. found that under drying process, the energy consumption of mushroom using EHD drying combined with hot air drying was much lower than that of the hot air drying [17]. In addition, many researchers have conducted extensive research on EHD drying combined with other drying techniques and achieved good results [18–20]. These studies showed that EHD drying combined with other drying techniques can achieve very good results, which provides a new idea for the development of EHD drying technology. However, there has been no detailed report on the drying characteristics of Chinese wolfberry fruits by EHD drying partially combined with oven drying. So, it is necessary to conduct in-depth research.

In this paper, the drying experiment of Chinese wolfberry fruits was investigated using the EHD partially combined with oven drying. The drying parameters such as drying rate, moisture content and effective diffusion coefficient of Chinese wolfberry fruits during drying process were measured. The effects of different drying conditions on the content of polysaccharides and flavonoids in dried Chinese wolfberry were studied. The microstructure changes during the drying process were studied by means of scanning electron microscopy and infrared spectroscopy. It provides an experimental and theoretical basis for the application of EHD combined with oven drying technology in the field of Chinese wolfberry fruits drying.

2. Materials and methods

2.1. Experimental equipment

The EHD drying experimental device is shown in Fig. 1. It is mainly composed of high voltage power supply (YD (JZ)-1.5/50, China), controller (KZX-1.5KVA, China) and multi-needle-to-plate electrode system. The high-voltage electrode is multiple sharp pointed needles and is connected to a high voltage power supply. The length and diameter of each needle are 20 mm and 1 mm, respectively. The distance between two needles is 40 mm. The grounded electrode is an 100 cm × 45 cm rectangular stainless steel plate. The distance between the emitting point and the grounded electrode is 100 mm. The power source can supply direct current (DC) high voltage or alternating current (AC) high voltage and is connected to a voltage regulator, with an adjustable voltage ranging from 0–50 kV for alternating current (AC) or 0–70 kV for direct current (DC) by a controller. A micro-ampere meter is placed in series between the lower plate and the ground to monitor the current change during the drying process.

Fig. 1.

Schematic diagram of electrohydrodynamic (EHD) drying. 1. Hygrometer 2. Thermometer 3. Sample 4. Ground electrode 5. Needle electrode 6. High voltage power supply 7. Control system 8. Microammeter.

2.2. Determination of initial moisture content

Fresh Chinese wolfberry fruits were purchased from a local farm in Tuoketuo County, Hohhot, Inner Mongolia, China, and immediately stored in a refrigerator at 4 °C. We took out the fresh Chinese wolfberry from the refrigerator and immersed in a 5% sodium carbonate solution at 50 °C. After 10 minutes, the fresh Chinese wolfberry were removed from solution. The excess water of Chinese wolfberry surface was completely wiped off with absorbent paper. Then, the initial moisture content of the pretreated Chinese wolfberry was measured with a rapid moisture tester (Sh10A, China). The initial moisture content of the pretreated Chinese wolfberry is 76 ± 1%.

2.3. Experimental method

The pretreated Chinese wolfberry fruits were placed in an EHD system at 30 kV for a period of time. Then, the pretreated Chinese wolfberry fruits were removed and placed in an oven at 60 °C. In other word, the EHD partially combined with oven drying of the pretreated Chinese wolfberry fruits was investigated. As comparison, the EHD drying, oven drying and ambient drying (the control) of Chinese wolfberry fruits were conducted, respectively. The ambient temperature was 25 ± 2 °C, the ambient humidity was 30 ± 2%, the ambient wind speed was 0 m/s. The drying time in the EHD system was 3 h, 6 h, 9 h, 12 h, 15 h, 18 h, respectively. Then the pretreated Chinese wolfberry fruits were taken out and placed in an oven to dry until the moisture content was 10%. The corresponding voltage in the EHD system and temperature in the oven were 30 kV and 60 °C, respectively.

The mass of the pretreated Chinese wolfberry fruits in the drying process was measured by an analytical balance (BS124S, China) every one hour. The moisture ratio and drying rate of Chinese wolfberry were calculated according to the corresponding formula. Each experiment was repeated three times independently, and the results were expressed as mean ± standard deviation (SD).

