Effects of high voltage electrostatic field and weak magnetic field assisted refrigeration on preservation of spinach

Abstract

With the improvement of food safety requirements, it is very important to find a safer and more effective physical method to replace the traditional chemical and physical processing of fruits and vegetables. Weak magnetic field and high voltage electrostatic field-assisted refrigeration is a new physical method of food preservation, which not only does not contaminate food but also avoids adverse effects on the human body. The effect of spinach preservation under different magnetic field strength and electric field strength assisted refrigeration was investigated, with conventional refrigerator preservation being used as a control. Results showed that the magnetic field intensity of 40Gs reduced the respiratory intensity, weight loss rate, and membrane permeability of spinach by 10.48%, 1.47%, and 1.98%, respectively, and increased the titratable acid content by 2.51%. The electric field intensity of 25 kV/m reduced the respiratory intensity, weight loss rate, membrane permeability, and titratable acid content of spinach by 16.6%, 3.4%, 5.5%, and 0.1%, respectively. That only lower magnetic field strengths and electric field strengths can have a positive effect on the preservation of spinach.

Introduction

Vegetables play a key role in people’s daily food supply system, providing the carbohydrates, minerals, vitamins and dietary fibre that humans need [1]. However, this situation is very serious in developing countries as some vegetables are prone to rotting due to their high water content [2]. According to the World Health Organisation, food contamination causes one in ten illnesses each year [3]. Therefore, it is very important to find a safe, reliable way to preserve vegetables.

In recent years, many researchers have begun to study the application of electromagnetic fields in the process of vegetable preservation, and have found that appropriate electromagnetic radiation has a bactericidal and disinfectant effect, which can slow down the aging of vegetables and reduce food decay [4]. Magnetic fields, as an electromagnetic technology, have the function of sterilising and disinfecting foodstuffs by using the non-thermal effect of magnetic fields to delay the ripening of fruits and vegetables, while maintaining the original colour, flavour and nutrition, and prolonging the shelf-life at room temperature without destroying the appearance of the fruits and vegetables [5]. Gao et al. [6] used alternating magnetic fields of different intensities to treat grapes, which significantly reduced the decay rate, decortication rate, sugar yield, pH and cell membrane permeability, and improved the freshness of grapes. Song et al. [7] investigated the effects of DC and alternating magnetic fields on the crystallisation process of carrots and found that both magnetic field methods reduced their damage to cell membranes and diminished the damage to carrot quality. Wang [8] tested the effects of different magnetic field strength and treatment time on the quality of honeydew melon, and the optimal strength and time of magnetic field treatment were obtained by determining its physiological and biochemical indexes, which provide some theoretical basis for application to other fruits and vegetables. Zhao et al. [4] proposed a preservation method combining a static magnetic field with low flux density and low-temperature water shock, and the experimental results showed that the temperature shock treatment and the electromagnetic effect had a positive impact on the pre-cooling, preservation quality and physicochemical properties of cucumber.

At present, electric field preservation devices are mainly divided into high-voltage electrostatic field, low-voltage electrostatic field, high-voltage variable frequency electric field, low-voltage alternating electric field and so on [9]. Kharel et al. [10] has treated a variety of fruits and vegetables including pears, plums, bananas, apples, and chillies with high voltage electrostatic field, the respiration rate of pears, plums and bananas decreased after the electric field treatment, and chillies retained its quality better and prolonged its storage time. Hu and Chen [11] investigated the effect pattern of high voltage electrostatic field on fresh lotus root, and the results showed that high voltage electrostatic treatment could inhibit the respiratory intensity of the cells to some extent during the storage process of lotus root, but the difference between the soluble protein content, reducing sugar and total phenol of lotus root in the test and the control group was very small. Lin et al. [12] investigated the preservation effect of nanosecond pulsed electric field on cherries under the conditions of field strength 0.9 kV/cm, frequency 6 kHz, and treatment time 60 s. Various indexes showed that the nanosecond pulsed electric field had a certain effect on the preservation of cherries.

In summary, the form and intensity of magnetic and electric fields affect different ingredients differently, and the temperature height and fluctuation during processing are also different, making it difficult to draw uniform conclusions. Therefore, it is particularly necessary to develop appropriate magnetic and electric field assistive devices for leafy vegetables represented by spinach and for refrigerators of different refrigeration types and models.

In this paper, the effect of different magnetic and electric fields on the preservation of spinach was investigated. The magnetic field strengths were 12.5Gs, 25Gs, 40Gs, 60Gs, 80Gs and 100Gs, and the electric field strengths were 25 kV/m, 50 kV/m, 75 kV/m and 100 kV/m, respectively. Spinach preservation was characterised in terms of respiratory strength, membrane permeability, titratable acid content and weight loss, respectively.

