The drying quality and energy consumption of Chinese wolfberry fruits under electrohydrodynamic system

Abstract

The aim of this work is to study the drying quality of Chinese wolfberry fruits and the effect of energy consumption on the drying process using electrohydrodynamic (EHD) drying. The drying rate, nutrient content and energy consumption of the Chinese wolfberry fruits which were dried in an EHD system at two levels of electric field intensity and five levels of the discharge gap were investigated and compared against oven drying and the control. The results showed that the drying rate have no significant difference at different voltages and discharge gaps under same electric field intensity. Experimentally, the polysaccharides content in the EHD system was improved when compared to oven drying. The specific energy consumption decreased with the decrease of the discharge gap and voltage. The drying rate can be enhanced and the energy consumption was greatly reduced at lower discharge gap and voltage in EHD system. The effect of degree of cell damage is smaller during EHD drying. Therefore, this work provides guidance for optimizing and improving the drying efficiency of Chinese wolfberry fruits in an EHD system.

Keywords

Electrohydrodynamic drying nutrient content energy consumption Chinese wolfberry

1. Introduction

Chinese wolfberry fruits are rich in nutrients, and have a medical role, therapeutic value and pharmacological effects [1, 2] such as enhancing the non specific immunity, anti-aging effect, lowering blood sugar, anti-tumor effect and anti-fatigue effect etc. But the fresh wolfberry fruits deteriorate rapidly after harvesting and the dried wolfberry is more popular because of longer shelf life and significant reduction in the volume of the product [3]. Through drying process, the moisture content of food products can be reduced in order to minimize the microbiological spoilage and deteriorative chemical reactions, also intended to decrease transportation and storage costs [4]. Therefore, it is necessary and great significance to dry the Chinese wolfberry fruits with short shelf life and economic value. Currently, many conventional drying techniques have been applied to the Chinese wolfberry such as solar drying [5], hot air [6] etc. But these drying methods cause loss of active ingredients and are a relatively energy intensive process. Therefore, it is significant to explore the new drying technology.

Figure 1.

(a) Schematic diagram of EHD drying. 1. Thermometer. 2. Hygrometer. 3. Sample. 4. Amperemeter. 5. Grounded electrode plate. 6. Needle electrode. 7. High voltage power source. 8. Voltage regulator. (b) Arrangement diagram of needle electrodes. 1. Needle electrodes. 2. Stainless steel wire. 3. Stainless steel frame. (c) Schematic diagram of needle electrodes.

Electrohydrodynamic (EHD) drying is an innovative and non-thermal drying technique [7, 8, 9]. The main mechanism of EHD drying is the production of corona wind in an electrostatic field and corona wind effects the moist surface and disturbs the saturated air layer; this phenomena leads to evaporation enhancement and consequently heat transfer augmentation [10]. Currently, many researchers have investigated its use for vegetables, fruits, seafood, grains, and other heat-sensitive materials [9, 11, 12, 13]. Hashinaga et al. showed that application of EHD accelerated the initial drying rate of apple slices by 4.5 times over the ambient air drying [14]. Alemrajabi et al. investigated carrot slices dried by EHD ${}^{+}$ drying, EHD ${}^{-}$ drying and oven drying, found that the energy consumption of EHD drying is far less than that of oven drying and the positive corona drying consumes less energy than the negative corona drying [11]. Taghian Dinani and Havet reported that the appropriate combination of electric field intensity with air velocity is a promising solution for reducing the substantial energy consumption of industries using convective drying technique [15]. Bai et al. showed that EHD drying consumed only 21.31% of the electric energy required for oven drying [16]. Bajgai and Hashinaga found that ascorbic acid contents in spinach were substantially higher after EHD drying than after oven drying [17]. Bai et al. found that sea cucumbers dried by EHD displayed much higher protein values, which is more than that of those dried oven [16]. In previous studies, parameters such as voltage, thickness and cross-sectional area of cooked beef, carotene content and rehydration ratio of carrots were investigated to determine the relationships and factors that affect EHD treatment, and many mathematical drying models were then compared to simulate drying curves of material [18, 19]. Both thickness and cross-sectional area of cooked beef had more impact on drying rate during the first half hour, and the Demir et al. model was the best mathematical model [18]. The drying rate of carrots was notably greater in the EHD system when compared to control, and quality, as determined by carotene content and rehydration ratio, was also improved when compared to oven drying [19]. We studied the drying characteristic of Chinese wolfberry fruits from the drying rate, the moisture rate, shrinkage rate, rehydration ratio, Vitamin C contents, and mathematical drying models under different voltages in an EHD system, and found that the drying rate was notably greater in the EHD system when compared to control, the EHD drying treatments have a significant effect on rehydration ratio, and Vitamin C contents [3]. The Parabolic model was best suited for describing the drying rate curve [3]. Despite the detail with which these studies treat the problem, few studies have systematically and comprehensively reported on retaining the nutritional quality of Chinese wolfberry fruits using EHD such as polysaccharides content, flavonoids content, etc, and the effect of the energy consumption on the drying process. And the drying efficiency and degree of cell damage at different voltages and discharge gaps between the emitting point and the grounded electrode under same electric field intensity was ignored in these studies.