2.4. Determination of relevant parameters

2.4.1. Moisture ratio

The moisture content and moisture ratio of the Chinese wolfberry fruit during the drying process are calculated using the following equations: $\begin{eqnarray}\displaystyle \text{} & \text{} & \displaystyle M_{i}=\frac{m_{i}-m_{g}}{m_{g}}\times 100\%\end{eqnarray}$ (1) $\begin{eqnarray}\displaystyle \text{} & \text{} & \displaystyle \mathit{MR}=\frac{M_{i}-M_{e}}{M_{0}-M_{e}}\end{eqnarray}$ (2) where m _g is the dry mass of Chinese wolfberry, m _i is the mass of the Chinese wolfberry fruit at time t _i, M ₀ is the water content of the Chinese wolfberry fruit at time t ₀, M _i is the water content of the Chinese wolfberry fruit (g water/g dry matter) at time t _i, M _e is the equilibrium water content of the Chinese wolfberry fruit, and MR is the moisture ratio of the Chinese wolfberry fruit.

2.4.2. Drying rate

The drying rate of Chinese wolfberry fruit in the drying process is calculated using the following equation [21]: $\begin{eqnarray}\displaystyle DR=\frac{M_{t}-M_{t+{\rm\Delta}t}}{{\rm\Delta}t} & & \displaystyle\end{eqnarray}$ (3) where DR is the drying rate (g water/g dry matter × h), M _t is the moisture content of the Chinese wolfberry fruit at time t, and M _t+Δt is the moisture content of the Chinese wolfberry fruit at time t + Δt.

2.4.3. Rehydration rate

Measurement method of rehydration rate is described by Esehaghbeygi et al. with made some modifications according to the method adopted by local farmers [22]. The dried Chinese wolfberry fruits was were soaked in a constant temperature water bath at 37 °C for 7 hours, then the Chinese wolfberry fruits was were removed. And the surface water of Chinese wolfberry fruits was completely absorbed by filter paper. The mass of the Chinese wolfberry before and after rehydration was measured with a Sartorius BS124S electronic balance. The rehydration rate of Chinese wolfberry fruits is defined as: $\begin{eqnarray}\displaystyle RR=\frac{m_{a}}{m_{b}} & & \displaystyle\end{eqnarray}$ (4) where RR is the rehydration rate of Chinese wolfberry fruits, m _a is the mass of Chinese wolfberry fruits after rehydration, m _b is the mass of Chinese wolfberry fruits before rehydration.

2.4.4. Shrinkage rate

The volumes of both fresh and dried Chinese wolfberry fruits were measured by the displacement method [23]. The shrinkage rate is defined as: $\begin{eqnarray}\displaystyle \mathit{SR}=\frac{V_{0}-V_{f}}{V_{0}}\times 100\% & & \displaystyle\end{eqnarray}$ (5) where SR is the shrinkage rate of Chinese wolfberry fruits, V ₀ is the volume of fresh Chinese wolfberry fruits, V _f is the volume when the moisture content of Chinese wolfberry achieves 10%.

2.4.5. Flavonoids contents

Total flavonoids content was determined with spectrophotometric method [24,25]. The dried Chinese wolfberry fruits was ground into fine powder. 1.0 g of fine powder was placed into 150 mL conical flask with stopper. Then, we added 30 mL of methanol solution. The conical flask was shook with 160 r/min for 2 h at 65 °C in a thermostatic water bath oscillators. Then the mixed liquor was filtered. And the filter liquor was transferred into 50 mL volumetric flask. Then, we added the methanol solution and made the total volume of solution up to 50 mL. We used the standard rutin solution to set standard curve. The absorbence of this solution was measured at 420 nm according to the step of standard curve.

2.4.6. Polysaccharides contents

Ultrasound-assisted extraction of polysaccharides was conducted following the method proposed by Yang et al. [26]. The dried Chinese wolfberry fruits was ground into fine powder. 1.0 g of Chinese wolfberry was placed into centrifuge tubes. Then, we added 5 mL of distilled water and 20 mL of absolute ethyl alcohol into centrifuge tubes. The samples were conducted the sonication using the ultrasonic processor for 30 min. After sonication, the extract was centrifuged at 4000 rpm for 10 min. We separated the residue and supernatant, and collected the precipitate. 10 mL of ethanol was added, washed and centrifuged. We were transferred the precipitate into the round-bottom flask and added 50 mL of distilled water. The samples were extracted in boiling water bath. After 2 h, the supernatant for the samples was transferred into a 100 mL volumetric flask and constant volumed with distilled water. We used the standard glucose solution to set standard curve. The absorbency of this solution was measured at 490 nm according to the step of standard curve.