Experimental apparatus and methods

Magnetic field experimental materials and setup design

Experimental materials

The material used for this experiment was fresh spinach, which was tested and stored in an air-cooled refrigerator at a set temperature of 4 °C, while the test environment was at 23 °C.

Seven groups were set up for the experiment and the quality of spinach in each group did not differ by more than 1%. The spinach was treated with magnetic field strengths of 0Gs, 12.5Gs, 25Gs, 40Gs, 60Gs, 80Gs, and 100Gs for 2 h during refrigeration. Bajpai et al. [13] used static magnetic fields to treat Escherichia coli and Staphylococcus epidermidis in sintered hydroxyapatite and found that magnetic field exposure for 2 h had the best antibacterial effect.

Therefore, the whole experiment started at 8 am and ended at 2 pm. Temperature equilibrium was carried out for 4 h prior to treating the spinach, which was treated from 12 noon, and after 2 h of treatment, the data was measured from 2 pm. for 7 consecutive days, testing the index at each interval of 1 day. Since a certain amount of heat is generated when the coil is energized, putting the coil in the cooler requires waiting for the internal temperature of the cooler to maintain equilibrium before putting the spinach in. In order to reduce the temperature difference between the treatment group and the control group, the two groups of samples were placed as shown in Fig. 1, with the treatment group in the lower coil and the control group in the upper coil, and a porous adiabatic uniform flow plate was placed in the middle, so that the heat generated in the lower coil was released to the upper coil through the uniform flow plate. Take 150Gs magnetic field strength as an example, test the magnetic field area, the control group area and the temperature change inside the refrigerator, where 1# is the magnetic field area, 3# is the control group area and 2# is the temperature change inside the refrigerator, the change curve is shown in Fig. 2, the upper and lower temperature difference is 0.5 °C.

Fig. 1

Schematic diagram of the experimental system. Where: 1—DC power supply, 2—Magnetic field area, 3—Air-cooled refrigerator, 4—Helmholtz coil, 5—Control group area, 6—Fiber optic thermometer, 7—Data acquisition system

Fig. 2

Internal temperature equilibrium curve of refrigerator at 150GS

The coil was made of 1 mm enameled copper wire. The power supply was WYK-10020 DC power supply, the output voltage can be adjusted within 0–100 V and the output current can be adjusted within 0–20 A.

Design of magnetic field devices

Magnetic field treatment devices can utilise the properties associated with the magnetic effect of electric current. A magnetic field is generated on an energised coil and is used to study the optimum treatment conditions for different fruits and vegetables. To make the magnetic field more homogeneous, the coil was wound in two circles in the form of a Helmholtz coil. Two sets of coaxial coils were connected in series in the same direction, spaced at the radius of the circular coil. A magnetic field area was formed between the two coils when the coil was energized. a, b and c were the locations of the cut-off lines. The specific model is shown in Fig. 3. The coils were wound from 1 mm diameter enameled copper wire, with a circular inner diameter of 0.03 m and a spacing of 0.015 m between the two coils, with each set of coils wound 800 times. A map of the magnetic field distribution in the area between the two coils was obtained. A stable magnetic field can be formed in the middle region of the Helmholtz coil at a certain voltage. The magnetic field distribution is shown in Fig. 4, and it can be found that the magnetic lines in the center region of the two coils were relatively uniform.

Fig. 3

Model and truncation locations

Fig. 4

Distribution of magnetic lines of force within a cylinder

It can be seen in Fig. 5 that the magnetic field in the center region of the two coils is relatively uniform, with one end of a circular coil as the starting point, and there is basically no gap in magnetic field strength near 0.015 m at the three truncated positions along the radius.

Fig. 5

Magnetic field distribution along the radius of the cylindrical circular section (25.5 V)

Electric field experimental materials and setup design

Experimental materials

The material used for this experiment was fresh spinach, which was tested and stored in an air-cooled refrigerator at a set temperature of 4 °C, while the test environment was at 23 °C.

The experiment was divided into five groups, each differing by no more than 1% in spinach mass, which were treated for 2 h at electric field strengths of 0 kV/m, 25 kV/m, 50 kV/m, 75 kV/m, and 100 kV/m, respectively. The whole experiment started at 8 am and ended at 2 pm. Temperature equilibrium was carried out for 4 h prior to treating the spinach, which was treated from 12 noon, and after 2 h of treatment, the data was measured from 2 pm. 7 days of continuous treatment, every interval of a day to test indicators, another group as a control. Since the electric field device does not generate additional heat during the energization process, the temperature measurement points are arranged as in Fig. 6, and the temperature equilibrium curves inside the refrigerator in the control group area and the treatment group area when the test electric field is turned on are shown in Fig. 7, and the schematic diagram of the experimental system is shown in Fig. 8. It can be seen through Fig. 7 that the temperature changes in the treatment group area and the control group area are the same when the electric field experiment is conducted.

