Method for preserving large leaf vegetables

Abstract

The present application belongs to the technical field of fruit and vegetable preservation, and specifically discloses a preservation method for large-leaf vegetables. The present application realizes the preservation of large-leaf vegetables by subjecting the large-leaf vegetables to low-temperature plasma treatment in slightly acidic electrolytic water. Compared with single treatment by slightly acidic electrolytic water or low-temperature plasma or two-stage treatment by the two, the preservation method provided by the present application not only has better sterilization efficiency, but also can better maintain the original color, texture, and the contents of nutrients such as chlorophyll, Vc, and cellulose of the vegetables, and can significantly prolong the storage period of the large-leaf vegetables. Compared with traditional thermal sterilization or chemical preservatives, the preservation method for large-leaf vegetables provided by the present application has the advantages of better preservation of nutritional and sensory quality, higher food safety, and better economic and environmental performance.

Description

Technical Field

[0001] This invention belongs to the field of fruit and vegetable preservation technology, specifically relating to a method for preserving leafy vegetables. Background Technology

[0002] Fruits and vegetables are important sources of vitamins, dietary fiber, and other nutrients in people's daily diets. Leafy green vegetables, in particular, are frequently used in salads and hamburger side dishes, playing a significant role in the daily diet. However, leafy green vegetables are highly susceptible to microbial contamination throughout their growth, harvesting, transportation, and storage, leading to quality deterioration and severely impacting consumers' purchasing intentions and eating experience. As people's living standards continue to improve, their demand for food has gradually shifted towards safety, nutrition, and health; therefore, food sterilization and quality control have become especially crucial.

[0003] Currently, common methods in food preservation and sterilization mainly include traditional high-temperature sterilization, chemical additive preservation, and physical low-temperature refrigeration. However, these methods all have significant limitations: high-temperature sterilization can damage the tissue structure, natural color, and heat-sensitive nutrients such as vitamins in leafy vegetables, leading to a significant reduction in the product's nutritional value and sensory quality; while physical low-temperature refrigeration can inhibit microbial growth to some extent, not all low-temperature conditions can achieve ideal preservation results; improper temperature control can not only increase cold chain storage costs but also fail to completely kill microorganisms, still leading to vegetable spoilage. Chemical disinfectants and preservatives are prone to producing chemical residues, posing potential risks to human health. In summary, current mainstream sterilization technologies struggle to simultaneously meet the core requirements of efficient sterilization, quality preservation, and safety and environmental protection. Summary of the Invention

[0004] In view of the above-mentioned problems existing in the prior art, the present invention provides a method for preserving leafy vegetables. The present invention employs a combined sterilization technology of slightly acidic electrolyzed water (SAEW) and low-temperature plasma (LTP). By regulating the sterilization mechanism of leafy vegetables, the method effectively inhibits the decline in vegetable quality, thereby maintaining the original flavor of leafy vegetables and ensuring food safety.

[0005] To achieve the above-mentioned objectives, the embodiments of the present invention employ the following technical solutions: This invention provides a method for preserving leafy vegetables by subjecting them to low-temperature plasma treatment in slightly acidic electrolyzed water.

[0006] Compared to single treatment methods such as slightly acidic electrolyzed water or low-temperature plasma, or a combination of both in stages, this invention uses a method of preserving leafy vegetables by treating them with low-temperature plasma in slightly acidic electrolyzed water. This method not only has better sterilization efficiency, but also better maintains the original color, texture, and nutrient content of the vegetables, such as chlorophyll, vitamin C, and cellulose, thus significantly extending the storage period of leafy vegetables.

[0007] Compared with traditional heat sterilization or chemical preservatives, the preservation method for leafy vegetables provided by this invention has advantages such as better preservation of nutritional and sensory quality, significantly improved food safety, and economic and environmental benefits.

[0008] Preferably, the slightly acidic electrolyzed water has a pH of 5.2-5.8 and an effective chlorine concentration of 15 mg / L-40 mg / L.

[0009] Preferably, the power of the low-temperature plasma treatment is 40W-60W, the electrode spacing is 0.7mm-5mm, and the treatment time is 3min-6min.

[0010] More preferably, after the leafy vegetables are subjected to low-temperature plasma treatment in slightly acidic electrolyzed water for 3-6 minutes, the plasma discharge is stopped, and the mixture is left to stand in the system for 7-16 minutes.

[0011] For example, the total processing time for the preservation method of leafy vegetables provided by the present invention is 10-20 minutes.

[0012] Preferably, the leafy vegetables include at least one of lettuce, romaine lettuce, lettuce leaves, endive, spinach, water spinach, or garland chrysanthemum.