2.4.7. The effective moisture diffusion coefficient

The effective moisture diffusion coefficient during the drying process of fresh Chinese wolfberry fruits is calculated using Fick’s second law. The expression is: $\begin{eqnarray}\displaystyle \frac{dM}{dt}=D_{\mathit{eff}}\frac{d^{2}M}{dr^{2}} & & \displaystyle\end{eqnarray}$ (6) for a long drying process, MR < 0.6, the equation can be expressed as: $\begin{eqnarray}\displaystyle MR=\frac{8}{{\pi}^{2}}\exp \left(-\frac{{\pi}^{2}D_{\mathit{eff}}∼t}{4L^{2}}\right) & & \displaystyle\end{eqnarray}$ (7) where D _eff is the effective moisture diffusion coefficient of the Chinese wolfberry fruit and L is the thickness of the Chinese wolfberry fruit.

2.4.8. The specific energy consumption

The specific energy consumption (SEC) is the energy required to evaporate 1 kg of water inside the Chinese wolfberry fruit. The expression is as follows: $\begin{eqnarray} \displaystyle \text{} & \text{} & \displaystyle \mathit{SEC}_{\mathit{EHD}}=\frac{V\times I}{m_{0}-m_{i}}\times {\rm\Delta}t_{1} \end{eqnarray}$ (8) $\begin{eqnarray}\displaystyle \text{} & \text{} & \displaystyle \mathit{SEC}_{Oven}=\frac{P}{m_{i}-m_{t}}\times {\rm\Delta}t_{2}\end{eqnarray}$ (9) $\begin{eqnarray}\displaystyle \text{} & \text{} & \displaystyle \mathit{SEC}=\mathit{SEC}_{\mathit{EHD}}+\mathit{SEC}_{Oven}\end{eqnarray}$ (10) where SEC _EHD is the energy required to evaporate 1 kg of water inside the Chinese wolfberry fruit by EHD drying. SEC _Oven is the energy required to evaporate 1 kg of water inside the Chinese wolfberry fruit during oven drying. V is the voltage, I is the current and P is the oven power. m ₀ is the initial mass of Chinese wolfberry fruit, m _i is the mass when Chinese wolfberry fruits were taken from the EHD system, and m _t is the mass when the moisture content of Chinese wolfberry achieves 10%. Δt ₁ is the drying time of Chinese wolfberry fruits in the EHD system and Δt ₂ is the drying time of Chinese wolfberry fruits in the oven.

2.4.9. Infrared spectrum

The measurement of infrared spectrum was conducted following the method proposed by Ni et al. [27]. The dried Chinese wolfberry fruit product was pulverized and mixed with potassium bromide, after sieving. It was placed in a tableting machine (HY-12, China) to form a tablet. The sample was scanned using a Fourier transform infrared spectrometer (Nicolet iS10, USA) to remove the interference of water and carbon dioxide, thereby obtaining a scanning spectrum.

2.4.10. Scanning electron microscopy

The measurement of scanning electron microscopy was conducted following the method proposed by Ni et al. [27]. In order to observe the effects and changes of different drying methods on the surface microstructure of Chinese wolfberry fruits, the dried Chinese wolfberry fruit products were sprayed with gold and then placed in a scanning electron microscope (S3400, Japan) for scanning and photographing, and then comparatively studied. The acceleration voltage and the magnification of scanning electron microscope is 30 kV and 5000 times, respectively.

2.4.11. Statistical analysis

Single-factor analysis of variance was used to calculate the moisture ratio and drying rate between the Chinese wolfberry fruits under EHD drying, different EHD drying partially combined with oven drying condition, oven drying and the control. The statistical difference was p (p < 0.05 means Statistical difference) value representation. The effective moisture diffusion coefficient, polysaccharide content and flavonoid content were also calculated between the Chinese wolfberry fruits treated under different EHD drying conditions and the control using Single-factor analysis of variance. The results reported in this study are presented as mean ± standard deviation (SD).