In this paper, to further investigate the potential of this method for optimizing and improving the drying efficiency in an EHD system, we studied the drying characteristic of Chinese wolfberry fruits from the nutrient content and energy consumption. To accomplish this, Chinese wolfberry fruits were died in EHD drying system with the same electric field intensity under the different discharge gaps between the multiple needles electrode and the grounded electrode. The drying rate, the specific energy consumption, polysaccharides content, flavonoids content, and the electrical conductivity of Chinese wolfberry fruits were investigated to study the energy consumption and product quality on drying process using EHD.

2. Materials and methods

2.1 Experimental equipment

The experimental setup for EHD drying was showed in Fig. 1a. It is made up of a top electrode with multiple sharp pointed needles and grounded metallic plate. The gap between the emitting point and the grounded electrode was 40, 60, 80, 100 and 120 mm, respectively. The power source which can provide alternating current (AC) high voltage or direct current (DC) high voltage was connected to the top electrode plate. In order to get the conceivable high voltage for EHD drying, the power was connected to a voltage controller, with an adjustable voltage ranging from 0–70 kV for direct current (DC) or 0–50 kV for alternating current (AC). Both the top flat plate and the grounded plate electrode were 32 cm $\times$ 24 cm rectangular stainless steel plate. Figure 1b and c present the arrangement and schematic diagrams of the needle electrodes, respectively. The needles were 60 mm long with a diameter of 1 mm. Both the gap between two stainless steel wires and gap between two needle electrodes were 60 mm.

2.2 Determination of initial moisture content

The Chinese wolfberry fruits were purchased from a local farm (Tuoketuo county, Hohhot, China) and then immediately stored in a refrigerator to prepare for drying experiment. The weighed and clean Chinese wolfberry fruits were placed in the moisture meter (Sh10A, Shanghai Luheng Instrument Co., Ltd., Shanghai, China) at 70 ${}^{\circ}$ C temperature until a constant weight was acquired. The initial moisture content of Chinese wolfberry fruits was (75 $\pm$ 1)%.

2.3 Drying experiment

The drying experiments were carried out under the following environmental conditions: a natural convection process was assumed in drying process, the drying relative humidity is (50 $\pm$ 5)% and the drying temperature is (29 $\pm$ 2) ${}^{\circ}$ C.