Fig. 6

Electric field experiment temperature measurement point arrangement

Fig. 7

Temperature change curve inside the refrigerator during the electric field experiment

Fig. 8

Diagram of the whole electric field equipment. Where: 1—High voltage DC power supply, 2—Electric field area 3—Cooler, 4—Acrylic cabinet, 5—Electrode plate, 6—Control area, 7—Data acquisition system, 8—Fibre optic pyrometer

Design of the electric field device

The electrostatic field experimental equipment was mainly composed of two parallel electrode plates and a high voltage DC power supply. The two electrode plates were divided into upper and lower electrodes, and there was a certain distance between them, the two poles of the high-voltage DC power supply were connected separately, where a more uniform high-voltage electrostatic field was generated between the poles. Among them, the upper pole plate can be different according to the shape so that different high-voltage electrostatic fields can be formed, for example, needle electrode, and plate electrode, while the lower pole plate used a plate electrode. In the case of needle and plate electrodes, a non-uniform electric field, also known as a corona field, was formed, in the case of two plate electrodes a uniform electric field was formed. In contrast, a uniform electric field formed by two plate electrodes was used in this study, with a maximum output voltage of + 50 kV/m from the DC power supply.

The intensity of the electric field between two parallel electrode plates was regulated by varying the output voltage. The electric field strength was calculated as follows.

$$E = \frac{U}{H},$$

(1)

where E electric field strength (kV/m), U output voltage of the high-voltage DC power supply (kV), H Two electrode plate spacing (m).

To ensure the safety of the experiment, one electrode plate used in the electric field was fixed to one side of the acrylic box, while another electrode plate was placed in the box with a slot, and the distance between the two electrode plates was 16 cm. Two steel plates were energized, plate 5a was connected to the positive pole and plate 5b to the negative pole, forming an electric field region in the middle as shown in Fig. 9. To ensure the condition that the difference in temperature between the control and treatment groups was small, the control group was placed on the other side of the chamber and no electric field was formed in the control area at this time.

Fig. 9

Electric field distribution diagram

Testing methods and indicators

Changes in the appearance of spinach

For fresh ingredients with and without the application of magnetic and electric fields, changes in the appearance and quality of the samples were observed at 1-day intervals in the test environment. The observation was made for 7 consecutive days and recorded [2].

Respiratory intensity measurement

Respiratory intensity is a common physiological index to evaluate the strength of respiration, and it is also one of the important indicators of the speed of metabolism of fruits and vegetables. It refers to the amount of carbon dioxide released per unit time and per unit weight of fruits and vegetables under certain temperature conditions. The storage potential of fruits and vegetables can be estimated from the respiration intensity, the storage capacity of fruits and vegetables is inversely proportional to the intensity of respiration, the greater the intensity of respiration, the faster the nutrients are consumed, and the faster the ripening and aging, and the shorter the storage time. This study uses the resting method to determine the respiratory intensity of spinach [14].

$${\text{Respiratory}}\,{\text{intensity}}\,\left( {{\text{CO}}_{2} {\text{mg}}/{\text{kg}}{\mkern 1mu} {\text{h}}} \right) = \frac{{\left( {V_{1} - V_{2} } \right) \times C \times 44}}{M \times T},$$

(2)

where V₁ the amount of oxalic acid used for experimental titration in the control group (ml), V₂ amount of oxalic acid used for experimental titration in the experimental group (ml), C concentration of oxalic acid (mol/l), M mass of spinach (kg), T measurement time (h).

Membrane permeability determination

Fruit and vegetable cell membranes play an important role in maintaining normal cell metabolism. The cell membrane has a selective permeability function. During the aging process of fruits and vegetables, the functional activity of the cytoplasmic membrane decreases, the membrane permeability increases, and the outward permeation of internal cellular dielectric occurs. When plant tissues are exposed to various adverse environmental hazards, the structure and function of cell membranes are the first to be injured, and cell membrane permeability increases. If the injured tissue is immersed in ion-free water, the amount of electrolytes in the extravasated fluid is greater than that in the normal tissue extravasated fluid, and the more severely the tissue is injured, the greater the increase in electrolyte content. By measuring the conductivity of fruit and vegetable tissue exudate, it is possible to understand the changes in cell membrane permeability of fruits and vegetables, reflecting the degree of injury to fruits and vegetables. The relative conductivity is generally used to express the cell membrane permeability [15].

$$L\left( \% \right) = \frac{{L_{1} }}{{L_{2} }} \times 100,$$

(3)

where L membrane permeability of spinach, L₁ conductivity of the exudate before tissue killing, L₂ conductivity value of the exudate after tissue killing.