3. Results

3.1. Moisture ratio

Figure 2 shows the moisture ratio of the Chinese wolfberry under different drying conditions. It can be seen from Fig. 2 that the decline of moisture ratio is different under different drying conditions. According to the decline of moisture content, the sequence of from high to low is 18 h (EHD drying) + 60 °C, 15 h (EHD drying) + 60 °C, 12 h (EHD drying) + 60 °C, 9 h (EHD drying) + 60 °C, EHD drying, 6 h (EHD drying) + 60 °C, 3 h (EHD drying) + 60 °C, oven drying and the control. The decline of moisture ratio treated with the EHD partially combined with oven drying was higher than that of the oven drying and the control, except for treatment with the 6 h (EHD drying) + 60 °C and the 3 h (EHD drying) + 60 °C. The longer the EHD drying time is, the faster the moisture ratio decreases. After the treatment of EHD drying, the decline of moisture ratio is obviously accelerated. It indicated that the internal structure of the Chinese wolfberry fruit was changed due to the treatment of EHD drying, which causes a rapid drop of the moisture ratio.

Fig. 2.

The moisture ratio of Chinese wolfberry fruits under different drying conditions.

3.2. Drying rate

Figure 3 shows the relationship between the drying rate and moisture content of Chinese wolfberry fruits under different drying conditions. It can be seen from Fig. 3 that the drying rate is higher when the moisture content is higher. In the initial stage, the drying rate treated with EHD drying was higher than that of the oven drying and the control. It can also be found that after the treatment of EHD drying, the drying rate of the Chinese wolfberry fruit in the oven is suddenly increased, which also indicates that the treatment of EHD drying has an influence on the internal structure of the Chinese wolfberry fruit. Compared with the EHD drying and oven drying, EHD partially combined with oven drying have significantly improved drying speed under some drying conditions. Bai et al. studied the EHD drying partially combined with vacuum freeze-dried sea cucumber and obtained very satisfactory results [15], which is consistent with our results.

Fig. 3.

The relationship between the drying rate and moisture content of Chinese wolfberry fruits under different drying conditions.

3.3. Rehydration rate

Rehydration rate is an index to measure the ability of dry products to restore the original shape after absorbing water, which reflects the influence of drying method on quality. The higher the rehydration rate is, the smaller the effect of the drying method on the internal structure and quality of the Chinese wolfberry fruit is. It can be seen from Fig. 4 that the rehydration rate of Chinese wolfberry treated with EHD drying was the highest (1.8613), and it was the lowest (0.43469) under oven drying. The rehydration rate treated with EHD partially combined with oven drying was significantly higher than that of the oven drying and the control. With the increase of drying time treated with EHD, the rehydration rate of Chinese wolfberry fruits increased significantly. It indicated that EHD drying has better rehydration ability compared with oven drying and the control, thus affecting the quality and taste of dried products.

Fig. 4.

The rehydration rate of Chinese wolfberry fruits under different drying conditions. Note: Data are shown as the mean ± SD. For each response, means with different lower case letters are significantly different (p < 0.05).

Fig. 5.

The shrinkage rate of Chinese wolfberry fruits under different drying conditions. Note: Data are shown as the mean ± SD. For each response, means with different lower case letters are significantly different (p < 0.05).

3.4. Shrinkage rate

Figure 5 shows the shrinkage rate of Chinese wolfberry fruits under different drying conditions. As can be seen from Fig. 5, the shrinkage rate of the dried Chinese wolfberry fruit products under the control, the oven drying, the EHD drying, the 3 h (EHD drying) + 60 °C, the 6 h (EHD drying) + 60 °C, the 9 h (EHD drying) + 60 °C, the 12 h (EHD drying) + 60 °C, the 15 h (EHD drying) + 60 °C and the 18 h (EHD drying) + 60 °C were 0.7842, 0.7797, 0.7971, 0.8202, 0.7884, 0.7697, 0.75971, 0.7691 and 0.7797, respectively. By ANOVA, the results showed that there was no significant difference between the treatments (p > 0.05), and indicated that the effects on the shrinkage rate of dried Chinese wolfberry were very small under different drying conditions. The shrinkage rate of Chinese wolfberry fruits was mainly related to moisture content and has little correlation with drying technology.