The Chinese wolfberry fruits samples with homogeneous size and complete appearance were selected and immersed in 400 mL of 5% sodium carbonate solution for 10 minutes. After, poured away the sodium carbonate solution, the excess water on the surface of the Chinese wolfberry fruits samples was blotted up with tissue paper. Then 50 g of pretreated Chinese wolfberry fruits samples were put into the EHD system for two different drying experiments. One is changed discharge gap each time at 4 cm, 6 cm, 8 cm, 10 cm and 12 cm with multiple needles-to-plate electrode for AC electric field, the corresponding voltage is 0 kV (control), 14 kV, 21 kV, 28 kV, 35 kV and 48 kV, respectively. The other is same electrode experimental conditions with AC electric field, but the corresponding voltage is 0 kV (control), 18 kV, 27 kV, 36 kV, 45 kV and 54 kV, respectively, under DC electric field. Oven drying experiment was conducted at 55 ${}^{\circ}$ C with 50 g of pretreated Chinese wolfberry fruits to detect the drying quality and compared with the EHD drying and control. The mass of Chinese wolfberry fruits samples in the drying process was measured by a Sartorius Analytical Balance BS124S (Goettingen, Germany) every 1 hour. Every experimental treatment was independently implemented three times in this research.

The drying rate (DR) was calculated using Eq. (1) [10]:

$\displaystyle\text{DR}=\frac{M_{i}-M_{i+1}}{t_{i+1}-t_{i}}$ (1)

where $t_{i}$ is the time (h), $M_{i}$ is moisture content of Chinese wolfberry fruits at $t_{i}$ (kg water/kg dry matter).

2.4 The specific energy consumption

The specific energy consumption for the EHD drying (SEC ${}_{\text{EHD}}$ ) was calculated in kJ/kg using Eq. (2) [3, 15]:

$\displaystyle\text{SEC}_{\text{EHD}}=\frac{V\times I}{\Delta m}\times\Delta t$ (2)

where $\Delta t$ is the drying time, $\Delta m$ is the weight of the water evaporated from Chinese wolfberry fruits, $V$ is the voltage, $I$ is the current.

2.5 Polysaccharides content

Ultrasound-assisted extraction of polysaccharides was carried out following the method proposed by Aguiló-Aguayo et al. and Zhu et al. [20, 21], with some modifications. An amount of 1.0 g of dried milled Chinese wolfberry fruits were weighed and placed into centrifuge tubes. 5 mL of distilled water and 20 mL of absolute ethyl alcohol were added. Sonication was conducted by means of the Ultrasonic Processor for 30 min. After sonication, the extract was centrifuged at 4000 rpm for 10 min. Then the residue and supernatant separated. The precipitate was collected. 10 mL of ethanol was added, washed and centrifuged. The precipitate was transferred into the round-bottom flask and 50 mL of distilled water was added. 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.

2.6 Flavonoids content

Total flavonoids content was determined with spectrophotometric method [22, 23]. An amount of 1.0 g of dried milled Chinese wolfberry fruits were weighed and placed into 150 mL conical flask with stopper. 30 mL of methanol solution was added. The conical flask was shook with 160 r/min for 2 h at 65 ${}^{\circ}$ C in a thermostatic water bath oscillators. Then the mixed liquor was filtered. And the filter liquor was transferred into 50 mL volumetric flask. The methanol solution was added. The total volume of solution reached 50 mL. We used the standard rutin solution to set standard curve. The absorbency of this solution was measured at 420 nm according to the step of standard curve.

Flavonoids content was calculated using the following equation:

$\displaystyle\omega=\frac{C\times V\times U\times 100}{m\times 1000\times(100-% H)}\times 100$ (3)

Where $\omega$ is the flavonoids content of Chinese wolfberry fruits, $C$ is the numerical value of the total flavonoid concentration of the test solution to be calculated by the standard curve, $U$ is the total dilution of the sample, $V$ is the volume of liquid to be tested, $m$ is the mass of the sample, $H$ is the mass fraction of water in the sample.

Figure 2.

The drying rate of Chinese wolfberry fruits at the same electric field intensity under the different discharge gaps. (a) 3.5 kV/cm under AC electric field; (b) 4.5 kV/cm under DC electric field.

2.7 The electrical conductivity disintegration index

The electrical conductivity of the sample was measured immediately after EHD treatment with multiple needles-to-plate electrode for 5 hours using a conduct meter (SIN-CT-TDS3031, Hangzhou, China). The degree of cell damage, expressed as the electrical conductivity disintegration index, $Z_{p}$ , was estimated on the basis of the electrical conductivity values according to the following equation [24, 25].