Determination of titratable acid content

Fruits and vegetables contain various organic acids, and different kinds of fruits and vegetables contain different kinds of organic acids, even if the same fruit and vegetable varieties, their different degrees of maturity, the content of organic acids also has a large difference. Therefore, the amount of acid in fruits and vegetables is one of the important indicators of quality, it is closely related to the flavor of fruits and vegetables. Its total acidity or titratable acid content is determined by the acid–base neutralization principle, spinach mainly contains oxalic acid [16].

$${\text{Titratable}}\,{\text{acid}}\,{\text{content}}\left( \% \right) = \frac{{B \times 100 \times C_{1} \times V_{{\text{A}}} \times V_{{\text{T}}} }}{{M_{1} \times V_{{\text{B}}} }},$$

(4)

where B Conversion factor, C₁ concentration of NaOH solution for titration (mol/l), V_A volume of NaOH solution for titration (ml), V_T Total volume of sample extract (ml), M₁ Fresh weight of the sample (g), V_B the volume of extract for titration (ml).

Determination of weight loss rate

The weight loss of the spinach is determined and the mass was tested every two days. The weighing is done using electronic scales, data recorded, and collated [17].

$${\text{Weight}}\,{\text{loss}}\,{\text{rate}}\left( \% \right) = \frac{{\left( {M_{1} - M_{2} } \right)}}{{M_{1} }} \times 100,$$

(5)

where M₁ Pre-storage, M₂ Post-storage.

Statistical analysis

The experiment results were analyzed using analysis of variance (ANOVA) carried out by SPSS software version 22 (SPSS Inc., Chicago, IL, USA). Differences between means were assessed by Duncan’s test with differences being considered significant at P < 0.05. Different capital letters indicate significant differences between groups at P < 0.05, and different lowercase letters indicate significant differences within groups at P < 0.05 [18].

Results and discussion

Results and discussion of magnetic fields at 40Gs, 60Gs, 80Gs and 100Gs

The appearance of spinach

The ideal appearance quality of spinach is mainly reflected in the turquoise color and adequate moisture in the leaves. As can be seen in Fig. 10, the freshness of spinach gradually deteriorates with storage time, Spinach treated with a magnetic field strength of 40Gs had the best preservation effect throughout the trial, with a better appearance from the beginning to the end than the control group, providing a preservation effect. At a magnetic field strength of 20Gs, the appearance of spinach was better than the control on the 1st, 5th, and 7th day and slightly worse than the control on the7th day. The magnetic field strength of 60Gs was essentially identical in appearance to the control group throughout the test period, In agreement with the results of [19] on fresh mangoes treated under different magnetic fields, there was a change in appearance with the increasing number of days, as well as a change in quality and hardness, with the added magnetic field providing a good preservation effect. It can therefore be seen that at lower magnetic field strengths can have a positive effect on the preservation of spinach.

Fig. 10

Changes in the appearance of spinach

Respiratory strength test results

Respiratory intensity is a commonly used physiological index to evaluate the strength of respiration and is one of the important indicators of the fast and slow metabolism of fruits and vegetables. It can be seen in Fig. 11 that with the increase of storage time, at the magnetic field strength of 60Gs, 80Gs, and 100Gs, the respiration intensity of spinach changed in the same trend as the control group. The respiratory intensity of 40Gs was slightly lower than that of the control group in the first 3 three days, but there was no significant difference between the two groups (P > 0.05), and on the seventh day there was a significant difference with the control group (P < 0.05), 63.59% reduction. The respiratory intensity at 60Gs and 80Gs was slightly higher than the control group on the first and fifth days, but there was no significant difference between the three groups (P > 0.05), both peaking on the first day with values of 119.22 mg/(kg h) and 121.68 mg/(kg h). In agreement with the results of Zhang et al. [2] on strawberries under different magnetic fields, the respiration intensity of the treated group was lower than that of the control group in the lower magnetic field environment. It can be seen that the lower intensity magnetic field inhibited the respiration of spinach and enhanced the freshness of spinach, while the higher intensity magnetic field promoted the respiration of spinach and made the nutrients depleted more rapidly, thus affecting the storage quality of spinach. This indicates that with the passage of storage time, the effect of the magnetic field can be more clearly manifested.