3.5. Polysaccharides contents

The polysaccharide of Chinese wolfberry fruits is one of the most effective nutrients, and has important functions of regulating immunity, anti-aging, anti-tumor and anti-oxidation [28–30]. Polysaccharide content is an important indicator for evaluating drying technology. Table 1 shows the polysaccharide content in Chinese wolfberry fruits under oven drying, EHD drying and the control. Table 2 shows the polysaccharide content in Chinese wolfberry fruits under EHD partially combined with oven drying. As can be seen from Table 1 and Table 2, the EHD drying can better preserve the polysaccharide compared to the oven drying and the control. In the EHD partially combined with oven drying, as the drying time treated with EHD increases, the polysaccharide content in the Chinese wolfberry increases. It shows that the EHD drying can preserve the polysaccharide content well, and the oven drying has a certain destructive effect on the polysaccharide inside the Chinese wolfberry fruits. Compared with the EHD drying and oven drying, the polysaccharide content was significantly improved under some EHD partially combined with oven drying conditions. It indicated that after the treatment of EHD partially combined with oven drying, the preservation ability of polysaccharide content is improved. The experimental results also showed that the polysaccharide content of Chinese Wolfberry is negatively correlated with drying time and drying temperature. As the drying time and drying temperature increased, the polysaccharide content decreases.

Table 1
Polysaccharides contents under EHD drying, oven drying and the control (g/100 g)

Control Oven drying EHD drying

Polysaccharides 12.9 ± 0.03^a 11.3 ± 0.06^a 16.0 ± 0.03^b

	Control	Oven drying	EHD drying
Polysaccharides	12.9 ± 0.03^a	11.3 ± 0.06^a	16.0 ± 0.03^b

Note: Data are shown as the mean ± standard deviation (SD). For each response in this table, means in each column not sharing the same lowercase letters are significantly different (p < 0.05).

Table 2

Polysaccharides contents with the EHD partially combined with oven drying (g/100 g)

EHD drying time + 60 °C	3 h	6 h	9 h	12 h	15 h	18 h
Polysaccharides	13.0 ± 0.04^a	13.4 ± 0.05^a	15.0 ± 0.04^b	15.7 ± 0.02^bc	17.6 ± 0.05^c	17.7 ± 0.03^c

Note: Data are shown as the mean ± standard deviation (SD). For each response in this table, means in each column not sharing the same lowercase letters are significantly different (p < 0.05).

3.6. Flavonoids contents

Chinese wolfberry flavonoids are the most important active ingredient of Chinese wolfberry fruit, which is the essence of Chinese wolfberry and is easily absorbed by the body. It has high anti-oxidation, anti-aging, anti-cancer, blood pressure lowering, blood fat reduction, blood sugar lowering, improving human immunity and preventing cardiovascular and other biological activities [31–33]. Table 3 and Table 4 show the flavonoids content in Chinese wolfberry fruits under different drying conditions. It can be seen from Table 3 and Table 4 that the flavonoid content in the treatment with 18 h (EHD drying) + 60 °C was the highest. Papoutsis et al. studied found that compared with the 70 °C drying, the 90 °C drying significantly increased the flavonoid content in the lemon [34]. Song et al. studied the freeze-dried and hot-air dried goji and found that the flavonoid content treated with the instant controlled pressure drop drying combined freeze-drying and instant controlled pressure drop drying combined with hot air drying was significantly increased compared to freeze drying and hot air drying [35]. The flavonoid content using the oven drying increased due to the high temperature, which is responsible for the oxidation of phenolic substances, resulting the liberation of some flavonoids [36,37]. The flavonoid content treated with 18 h (EHD drying) + 60 °C was higher than that of other treatment, indicating that oven drying after EHD drying treatment may release more flavonoids.

Table 3
Flavonoids contents under EHD drying, oven drying and the control (g/100 g)

Control Oven drying EHD drying

Flavonoids 0.11 ± 0.003^a 0.13 ± 0.006^ab 0.10 ± 0.003^a

	Control	Oven drying	EHD drying
Flavonoids	0.11 ± 0.003^a	0.13 ± 0.006^ab	0.10 ± 0.003^a

Note: Data are shown as the mean ± standard deviation (SD). For each response in this table, means in each column not sharing the same lowercase letters are significantly different (p < 0.05).