[0013] This invention uses lettuce as an example to illustrate the preservation effect of the preservation method. When other leafy vegetables are treated using the preservation method provided by this invention, the preservation effect is basically the same as that of lettuce.

[0014] Preferably, the mass ratio of the leafy vegetables to the slightly acidic electrolyzed water is 1:4 to 1:8.

[0015] More preferably, the low-temperature plasma treatment employs a dielectric barrier discharge mode.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Significantly improved sterilization efficacy Compared to traditional heat sterilization, this invention utilizes slightly acidic electrolyzed water with strong oxidizing properties under low-temperature conditions. Through the dual effects of ionization and oxidation, sterilization is achieved, resulting in more efficient elimination of various microorganisms on the surface of vegetables, especially effective against some heat-resistant microorganisms. This method achieves a higher sterilization rate, effectively inhibits microbial growth and reproduction, and significantly extends the shelf life of leafy vegetables.

[0017] 2. Better preservation of nutritional and sensory qualities Compared to traditional heat sterilization, the processing conditions of this invention are gentler, causing less damage to the nutrients and active substances in vegetables such as vitamins and chlorophyll, thus better preserving the nutritional quality and original taste of the vegetables.

[0018] 3. Food safety has been significantly improved. Compared to the risk of chemical residues associated with traditional chemical sterilization methods, this invention uses a combination of slightly acidic electrolyzed water and low-temperature plasma for sterilization. The treatment medium used can be gradually degraded into harmless substances, leaving no chemical residues and making it safe and harmless to human health and the environment.

[0019] 4. Economic and environmental protection Compared to traditional high-temperature treatment and chemical sterilization techniques, the preservation method provided by this invention can reduce energy consumption and production costs, while also reducing environmental pollution and having good environmental benefits.

[0020] 5. Excellent preservation effect The preservation method provided by this invention not only has better sterilization effect, but also better maintains the original color, texture and nutrient content of vegetables, effectively extending the storage period of leafy vegetables. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This illustrates the effect of different low-temperature plasma treatment conditions on the total bacterial count in Example 1 of the present invention. Figure 2 The effect of different slightly acidic water electrolysis parameters on the total bacterial count is shown in Example 1 of the present invention. Figure 3 The results of measuring the total number of bacterial colonies in different groups during the combined sterilization treatment in Example 1 of the present invention; Figure 4 The results of the determination of the total number of colonies in the samples treated by different preservation methods in Example 1 of the present invention during the storage period; Figure 5 The results of the determination of the total number of natural colonies in different groups of samples during the storage period in Example 3 of the present invention; Figure 6 This is an example of the changes in the appearance of different groups of lettuce during the storage period in Example 3 of the present invention; Figure 7The results of the determination of the textural changes of lettuce during the storage period in Example 3 of the present invention; Figure 8 The results of measuring the color difference of lettuce during the storage period in Example 3 of the present invention; Figure 9 The results of the weight loss rate of lettuce during the storage period in Example 3 of the present invention are shown. Figure 10 The results of chlorophyll content determination of different groups of samples during storage in Example 3 of the present invention; Figure 11 The results of the determination of vitamin C content in different groups of samples during the storage period in Example 3 of the present invention; Figure 12 The results of cellulose content determination in different groups of samples during storage are shown in Example 3 of the present invention. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0024] The fresh vegetables used in this invention were purchased from a farmers' market in Shijiazhuang City, Hebei Province, and were washed and stored in a 4°C cold storage for later use; among them, the lettuce used was romaine lettuce.

[0025] The acidic electrolyzed water used in this invention is self-made. Specifically, a self-made electrolyzed water generator is used in the laboratory to produce slightly acidic electrolyzed water with sodium chloride solution as the electrolyte. The pH value of the electrolyzed water is measured using a multi-parameter analyzer ThermoOrion 5-Star 510M-01, and the effective chlorine concentration (ACC) is determined by iodometric titration.

[0026] The strain of Escherichia coli used in the experiment of this invention is Escherichia coli ATCC25922, which was purchased from Beijing Baocang Biotechnology Co., Ltd.

[0027] This invention uses a CTP-2000K low-temperature plasma experimental power supply (plasma generator) to process the sample.

[0028] Example 1 This invention provides a method for preserving leafy vegetables, as detailed below: Take 50g of lettuce and place it in 300mL of slightly acidic electrolyzed water. Treat it for 3min under low-temperature plasma conditions with dielectric barrier discharge mode, output power of 45W, and electrode spacing of 1mm. Then, stop the plasma discharge and let it stand in this system for 12min.