Fig. 11

Respiratory strength test results at different magnetic field strengths

Membrane permeability test results

Membrane permeability is one of the important indicators affecting the maturity and senescence of fruit and vegetable tissues. As can be seen from Fig. 12, the membrane permeability of spinach increased with storage time, which is due to the gradual senescence of the leaves over time and the accelerated outward permeation of the dielectric in the cells, all reaching a peak on the seventh day with values of 23.89%, 21.23%, 22.62%, 22.48% and 24.18%, respectively. Among them, the membrane permeability test results were smaller in the 40Gs magnetic field treatment group than in the control group, and there was a significant difference (except for the first day) between the two groups (P < 0.05). The membrane permeability was slightly higher in the 60Gs and 80Gs magnetic field treatment groups than in the control group, but there was no significant difference between the three groups (P > 0.05). The results of membrane permeability testing were greater in the 100Gs magnetic field treatment group than in the control group, and there was a significant difference between the two groups (P < 0.05). And the 100Gs magnetic field treatment group had the largest membrane permeability test result and the worst effect. The rate of increase in membrane permeability of spinach for each treatment group can also be seen in the figure, with the fastest increase in membrane permeability for each treatment group from the first day to the third day, increasing by 52.95%, 53.33%, 61.63%, 69.1%, and 71.33%, respectively. Overall, the membrane permeability of the 40Gs magnetic field treated group was always lower than that of the control group throughout the storage process. This is consistent with the effective reduction in respiration intensity, decay rate, and membrane permeability when cantaloupe was treated with an alternating magnetic field for 15 min by Wang [8]. Therefore, the 40Gs magnetic field can significantly inhibit the increase in membrane permeability of spinach during low-temperature storage.

Fig. 12

Membrane permeability test results at different magnetic field strengths

Test results for titratable acid content

Fruits and vegetables contain various organic acid, and different types of fruits and vegetables contain different types of organic acids, and even for the same fruit and vegetable species with different maturity levels, the content of organic acids varies greatly. From Fig. 13, it can be seen that the experimental and control groups of spinach during storage could titratable acid content showed the same trend of gradual decrease, and the titratable acid content all reached the peak on the first day with 15.33%, 15.78%, 15.2%, 14.49%, and 14.03%, respectively. The titratable acid content of spinach in the 40Gs magnetic field treatment group decreased slowly and was significantly different from that of the control group (P < 0.05), and the content was much higher than that of the control group for the same number of storage days, maintaining a better storage quality, and the titratable acid content increased by 88.37% in the seventh day compared to the control group, implying a better preservation effect. Spinach in the 60Gs magnetic field treatment group had slightly higher titratable acid content than the control group at days 3 and 5 of storage, but the results were not significantly different between the two groups (P > 0.05) and were higher than the control group at day 7. Spinach in the 80Gs and 100Gs magnetic field treatment groups had lower titratable acid content on days 1, 3, and 5 relative to the control group, and slightly higher than the control group on day 7, but the results were not significantly different between the three groups (P > 0.05), so the freshness preservation effect was not significant. Gao et al. [6] treated grapes using different magnetic fields and the titratable acid content of grapes increased significantly, in agreement with the experiment on spinach. Therefore, when titratable acid content was used as a criterion, the best results were obtained in the 40Gs magnetic field treatment group, followed by the 60Gs magnetic field treatment group, the control group, the 80Gs magnetic field treatment group, and the 100Gs magnetic field treatment group.

Fig. 13

Test results for titratable acid content at different magnetic field strengths

Weight loss rate test results

Weight loss is the most important sensory quality which directly reflects the quality and shelf life. As can be seen from Fig. 14, the weight loss rate of spinach continued to increase in all groups during the test period, and according to the data, the rate of increase in weight loss of spinach varied considerably with the increase in storage time, with the control group having 20%, 63% and 35% increase in weight loss throughout the storage period, respectively. This also indicates that the nutrient loss in spinach varies greatly during storage. All peaked on the seventh day with weight loss rates of 18.33%, 16.04%, 18.6%, 23.16% and 24.13%, respectively. Among them, spinach in the 40Gs magnetic field treatment group lost less weight and lost material more slowly than the control group throughout the experimental period, and there was a significant difference (P < 0.05) between them and the control group (except for the first day). The rate of weight loss in the early experimental period was slightly higher in the 60Gs magnetic field treatment group than in the control group, but there was no significant difference (P > 0.05) between the two groups (except for the third day). The spinach in the 80Gs and 100Gs magnetic field treatment groups had greater weight loss throughout the experimental period compared to the control group, and the 100Gs magnetic field treatment group kept a higher gap in weight loss throughout the storage period compared to the other groups, while the 80Gs magnetic field treatment group only kept a smaller range of weight loss with the other groups in the later stages of storage, and there was no significant difference between the 80Gs and 100Gs groups (P > 0.05), but the results were significantly different compared to the control group (P < 0.05). On day 7, weight loss increased by 1.4%, 26% and 31.6% at 60Gs, 80Gs and 100Gs, respectively, and decreased by 12.5% at 40Gs relative to the control group. Liu and Li [20] reached consistent conclusions in experiments with tomatoes, mangoes, and bananas. The weight loss rate would continue to increase continuously, but the treatment group with lower magnetic field increased more slowly than the control group, delaying the decay of spinach. Therefore, it can be found that the magnetic field of lower intensity has a good effect on the freshness of spinach, while the magnetic field of higher intensity than about 60Gs will greatly reduce the freshness storage time of spinach.

Fig. 14

Weight loss rate test results at the same magnetic field strength

Comparison of the results of different magnetic field strengths

The differences between the treatment groups and the control group were combined for different strengths of magnetic fields to identify the magnetic field strengths that had the most significant effect on spinach quality. Table 1 shows the average differences between the experimental results of several treatment groups and the results of the control group.