Table 4

Flavonoids contents with the EHD partially combined with oven drying (g/100 g)

EHD drying time + 60 °C	3 h	6 h	9 h	12 h	15 h	18 h
Flavonoids	0.11 ± 0.004^a	0.12 ± 0.008^a	0.13 ± 0.004^ab	0.13 ± 0.007^ab	0.11 ± 0.005^a	0.15 ± 0.002^bc

Note: Data are shown as the mean ± standard deviation (SD). For each response in this table, means in each column not sharing the same lowercase letters are significantly different (p < 0.05).

3.7. The effective moisture diffusion coefficient

Table 5 shows the effective moisture diffusion coefficient of Chinese wolfberry fruits under different drying conditions. It can be seen from Table 5 that the effective moisture diffusion coefficient treated with the 18 h (EHD drying) + 60 °C is the highest (5.392981), and it was the lowest (2.679490) under the control. Both the EHD drying and the oven drying can increase the effective moisture diffusion coefficient in Chinese wolfberry fruits. In the early stage of drying process, EHD drying can destroy the cell membrane, cause the membrane to disintegrate, change the permeability and accelerate the evaporation of water in the Chinese wolfberry. The greater the electric field intensity is, the more holes there are in the cell membrane and the greater the permeability will be. On this foundation, oven drying further improve the internal moisture of the material from the cells to the outside, and accelerate the drying rate and reduce the drying time in the later stage of drying process.

Table 5
The effective moisture diffusion coefficient under different drying conditions

Conditions D _eff (10⁻¹⁰ m²∕s)

Control 2.679490 ± 0.000014^a

Oven drying 3.541590 ± 0.000020^b

EHD drying 4.609960 ± 0.000018^cde

3 h (EHD drying) + 60 °C 3.910260 ± 0.000031^bc

6 h (EHD drying) + 60 °C 4.251290 ± 0.000015^bcd

9 h (EHD drying) + 60 °C 4.828410 ± 0.000019d^e

12 h (EHD drying) + 60 °C 5.001550 ± 0.000036d^e

15 h (EHD drying) + 60 °C 5.189110 ± 0.000017^e

18 h (EHD drying) + 60 °C 5.392981 ± 0.000023^e

Conditions	D _eff (10⁻¹⁰ m²∕s)
Control	2.679490 ± 0.000014^a
Oven drying	3.541590 ± 0.000020^b
EHD drying	4.609960 ± 0.000018^cde
3 h (EHD drying) + 60 °C	3.910260 ± 0.000031^bc
6 h (EHD drying) + 60 °C	4.251290 ± 0.000015^bcd
9 h (EHD drying) + 60 °C	4.828410 ± 0.000019d^e
12 h (EHD drying) + 60 °C	5.001550 ± 0.000036d^e
15 h (EHD drying) + 60 °C	5.189110 ± 0.000017^e
18 h (EHD drying) + 60 °C	5.392981 ± 0.000023^e

Note: Data are shown as the mean ± standard deviation (SD). For each response in this table, means in each column not sharing the same lowercase letters are significantly different (p < 0.05).

Fig. 6.

The specific energy consumption of Chinese wolfberry fruits under different drying conditions. Note: Data are shown as the mean ± SD. For each response, means with different lower case letters are significantly different (p < 0.05).

3.8. Specific energy consumption

Figure 6 shows the specific energy consumption when moisture content of Chinese wolfberry ranges from 76% to 10% under different drying conditions. As can be seen from Fig. 6, the energy consumption of the EHD drying is only 543.0211 (the lowest), but it is 7987690.4860 (the highest) under oven drying. The specific energy consumption treated with EHD partially combined oven drying is reduced in turn when the drying time of EHD is from 3 h to 18 h. Dinani et al. studied the convective combined with electrohydrodynamic dried mushroom slices and found that the energy consumption of hot air drying is 723.51 ∼ 2817.74 times higher than that of EHD drying when using convective combined with electrohydrodynamic drying system to dry mushrooms [20]. Bai et al. studied the EHD partially combined with vacuum freeze drying sea cucumbers and found that the EHD partially combined with vacuum freeze-drying technology can effectively save energy consumption compared to the vacuum freeze-drying [15]. These findings were similar to our experimental results. The electric field under EHD drying process can change the permeability of the material and promote the penetration of water into the surface of the cell, and then continuously blow the water into the environment by the action of ionic wind. The mechanism of EHD drying is the interaction of ionic wind and a nonuniform electric field [27], which is different from the mechanism of the traditional drying. So, the energy consumption during EHD partially combined oven drying process was lower than that under oven drying.