[0029] The effective chlorine concentration of the slightly acidic electrolyzed water is 40 mg / L, and the pH value is 5.5.

[0030] Dry the surface moisture of the processed lettuce and store it at room temperature.

[0031] Example 2 This invention provides a method for preserving leafy vegetables, as detailed below: Take 50g of lettuce and place it in 200mL of slightly acidic electrolyzed water. Treat it for 4min under low-temperature plasma conditions with dielectric barrier discharge mode, output power of 40W, and electrode spacing of 5mm. Then, stop the plasma discharge and let it stand in this system for 16min.

[0032] The effective chlorine concentration of the slightly acidic electrolyzed water is 15 mg / L, and the pH value is 5.2.

[0033] Dry the surface moisture of the processed lettuce and store it at room temperature.

[0034] Example 3 This invention provides a method for preserving leafy vegetables, as detailed below: Take 50g of garland chrysanthemum and place it in 400mL of slightly acidic electrolyzed water. Treat it for 5min under low-temperature plasma conditions with dielectric barrier discharge mode, output power of 60W, and electrode spacing of 3mm. Then, stop the plasma discharge and let it stand in this system for 15min.

[0035] The effective chlorine concentration of the slightly acidic electrolyzed water is 30 mg / L, and the pH value is 5.8.

[0036] After processing, dry the surface moisture of the garland chrysanthemum and store it at room temperature.

[0037] Example of effect 1 To investigate the sterilization effects of different preservation methods, this invention employs the following procedures: Fresh lettuce is washed and cut into 3cm × 3cm pieces, with three parallel samples per group; each leaf piece is inoculated with 1mL of Escherichia coli ATCC25922 bacterial suspension and co-cultured in a 37℃ biochemical incubator for 24 hours to obtain lettuce artificially contaminated with E. coli; then, different preservation methods are applied separately, and after treatment, the surface moisture of the lettuce is dried, and it is placed in physiological saline test tubes and sonicated. The test solution is serially diluted, inoculated onto eosin methylene blue medium, and cultured for 24 hours before counting. The number of viable E. coli in the lettuce leaves of each group is compared. The preparation method of the E. coli bacterial suspension is as follows: activated and expanded-cultured E. coli is removed from the culture medium, washed three times with sterile physiological saline, resuspended, and the OD of the bacterial suspension is adjusted to 1.000±0.050 at a wavelength of 600nm using a spectrophotometer. In this invention, dielectric barrier discharge mode is used for low-temperature plasma treatment.

[0038] I. Single-factor experiment of low-temperature plasma (LTP) treatment A CTP-2000K low-temperature plasma experimental power supply was used. 5g of lettuce artificially contaminated with *E. coli* was placed in a glass petri dish containing 30mL of physiological saline. The plasma output power (e.g., 0W, 30W, 45W, or 60W), plasma treatment time (e.g., 0min, 2min, 3min, or 4min), and electrode spacing (e.g., 1mm, 3mm, 5mm, or 7mm) were adjusted, and the effects of different parameters on the sterilization effect were recorded. After treatment, the lettuce was dried, placed in a physiological saline test tube, and sonicated. The test solution was serially diluted, inoculated into eosin methylene blue medium, and incubated at 37℃ for 24h before counting. The effects of different low-temperature plasma treatment conditions on the total bacterial count are shown below. Figure 1 As shown.

[0039] Depend on Figure 1 It can be seen that the bactericidal effect of low-temperature plasma (LTP) on Escherichia coli gradually increases with the increase of power, the length of treatment time, and the reduction of the electrode spacing.

[0040] II. Single-factor experiment on the treatment of slightly acidic electrolyzed water (SAEW) Five grams of lettuce artificially contaminated with *E. coli* were placed in a glass petri dish containing 30 mL of slightly acidic electrolyzed water. Unless otherwise specified, under conditions where the available chlorine concentration (ACC) was fixed at 30 mg / L or the treatment time was 15 min, the effects of different parameters on the bactericidal efficacy were recorded by varying the available chlorine treatment time (5 min, 10 min, 15 min, or 20 min) or ACC concentration (15 mg / L, 20 mg / L, 30 mg / L, or 40 mg / L). After treatment, the lettuce was dried, placed in a physiological saline test tube, and sonicated. The test solution was then serially diluted and inoculated into eosin methylene blue medium and incubated at 37°C for 24 h before counting. The effects of different slightly acidic electrolyzed water parameters on the total bacterial count are shown below. Figure 2 As shown.