Table 1 Mean of the difference in results between the treatment and control groups at different times

As can be seen from Table 1, the treatment group with a magnetic field strength of 40Gs was better than the control group in all indicators. The treatment group with a magnetic field strength of 60Gs was better than the control group in terms of titratable acid content and worse than the control group in all the remaining indicators, although a small difference could be seen between them. The treatment groups with a magnetic field strength of 80Gs and 100Gs were worse than the control group in all indicators, especially in the respiratory strength indicator. In summary, the best preservation was achieved at a magnetic field strength of 40Gs.

Results and discussion of the 12.5Gs and 25Gs magnetic fields

In order to further investigate the preservation effect of spinach at lower magnetic field strengths, spinach was next studied further at 12.5Gs and 25Gs magnetic field strengths respectively and compared with at magnetic field strengths of 40Gs, 60Gs, 80Gs and 100Gs.

Appearance of spinach

From the appearance graph of spinach in Fig. 15, it can be seen that the appearance of freshness of spinach changed with the extension of preservation time at different magnetic field strengths, and the preservation effect of 12.5Gs was slightly better than that of 25Gs at the magnetic field strengths of 12.5Gs and 25Gs. Lin et al. [12] investigated the effect of static and alternating magnetic field treatment on the thermal properties and freshness of cherry tomatoes, and the results showed that the static magnetic field at 40Gs showed the least color difference and all other indicators were better than the control group, which delayed the ripening process and maintained the freshness of cherry tomatoes after harvesting to some extent.

Fig. 15

Changes in the appearance of spinach

As can be seen in Figs. 16, 17, 18, 19, there was a significant difference (P < 0.05) in the preservation effect on spinach at different magnetic field strengths, with respiration intensity at 12.5Gs magnetic field strength being lower than the control group on days 1, 3 and 5 and higher than the control group on day 7 with a value of 209.77 mg/(kg h), an increase of 27% relative to the control. The membrane permeability was lower than the control group on days 1, 3, and 7 and higher than the control group on day 5 with a value of 63.15%, which was a 12% increase relative to the control group. The titratable acid content was higher than the control group on day 5 and lower than the control group the rest of the time and peaked on day 1. The weight loss rate increased with storage time and the trend was consistent with the control group and the results were not significantly different from those of the control group (P > 0.05). The respiratory intensity was always higher than that of the control group at a magnetic field strength of 25Gs and reached a peak value of 222.43 mg/(kg h) on day 3. The results of membrane permeability were lower than those of the control group, reaching a peak value of 54.14% on day 5, and decreasing by 15% on day 7 relative to the control group. The results of the control group were not significantly different from those of the control group (P > 0.05). Zhang et al. [2] investigated to explore the effect of a combined pulsed magnetic field (PMF) and cold water shock pretreatment on postharvest cucumber quality. The results showed that the combined PMF and cold water pretreatment had better preservation quality, and when the magnetic flux density of PMF was 40Gs, the samples showed the lowest color change and decay rate, inducing higher film integrity and prolonging their shelf life. Therefore, it can be concluded that low magnetic field strength played a small role in inhibiting the decay of spinach and the preservation effect was not significant.

Fig. 16

Respiratory strength test results under different magnetic field strength

Fig. 17

Membrane permeability test results under different magnetic field strengths

Fig. 18

Test results of titratable acid content under different magnetic field strength

Fig. 19

Weight loss rate test results under different magnetic field strength

Comparison of the results of different magnetic field strengths

Combining the differences between the treatment and control groups with different strengths of magnetic fields to find the magnetic field strength that has the most significant effect on spinach quality. Table 2 shows the mean difference between the experimental results of several treatment groups and the results of the control group.

Table 2 Mean of the difference in results between the treatment and control groups at different times

As can be seen from Table 2, the treatment group with a magnetic field strength of 12.5Gs was better than the control group in terms of weight loss rate and membrane permeability, and the rest of the indexes were slightly worse than the control group. The treatment group at a magnetic field strength of 25Gs was better than the control group in weight loss rate, membrane permeability, and titratable acid content, and slightly worse than the control group in respiratory intensity, but a small difference could be seen between them. In contrast, the treatment group with a magnetic field strength of 40Gs was better than the control group in all indexes, especially in the index of respiratory strength, the difference between the two groups reached 10.48%, which played a preservation effect. Combined with Table 1, it can be seen that the magnetic field strength at 40Gs plays a role in the preservation of spinach and inhibits its decay of spinach.

Results and discussion of the electric field

Appearance of spinach

In the experiments with electric fields, the experiments were conducted twice 0 kV/m, 25 kV/m, 50 kV/m, and 0 kV/m, 75 kV/m, and 100 kV/m. Therefore, different control groups were chosen to measure the data and to compare the trends of each indicator to reach the optimal conclusion.