$\displaystyle Z_{p}=\frac{\sigma-\sigma_{i}}{\sigma_{d}-\sigma_{i}}$ (4)

where $\sigma$ is the electrical conductivity at different EHD parameters, $\sigma_{i}$ is the conductivity of intact tissue, and $\sigma_{d}$ is the sample conductivity corresponding to completely destroyed sample treated by electric field. Thus, the cell disintegration index $Z_{p}$ can take values from 0 to 1 for intact and completely damaged samples, respectively. Electrical conductivity was measured continuously for at least 5 min and this measurement was performed in triplicate.

2.8 Statistical analysis

A factorial experiment in a Randomized Complete Block Design with two levels of electric field intensity was applied to contrast ten EHD treatments (4 cm–3.5 kV/cm, 6 cm–3.5 kV/cm, 8 cm–3.5 kV/cm, 10 cm–3.5 kV/cm and 12 cm–3.5 kV/cm under AC electric field, and 4 cm–4.5 kV/cm, 6 cm–4.5 kV/cm, 8 cm–4.5 kV/cm, 10 cm–4.5 kV/cm and 12 cm–4.5 kV/cm under DC electric field) with each other. Single-factor analysis of variance was applied to calculate the drying rate between the Chinese wolfberry fruits under alternating current electric field, direct current electric field and without electric field (control). The differences in drying rate are considered statistically significant when $p<$ 0.05. Polysaccharides content and flavonoids content also calculated between the Chinese wolfberry fruits treated under EHD drying and the control, oven drying using single-factor analysis of variance. The results reported in this study are presented as means $\pm$ standard deviation (SD).

3. Results and discussion

3.1 Drying rate analysis

Figure 2 depicts that variation of drying rate of Chinese wolfberry fruits with time at the same electric field intensity under the different discharge gaps. Figure 2a shows that all the drying rate of Chinese wolfberry fruits dried by EHD drying system with the five treatments (4 cm–3.5 kV/cm, 6 cm–3.5 kV/cm, 8 cm–3.5 kV/cm, 10 cm–3.5 kV/cm and 12 cm–3.5 kV/cm under AC electric field) were much faster than that of the control (0 kV) in the 25 h. In other words, EHD drying system had a major impact on enhancing the drying rate and improved by 1.8818, 1.7426, 1.8731, 1.6567 and 1.6726 times, respectively, at 4 cm–3.5 kV/cm, 6 cm–3.5 kV/cm, 8 cm–3.5 kV/cm, 10 cm–3.5 kV/cm and 12 cm–3.5 kV/cm treatments compared to that of the control in the 25 h. This result coincides with what has been found in other studies [10]. The reason for this is that corona wind generated by applied a high voltage can improve the convective heat transfer coefficient and evaporation rate on the sample surface exposed to convective flow, resulting in heat and mass transfer enhancement in the sample [4, 10]. There is a slight difference among 4 cm–3.5 kV/cm, 6 cm–3.5 kV/cm, 8 cm–3.5 kV/cm, 10 cm–3.5 kV/cm and 12 cm–3.5 kV/cm treatments on the drying rate. By ANOVA, the results indicate that there was no significant difference among 4 cm–3.5 kV/cm, 6 cm–3.5 kV/cm, 8 cm–3.5 kV/cm, 10 cm–3.5 kV/cm and 12 cm–3.5 kV/cm treatments in terms of the drying rate. Figure 2b depicts the same rule with Fig. 2a, and by ANOVA, the two groups have no significant difference ( $p>$ 0.05). This shows that the treatments at same electric field intensity have same drying rate.