[0041] Depend on Figure 2It was found that increasing the treatment time of SAEW and the concentration of ACC were positively correlated with the bactericidal effect on Escherichia coli. The optimal bactericidal effect was achieved when the treatment time was fixed at 30 mg / L for 20 min, and at 15 min with an ACC concentration of 40 mg / L; the total bacterial count decreased from the initial 7.28 lg CFU / mL and 7.26 lg CFU / mL to 0.39 lg CFU / mL and 1.35 lg CFU / mL, respectively. These results indicate that the bactericidal efficacy of slightly acidic electrolyzed water is time-dependent and effective chlorine concentration-dependent, and the inactivation ability against E. coli is more significant under conditions of longer treatment time and higher concentration. Compared with traditional food preservatives, the slightly acidic electrolyzed water used in this invention is more efficient and has a broader spectrum of bactericidal activity, and is safe and leaves no residue; after sterilization, SAEW decomposes into odorless substances and does not leave harmful residues on the surface of vegetables.

[0042] III. Combined sterilization treatment This invention investigates the bactericidal effects of three combined modes of slightly acidic electrolyzed water and low-temperature plasma on Escherichia coli on the surface of vegetables. The total treatment time was set to 15 min and 20 min, and included three treatment modes: SAEW followed by LTP, LTP followed by SAEW, and a combined LTP and SAEW treatment. The control group consisted of lettuce artificially contaminated with E. coli without any preservation treatment. Specific settings are as follows: The first group was treated with 30 mL of slightly acidic electrolyzed water (ACC 30 mg / L, pH 5.5) for 12 min with 5 g of artificially contaminated lettuce containing *E. coli*. Then, the lettuce was subjected to low-temperature plasma treatment for 3 min at an output power of 45 W and an electrode spacing of 1 mm. After treatment, the lettuce was dried, placed in a saline tube, and sonicated. The test solution was then serially diluted and inoculated into eosin methylene blue medium. The medium was incubated at 37°C for 24 h, and the cells were counted. This group was designated S12-L3.

[0043] The second group was first treated with 30 mL of physiological saline, followed by low-temperature plasma treatment of 5 g of artificially contaminated lettuce with E. coli for 3 min at an output power of 45 W and an electrode spacing of 1 mm. The lettuce was then removed and treated with 30 mL of slightly acidic electrolyzed water (ACC 30 mg / L, pH 5.5) for 12 min. After treatment, the lettuce was dried, placed in a physiological saline test tube, and sonicated. The test solution was then serially diluted and inoculated into eosin methylene blue medium. The medium was incubated at 37 °C for 24 h, and the samples were counted. This group was designated L3-S12.

[0044] The third group added 30 mL of slightly acidic electrolyzed water (ACC 30 mg / L, pH 5.5) to 5 g of lettuce artificially contaminated with E. coli. The lettuce was then subjected to low-temperature plasma treatment for 3 min at an output power of 45 W and an electrode spacing of 1 mm. The plasma discharge was then stopped, and the lettuce was allowed to stand for 12 min. After treatment, the lettuce surface moisture was dried, and the lettuce was placed in a physiological saline tube and sonicated. The test solution was then serially diluted and inoculated into eosin methylene blue medium. The mixture was incubated at 37°C for 24 h, and the samples were counted. This group was designated L+S15.

[0045] The fourth group was first treated with 30 mL of slightly acidic electrolyzed water (ACC 30 mg / L, pH 5.5) containing 5 g of artificially contaminated lettuce for 16 min, followed by low-temperature plasma treatment at 45 W output power and 1 mm electrode spacing for 4 min. After treatment, the lettuce was dried, placed in physiological saline tubes, and sonicated. The test solution was then serially diluted, inoculated into eosin methylene blue medium, and incubated at 37 °C for 24 h before counting. This group was designated S16-L4.

[0046] In the fifth group, 30 mL of physiological saline was added, and the 5 g of artificially contaminated lettuce with E. coli was treated with low-temperature plasma at an output power of 45 W and an electrode spacing of 1 mm for 4 min. After removing the lettuce, it was treated with 30 mL of slightly acidic electrolyzed water with an ACC of 30 mg / L and a pH of 5.5 for 16 min. After treatment, the surface moisture of the lettuce was dried, and it was placed in a physiological saline test tube and sonicated. The test solution was then serially diluted and inoculated into eosin methylene blue medium and incubated at 37 °C for 24 h before counting. This group was designated L4-S16.

[0047] In the sixth group, 30 mL of slightly acidic electrolyzed water with an ACC of 30 mg / L and a pH of 5.5 was added to 5 g of lettuce artificially contaminated with E. coli. The lettuce was then treated with low-temperature plasma for 4 min at an output power of 45 W and an electrode spacing of 1 mm. The plasma discharge was then stopped, and the lettuce was allowed to stand for 16 min. After treatment, the lettuce was dried, placed in a physiological saline tube, and sonicated. The test solution was then serially diluted and inoculated into eosin methylene blue medium. The mixture was incubated at 37°C for 24 h, and the results were counted. This group was designated L+S20.