The ideal appearance quality of spinach is mainly reflected in the turquoise color and adequate moisture in the leaves. As can be seen from Fig. 20, the freshness of spinach gradually deteriorates with storage time, and spinach treated with an electric field intensity of 25 kV/m had better freshness than the control group throughout the test period. At an electric field intensity of 50 kV/m, the appearance of spinach was better than that of the control on the 1st, 3rd, and 5th days, and significantly worse than that of the control on the 7th day. The electric field intensities of 75 kV/m and 100 kV/m were worse in appearance than the control group throughout the test period. The loss of appearance in the 75 kV/m electric field treatment group remained within a small range of variation during the period of storage, and at 100 kV/m there was a significant change in the appearance of the spinach in the later stages, which played an inverse role in preservation. Dai [21] also found that the color appearance of fresh-cut peaches changed as the storage time became longer, and the changes were more pronounced in 3–7 days, comparing the changes with the addition of electric field, the high voltage electrostatic field treatment could alleviate the appearance changes during storage of fresh-cut peaches at the same and temperature. So it can be found that the lower intensity of electric field strength has a good effect on the freshness of spinach.

Fig. 20

Changes in appearance of spinach

Weight loss rate test results

As can be seen in Fig. 21, the weight loss rate of spinach increased continuously with the storage time. There was a significant difference between the treated and control groups (except for the first day) (P < 0.05) when the spinach was treated at 25 kV/m electric field strength, the weight loss rate was lower than the control group on day 1, 5 and 7, slightly higher than the control group on the third day, and decreased by 30.8% relative to the control group on day 7. At 50 kV/m electric field strength, both the control and treated groups reached their peak on day 7, respectively the electric field intensity of 50 kV/m was slightly less effective than that of 25 kV/m. The electric field intensity of 75 kV/m and 100 kV/m had a greater weight loss than that of the control group throughout the test period, and the results were significantly different from the control group with the increase of storage time (except the first day) (P < 0.05), and both reached the peak on the 7th day, with an increase of 34% and 68.49%, respectively, relative to the control group. Therefore, it can be found that the lower intensity of electric field strength has a good effect on the freshness preservation of spinach, while the electric field of strength higher than about 50 kV/m will greatly shorten the freshness storage time of spinach.

Fig. 21

Weight loss rate test results under different electric field strength

Respiratory strength test results

As can be seen in Fig. 22, the test results were slightly higher than those of the control group on day 3 at 25 kV/m, but there was no significant difference between the two groups (P > 0.05), and the respiratory intensity was lower than that of the control group at all other times, reaching a peak on day 7, which was 27% lower than that of the control group. The respiratory intensity of the electric field strength of 50 kV/m was higher than that of the control group on day 3, and the results were significantly different between the two groups (P < 0.05), with an increase of 31.82% relative to the results of the control group, and the rest of the time was lower than that of the control group, and the preservation effect was slightly worse than that of 25 kV/m. The electric field intensity of 75 kV/m was lower than the control group in respiration intensity during the storage of spinach on day 7, and the results were significantly different between the two groups (P < 0.05), and the respiration intensity was higher than the control group for the rest of the time, with a peak of 43.99% increase on day 5. The respiration intensity of spinach in the 100 kV/m treatment group, on the other hand, was always higher than the control group, and the results were significantly different between the two groups (P < 0.05), with the increase in storage days, and respiratory intensity increased by 41%, 264.4%, 29.93%, and 60.76%, respectively. This shows that with 50 kV/m as the boundary, the lower intensity electric field inhibits spinach respiration and enhances the freshness of spinach, while the higher intensity electric field promotes spinach respiration and makes spinach nutrients depleted more quickly, thus affecting the storage quality of spinach. Yang et al. [22] used 80 kV/m high voltage electrostatic field to treat Red Star apples for 1 min and then stored them for 3 months (0 °C, 90% humidity), and their hardness and soluble solids content increased by 10% and 1.8%, respectively, and their respiratory intensity decreased by about 20% compared to the control group. This is consistent with the conclusions drawn in the study. This also indicates that the effect of electric field preservation can be more obvious with the storage time.