3.2 Polysaccharides content

Table 1 shows the polysaccharides content of Chinese wolfberry fruits dried by four different treatments. The polysaccharides have been identified as main active components of Chinese wolfberry fruits [26]. The high quality dried Chinese wolfberry fruits is an important prerequisite to realize all its promising applications. Therefore, the polysaccharides content of Chinese wolfberry fruits is an important evaluation index of drying technology. The Chinese wolfberry fruits dried by oven had the lowest polysaccharides content. In other words, the Chinese wolfberry fruits were very easy to lose polysaccharides when they were dried by oven. Results show that the polysaccharides content of Chinese wolfberry fruits exposed to oven process was significantly ( $p<$ 0.01) less than that of the alternative current electric field, the direct current electric field and control, but there was no significant difference ( $p>$ 0.05) in the mean values of the polysaccharides content of the alternative current electric field, the direct current electric field and control. In fact, compared to oven, there has a increase by 32.55%, 29.55%, and 37.43% in the polysaccharides content of dried Chinese wolfberry fruits, respectively, at the alternative current electric field, the direct current electric field and control (Table 1).

Table 1
Polysaccharides contents of dried Chinese wolfberry fruits

Control	AC electric field	DC electric field	Oven
13.3020 $\pm$ 0.1972 ${}^{\text{b}}$	12.3411 $\pm$ 0.2533 ${}^{\text{b}}$	11.8160 $\pm$ 0.0995 ${}^{\text{b}}$	8.3240 $\pm$ 0.0460 ${}^{\text{a}}$

Data are shown as the mean $\pm$ standard deviation (SD). For each treatment, means with different lower case letters are significantly different ( $p<$ 0.01).

3.3 Flavonoids content

Table 2 depicts that the alternative current electric field, the direct current electric field and oven had no impact on the flavonoids content of Chinese wolfberry fruits. The flavonoids of Chinese wolfberry have many physiological activities, such as anti-cancer, anti-inflammation, and anti-atherosclerosis, which have been well documented [27]. Therefore, it is very important to keep the content of flavonoids in the drying process. By ANOVA, the results showed that the flavonoids contents of Chinese wolfberry treated at the alternative current electric field, the direct current electric field with multiple needles-to-plate electrode and oven have no significant difference ( $p>$ 0.05), compared to control.

Table 2
Flavonoids contents of dried Chinese wolfberry fruits

Control	AC electric field	DC electric field	Oven
0.310000 $\pm$ 0.000882 ${}^{\text{a}}$	0.308500 $\pm$ 0.000612 ${}^{\text{a}}$	0.300000 $\pm$ 0.000098 ${}^{\text{a}}$	0.387500 $\pm$ 0.000001 ${}^{\text{a}}$

Data are shown as the mean $\pm$ standard deviation (SD). For each treatment, means with same lower case letters are significantly different ( $p>$ 0.05).

3.4 The electrical conductivity disintegration index

Figure 3 depicts the electrical conductivity disintegration index of Chinese wolfberry fruits variation versus the different discharge gap under different treatments at five levels of discharge gap (4, 6, 8, 10 and 12 cm) and two levels of electric field intensity (3.5 kV/cm for AC electric field and 4.5 kV/cm for DC electric field). This figure shows that the cell disintegration index of Chinese wolfberry fruits at 3.5 kV/cm under AC electric field was higher than that of Chinese wolfberry fruits at 4.5 kV/cm under DC electric field. In addition, the electrical conductivity disintegration index of Chinese wolfberry fruits at 4 cm–3.5 kV/cm, 6 cm–3.5 kV/cm, 8 cm–3.5 kV/cm under AC electric field have very slight difference ( $p>$ 0.05) and the values of them were all 4.683%. There were a major effect at 10 cm–3.5 kV/cm and 12 cm–3.5 kV/cm under AC electric field. The electrical conductivity disintegration index under DC electric field increased with the increase of discharge gap, but there was dropped at 12 cm–4.5 kV/cm. Figure 4 represents the relationship between the electrical conductivity disintegration index and energy delivered to the tissue. Generally, the electrical conductivity disintegration index increased with the increase of energy. However, the values of this parameter treated by EHD at 4.5 kV/cm under DC electric field have very slight difference. And the energy delivered to the sample has a major effect on electrical conductivity disintegration index at 3.5 kV/cm under AC electric field. From Figs 3 and 4, we found that the electrical conductivity disintegration index is less than 0.13. This can be explained that the degree of cell damage may enhance transport processes of water in Chinese wolfberry fruits, but the effect is smaller during EHD drying.