Fig. 7.

The infrared spectrum of Chinese wolfberry fruits under different drying conditions. a. Oven drying, b. 3 h (EHD drying) + 60 °C, c. 6 h (EHD drying) + 60 °C, d. 9 h (EHD drying) + 60 °C, e. 12 h (EHD drying) + 60 °C, f. 15 h (EHD drying) + 60 °C, g. 18 h (EHD drying) + 60 °C, h. Control, i. EHD drying.

3.9. Infrared spectrum

Figure 7 shows the infrared spectrum of dried Chinese wolfberry products under different drying conditions. It can be seen from Fig. 7 that there generally have the same peak intensity, peak position and the same chemical composition under the EHD drying and the control. And there generally have the same peak intensity, peak position and the same chemical composition under other drying process. In addition, the characteristic peak intensity of the EHD drying and the control was very high near the wave number 1350 cm⁻¹. However, the peak intensity of other drying conditions is very weak. This may be explained that the temperature caused the peak intensity and peak position change of the infrared spectrum of Chinese wolfberry fruits during the oven drying process. The main characteristic absorption peaks in infrared spectrum of Chinese wolfberry were identified and compared. Some laws can be found. The vicinity of the wave number 3420 cm⁻¹ is the stretching vibration of N-H and O-H of polysaccharides, glycosides, amino acids, proteins, and sugar alcohols. The vicinity of the wave number 2927 cm⁻¹ and wave number 2855 cm⁻¹are the telescopic vibration of C-H of methylene and methyl. The vicinity of the wave number 1740 cm⁻¹ is telescopic vibration of C=O of carboxylic acids or esters. The wave number 1630 cm⁻¹, wave number 1380 cm⁻¹ and wave number 1250 cm⁻¹ are the vibration of amino acid, protein amide I band, III band, alkaloids, unsaturated esters. The broad-strong peak near the wave number of 1060 cm⁻¹ is mostly the bending vibration of C-OH of carbohydrates such as glycosides and polysaccharides. According to Fig. 7, we can also know that there have certain effects on the wave number range from 2860 cm⁻¹ to 3400 cm⁻¹ and from 1050 cm⁻¹ to 1740 cm⁻¹ under different drying conditions. It indicated that the internal active components of Chinese wolfberry dried products was affected during drying process, especially for the polysaccharides, glycosides, amino acids, proteins, and sugar alcohols. This will be of great help to the further study mechanisms of EHD drying combined with other drying technology.

3.10. The surface microstructure

Figure 8 depicts the effect of different drying methods on the surface microstructure of dried Chinese wolfberry products. The results showed that the surface texture of Chinese wolfberry fruits in the control was relatively regular, but the surface texture of Chinese wolfberry fruits treated with oven drying became obviously disordered, while the surface texture of Chinese wolfberry fruits treated with EHD drying showed a large number of holes of different sizes. The surface microstructure of Chinese wolfberry fruits under the EHD partially combined with oven drying becomes more and more irregular, and many holes appear and gradually become larger when the drying time treated with EHD is from 3 h to 18 h. This indicated that the EHD drying mainly causes small holes on the surface of Chinese wolfberry. The microstructure analysis further verified that the EHD partially combined with oven drying had a great impact on the surface structure of Chinese wolfberry, which in turn affects the drying speed, quality and various indexes.

Fig. 8.

The surface microstructure of Chinese wolfberry fruits under different drying conditions. (a) The control, (b) Oven drying, (c) EHD drying, (d) 3 h (EHD drying) + 60 °C, (e) 6 h (EHD drying) + 60 °C, (f) 9 h (EHD drying) + 60 °C, (g) 12 h (EHD drying) + 60 °C, (h) 15 h (EHD drying) + 60 °C, (i) 18 h (EHD drying) + 60 °C.

4. Discussion

Under the action of EHD drying, the transmembrane voltage at both ends of the cell membrane in the Chinese wolfberry fruits changes. Kotnik et al. thought that it can be described by the following formula [38]: $\begin{eqnarray}\displaystyle {\rm\Delta}{\varphi}_{m}=f\cdot R\cdot E\cdot \cos {\theta}(1-e^{-t/{\tau}}) & & \displaystyle\end{eqnarray}$ (11) where Δφ_m is the induced transmembrane voltage caused by a applied electric field, f is a function reflecting the electric and dimensional properties of the cell and the surrounding medium, R is the cell radius, E is the electric field strength, θ is the polar angle measured with respect to the direction of the field and τ is the time constant of the membrane which reads.