Fig. 22

Respiratory strength test results under different electric field strengths

Membrane permeability test results

It can be seen from Fig. 23 that the membrane permeability of spinach increased with storage time, which is due to the gradual senescence of the leaves over time and the faster outward permeation of dielectric in the cells. Among them, the electric field intensity of 25 kV/m treated groups all had lower membrane permeability than the control group and the results were significantly different between the two groups (P < 0.05), reaching a peak value of 55.4% on day 7 and the most significant preservation effect on day 5, with a 10.7% reduction in membrane permeability relative to the control group. The membrane permeability test results of the treated group at 50 kV/m electric field intensity were higher than the control group on day 1 and lower than the control group for the rest of the time, reaching a peak value of 59.37% on day 5 and the most significant preservation effect on day 7, with a reduction of 8.41% relative to the results of the control group. The membrane permeability of spinach was higher than that of the control throughout the preservation period at an electric field strength of 75 kV/m, and at an electric field strength of 100 kV/m, the membrane permeability was lower than that of the control only on the third day. Dai [21] studied the changes in fresh-cut peaches at different temperatures with different electric field intensities, and the strong electric field induced a rise in cell permeability of fresh-cut peaches under low-temperature refrigeration conditions, and the cell permeability with the electric field was lower than that of without the electric field, which is consistent with the conclusions obtained from this experiment. Therefore, it can be concluded that the electric field intensity treatment group of 25 kV/m can significantly inhibit the rise of membrane permeability of spinach during low-temperature storage.

Fig. 23

Membrane permeability test results under different electric field strengths

Test results of titratable acid content

From Fig. 24, it can be seen that the experimental and control spinach groups showed the same trend of change in titratable acid content during storage, in which the titratable acid content of spinach in the 25 kV/m electric field intensity treatment group was slightly lower than that of the control group on days 1 and 3, and slightly lower than that of the control group on days 5 and 7, but there was no significant difference in the results between the two groups (P > 0.05). Spinach in the 50 kV/m electric field intensity treatment group had higher titratable acid content than the control group on days 1, 5, and 7 of storage, and lower at day 3. The titratable acid content was slightly higher than the control on the first day at an electric field intensity of 75 kV/m and lower than the control for the rest of the day, with a significant difference in the results between the two groups (P < 0.05), the peak was reached on day 1, with a value of 9.29%, indicating a poor preservation effect. The electric field intensity of 100 kV/m was higher than the control for titratable acid on days 1 and 3, approximated to the control on day 5, but was again lower than the control on day 7. Therefore, when using the titratable acid content as a criterion, the treatment group with an electric field intensity of 25 kV/m was the most effective, followed by 50 kV/m and 75 kV/m, and the electric field intensity of 100 kV/m was the least effective at preserving freshness.

Fig. 24

Test results of titratable acid content under different electric field strength

Comparison of the results of different electric field strengths affecting spinach quality

The difference between the different intensity electric field treatment groups and the control group was combined to identify the electric field intensity that had the most significant effect on spinach quality. Table 3 shows the average difference between the experimental results of several treatment groups and the results of the control group.

Table 3 Mean of the difference in results between the treatment and control groups at different times

As can be seen from Table 3, the treatment group with 25 kV/m electric field strength was better than the control group in the indexes of weight loss rate, membrane permeability, and respiratory strength, and slightly worse than the control group in titratable acid content, but it can be seen that the difference between them were small. The treatment group with 50 kV/m electric field strength was better than the control group in terms of membrane permeability and respiratory strength; the treatment groups with 75 kV/m and 100 kV/m electric field strength were worse than the control group in all indicators, and the difference was larger.

Conclusion

In order to study the effect of different magnetic and electric field strengths on the storage quality of spinach, the storage quality of spinach was studied using different strength magnetic and electric field strength treatments in the spinach preservation process, and after the experiments, specific results were obtained as follows.

• (1) The quality of spinach treated with magnetic field was significantly different from that of control. 40Gs had a positive effect on the quality of spinach, while 60Gs, 80Gs and 100Gs accelerated the decay of spinach. In the case of 40Gs, the respiration intensity was reduced by an average of 10.48% with respect to the control. At 60Gs, 80Gs and 100Gs magnetic field strength, respiration intensity increased by 3.65%, 15.04% and 23.08%, respectively, and weight loss, membrane permeability and titratable count content showed negative effects. It can be seen that the high magnetic field intensity accelerated the decay of spinach and played a negative role in preservation.

• (2) The quality of spinach after electric field treatment showed that the electric field intensity of 25 kV/m had a positive effect on the preservation of spinach, whereas the 50 kV/m, 75 kV/m and 100 kV/m electric fields accelerated the decay of spinach. At 25 kV/m electric field strength, respiration intensity was reduced by an average of 16.6% relative to the control, and weight loss, membrane permeability and titratable count content were also significantly improved. At an electric field strength of 50 kV/m, membrane permeability and respiration intensity were reduced by 2.4% and 10.3%, respectively, weight loss rate increased by 1.3%, and titratable acid content decreased by 0.5%. At electric field strengths of 75 kV/m and 100 kV/m, respiration intensity increased by 27% and 40.7%, respectively, and weight loss rate, membrane permeability and titratable acid content increased. It can be seen that the high electric field intensity accelerated the decay of spinach and played a negative role in preservation.

Based on the above conclusions, lower electric and magnetic field assisted refrigeration has a positive effect on the preservation of spinach, This discovery helps to provide good techniques for preserving large leafy types of vegetables represented by spinach.