Figure 3.

The electrical conductivity disintegration index under the different discharge gap.

Figure 4.

The electrical conductivity disintegration index under different energy delivered to the sample.

Table 3

Effect of voltage and discharge gap on average drying rate and the specific energy consumption (SEC ${}_{\text{EHD}}$ ) of Chinese wolfberry fruits dried by EHD drying system

The treatments	Drying rate(g kg ${}^{-1}$ water/dry matter*hour)	SEC ${}_{\text{EHD}}$ (kJ/kg water)
4 cm–18 kV (DC)	0.0326 $\pm$ 0.0043 ${}^{\text{a}}$	633.6
4 cm–14 kV (AC)	0.0336 $\pm$ 0.0044 ${}^{\text{a}}$	1235.6
6 cm–27 kV (DC)	0.0311 $\pm$ 0.0034 ${}^{\text{a}}$	1000.9
6 cm–21 kV (AC)	0.0316 $\pm$ 0.0005 ${}^{\text{a}}$	3181.2
8 cm–36 kV (DC)	0.0323 $\pm$ 0.0009 ${}^{\text{a}}$	1463.6
8 cm–28 kV (AC)	0.0334 $\pm$ 0.0012 ${}^{\text{a}}$	4840.6
10 cm–45 kV (DC)	0.0309 $\pm$ 0.0009 ${}^{\text{a}}$	2290.2
10 cm–35 kV (AC)	0.0298 $\pm$ 0.0016 ${}^{\text{a}}$	9414.3
12 cm–54 kV (DC)	0.0318 $\pm$ 0.0011 ${}^{\text{a}}$	3099.5
12 cm–42 kV (AC)	0.0303 $\pm$ 0.0013 ${}^{\text{a}}$	12426.8

Data are shown as the mean $\pm$ standard deviation (SD). For each treatment, means with same lower case letters are no significantly different ( $p>$ 0.05).

Figure 5.

The specific energy consumption of EHD drying versus different discharge gaps.

3.5 The optimization of EHD system on specific energy consumptions

Figure 5 indicates that the specific energy consumption of EHD drying at the same electric field intensity was significantly influenced by the discharge gap. The specific energy consumption increased with an increase in discharge gap both 3.5 kV/cm under AC electric field and 4.5 kV/cm under DC electric field. The energy consumption at 3.5 kV/cm under AC electric field was faster than that at 4.5 kV/cm under DC electric field. The average specific energy consumption of Chinese wolfberry fruits drying with 12 cm–3.5 kV/cm, 10 cm–3.5 kV/cm, 8 cm–3.5 kV/cm and 6 cm–3.5 kV/cm under AC electric field treatments were 10.06, 7.62, 3.92 and 2.57 times, respectively, compared to 4 cm–3.5 kV/cm under AC electric field treatment (Table 3). The average specific energy consumption with 12 cm–4.5 kV/cm, 10 cm–4.5 kV/cm, 8 cm–4.5 kV/cm, 6 cm–4.5 kV/cm under DC electric field treatments were 4.89, 3.61, 2.31 and 1.58 times, respectively, compared to the 4 cm–4.5 kV/cm under DC electric field treatment. The specific energy consumption of Chinese wolfberry fruits dried by EHD decreased with the decrease of the discharge gap (Table 3). In addition, the specific energy consumption of Chinese wolfberry of the 4 cm–3.5 kV/cm under AC electric field treatment is 1.95 times as much as that of 4 cm–4.5 kV/cm under DC electric field treatment. The same law is expressed in other groups, the ratio of the specific energy consumption between them are 3.18, 3.31, 4.11 and 4.00, respectively, at 6 cm, 8 cm, 10 cm and 12 cm. It is most surprising that the specific energy consumption of Chinese wolfberry of the 12 cm–3.5 kV/cm under AC electric field treatment is 19.61 times as much as that of 4 cm–4.5 kV/cm under DC electric field treatment. Therefore, those results show that the treatments with EHD drying at the same electric field intensity can obtain the same drying rate, and the energy consumption decrease with the decrease of the discharge gap and voltage between the two electrodes. And the specific energy consumption of EHD drying under DC electric field is less than that under AC electric field when they have same drying rate (Table 3).