Under the action of high voltage electric field, the maximum membrane voltage is quickly reached on the cell membrane, and the cell membrane is thinned by the electric field force. When the transmembrane voltage reaches a certain critical value, the cell membrane will be broken down. It caused the cell membrane to begin to disintegrate to produce fine voids, and led to the entry of extracellular substances. The cells rupture, which in turn changes the permeability of the cell membrane and accelerates the evaporation of water inside the cells. The larger the pores of the cell membrane, the better the permeability. Which affects the effective moisture diffusion coefficient.

After EHD drying, the internal structure and the effective moisture diffusion coefficient of the Chinese wolfberry were changed, which can significantly accelerate the drying effect. On this basis, combined with oven drying, the drying effect can be improved. It is also mentioned in the literature that the drying effect can be significantly improved by EHD drying pretreatment and then drying with other drying techniques [39–44], which is consistent with our experimental results.

Polysaccharide is a heat-sensitive chemical component. The volatilization of Chinese wolfberry polysaccharide components has a great correlation with the temperature and the drying time. In the EHD partially combined with oven drying process, the drying speed of the Chinese wolfberry fruit is obviously improved, and the drying time is reduced, so that the polysaccharide content can be more effectively maintained. In the previous literature studies, it was also found that the polysaccharide content of Chinese wolfberry fruits was significantly increased after EHD drying compared to oven drying [30].

The EHD partially combined with oven drying is a new composite drying technology, which can make up for the disadvantages of EHD drying and oven drying, give full play to the advantages of both, and open up a new way for the composite drying technology.

5. Conclusions

Compared with EHD drying, the EHD partially combined with oven drying can significantly compensate for the disadvantage of drying speed in the latter half, improve the drying rate of Chinese wolfberry in the latter half, thus reducing drying time and better preservation of nutrients. Compared to oven drying, the EHD partially combined with oven drying not only has a faster drying speed and better drying quality. The EHD partially combined with oven drying technology has a high drying rate, and improves the drying quality of the material and reduces the energy consumption. So, it is a very potential drying technology.

Footnotes

Acknowledgements

This research was funded by National Natural Science Foundations of China (Nos. 51467015, 51767020 and 61961032), Natural Science Foundation of Inner Mongolia Autonomous Region of China (No. 2017MS(LH)0507), and Graduate Students’ Research and Innovation Funding Project of Inner Mongolia Autonomous Region (No. S2018111941Z). The authors also would like to express their gratitude to the anonymous referees for their valuable comments and suggestions.

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Effect of electrohydrodynamic partially combined with oven drying on Chinese wolfberry

Abstract

Keywords

1. Introduction

2. Materials and methods

2.1. Experimental equipment

2.3. Experimental method

2.4. Determination of relevant parameters

2.4.1. Moisture ratio

2.4.6. Polysaccharides contents

2.4.7. The effective moisture diffusion coefficient

2.4.10. Scanning electron microscopy

2.4.11. Statistical analysis

3. Results

3.1. Moisture ratio

3.5. Polysaccharides contents

Table 1 Polysaccharides contents under EHD drying, oven drying and the control (g/100 g) Control Oven drying EHD drying Polysaccharides 12.9 ± 0.03a 11.3 ± 0.06a 16.0 ± 0.03b

Table 3 Flavonoids contents under EHD drying, oven drying and the control (g/100 g) Control Oven drying EHD drying Flavonoids 0.11 ± 0.003a 0.13 ± 0.006ab 0.10 ± 0.003a

3.10. The surface microstructure

Footnotes

Acknowledgements

References

Table 1
Polysaccharides contents under EHD drying, oven drying and the control (g/100 g)

Control Oven drying EHD drying

Polysaccharides 12.9 ± 0.03^a 11.3 ± 0.06^a 16.0 ± 0.03^b

Table 3
Flavonoids contents under EHD drying, oven drying and the control (g/100 g)

Control Oven drying EHD drying

Flavonoids 0.11 ± 0.003^a 0.13 ± 0.006^ab 0.10 ± 0.003^a