Figure 6 showed the variation of ratio of the specific energy consumption (SEC ${}_{\text{oven}}$ ) of Chinese wolfberry fruits dried by oven and the specific energy consumption (SEC ${}_{\text{EHD}}$ ) of Chinese wolfberry fruits dried by EHD drying system with the two levels of electric field intensity (3.5 kV/cm under AC electric field and 4.5 kV/cm under DC electric field) and five levels of discharge gap (4 cm, 6 cm, 8 cm, 10 cm, 12 cm). This figure depicted that the ratio of SEC ${}_{\text{oven}}$ and SEC ${}_{\text{EHD}}$ decreased with the increase of discharge gap. The value of SEC ${}_{\text{oven}}$ /SEC ${}_{\text{EHD}(\text{12∼{}cm}-\text{3.5∼{}kV/cm})}$ is 4.85, which is the lowest average ratio. The value of specific energy consumption of Chinese wolfberry fruits dried by oven is 4.85 times as much as that of 12 cm–3.5 kV/cm under AC electric field treatment. This is in agreement with previous analysis of energy used up in EHD drying and oven drying [9]. But the values of SEC ${}_{\text{oven}}$ /SEC ${}_{\text{EHD}(\text{4∼{}cm}-\text{4.5∼{}kV/cm})}$ and SEC ${}_{\text{oven}}$ /SEC ${}_{\text{EHD}(\text{4∼{}cm}-\text{3.5∼{}kV/cm})}$ are up to 92.04 and 47.22, respectively. The results showed that the appropriate combination of the DC high voltage electric field with low discharge gap and low voltage can greatly reduce the energy consumption, and it can save energy consumption.

Figure 6.

Ratio of the specific energy consumption of oven and the specific energy consumption of EHD drying.

4. Conclusion

The EHD drying system had a major impact on enhancing the drying rate. The drying rate of Chinese wolfberry fruits exposed to EHD process on the same electric field intensity among different discharge gaps was the same.

Polysaccharides content, total flavonoids content of Chinese wolfberry fruits dried by the EHD drying system have no significant difference compared to control. But the polysaccharides content of Chinese wolfberry fruits exposed to oven was significantly less than that of the AC electric field, the DC electric field and control. The drying rate, under the same electric field intensity of the different discharge gaps were the same, but the specific energy consumption decreased with the decrease of the discharge gap.

The appropriate combination of the DC high voltage electric field with low discharge gap and low voltage can greatly reduce the energy consumption and can more save energy consumption.

Footnotes

Acknowledgments

This work was supported by National Natural Science Foundations of China (No. 51467015 and 61405099), Natural Science Foundation of Inner Mongolia Autonomous Region of china (No. 2017MS(LH)0507, 2015BS0311 and 2015MS0119) and Science Foundation of Inner Mongolia of Technology (No. ZD201311). The authors also would like to express their gratitude to the anonymous referees for their valuable comments and suggestions.

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The drying quality and energy consumption of Chinese wolfberry fruits under electrohydrodynamic system

Abstract

Keywords

1. Introduction

2.1 Experimental equipment

2.2 Determination of initial moisture content

2.3 Drying experiment

2.6 Flavonoids content

3. Results and discussion

3.1 Drying rate analysis

3.2 Polysaccharides content

Table 1 Polysaccharides contents of dried Chinese wolfberry fruits

Table 2 Flavonoids contents of dried Chinese wolfberry fruits

Footnotes

Acknowledgments

References

Table 1
Polysaccharides contents of dried Chinese wolfberry fruits

Table 2
Flavonoids contents of dried Chinese wolfberry fruits