[0048] The results of the total bacterial count determination in different groups during the combined sterilization treatment are as follows: Figure 3 As shown.

[0049] Depend on Figure 3 It can be seen that, with a total treatment time of 15 minutes, compared with the S12-L3 group or the L3-S12 group which were treated with slightly acidic electrolyzed water or low-temperature plasma in stages, the L+S15 group which was treated with low-temperature plasma in slightly acidic electrolyzed water had a significantly better bactericidal effect on Escherichia coli.

[0050] With a total treatment time of 20 minutes, compared to the S16-L4 group or the L4-S16 group which were treated with slightly acidic electrolyzed water or low-temperature plasma in stages, the L+S20 group, which was treated with low-temperature plasma in slightly acidic electrolyzed water, showed significantly better bactericidal effect on Escherichia coli and could achieve commercial sterility.

[0051] Overall, the sterilization effect of a total treatment time of 20 minutes was significantly better than that of a total treatment time of 15 minutes. When both groups used slightly acidic electrolyzed water or low-temperature plasma treatment in stages, the sterilization effect of the S16-L4 group or the L4-S16 group was significantly better than that of the S12-L3 group or the L3-S12 group.

[0052] IV. Effects of different treatment methods on total bacterial count during storage This invention measures the total bacterial count of samples treated by the above-mentioned different preservation methods during storage, and the results are as follows: Figure 4 As shown.

[0053] The control group consisted of lettuce artificially contaminated with E. coli that had not undergone any preservation treatment. Group L involved placing 5g of lettuce artificially contaminated with E. coli into a glass culture dish containing 30mL of physiological saline, and then treating it with low-temperature plasma for 3min at a plasma output power of 45W and an electrode spacing of 1mm. After that, the plasma discharge was stopped, and the mixture was left to stand in the system for 12min. Group S involved placing 5g of lettuce artificially contaminated with E. coli into a glass petri dish containing 30mL of slightly acidic electrolyzed water (ACC = 30mg / L, pH = 5.5) for 15min. Group LS refers to groups L3-S12 mentioned above; Group SL refers to the aforementioned groups S12-L3; The S+L group is the L+S15 group mentioned above.

[0054] Depend on Figure 4It was found that at the initial stage of storage, the total bacterial counts of the controll group, L group, S group, and L+S group were 2.74 lg CFU / mL, 2.12 lg CFU / mL, 1.25 lg CFU / mL, and 0 lg CFU / mL, respectively. After 1 day of storage, the total bacterial counts of the controll group, L group, S group, and L+S group were 3.46 lg CFU / mL, 2.99 lg CFU / mL, 2.66 lg CFU / mL, and 0, respectively. Furthermore, although the total bacterial counts of the LS group and SL group were comparable to those of the L+S group at the initial stage of storage, after 1 day of storage, the total bacterial counts of the LS group (1.53 lg CFU / mL) and SL group (0.73 lg CFU / mL) were significantly higher than those of the L+S group. The bactericidal efficacy of the combined LTP and SAEW treatment mode is significantly better than that of a single treatment method. Compared with the phased use of LTP or SAEW, the group that uses LTP in SAEW (L+S15) has a significantly better bactericidal effect on Escherichia coli.

[0055] Example 2 Based on Example 1, this invention uses different preservation methods to treat leafy vegetables and measures the physicochemical properties and colony count of the products during the initial treatment and storage periods. The physicochemical properties measured in this invention include the texture of the samples, color difference changes, and nutritional components such as vitamin C, chlorophyll, and cellulose.

[0056] I. Specific Measurement Methods for Different Indicators 1. Texture determination A TA-XT Plus texture analyzer with a P / 5R probe was used. Three leaves were selected for each sample and the measurements were repeated. The average value was taken. The results directly reflect the aging degree, freshness, and storage period of lettuce.

[0057] 2. Color difference measurement A CR-400 colorimeter was used to measure the total color difference (ΔE), brightness (L), red-green axis (a), and yellow-blue axis (b) at three uniform sites on the middle leaves of the sample. The average value was taken as the result. The color difference changes in the experimental data can directly reflect the chlorophyll degradation rate, cell integrity, and post-harvest senescence, and are important indicators for evaluating appearance quality, freshness, and preservation effect.

[0058] 3. Determination of Vitamin C, Chlorophyll, and Fiber Content (1) The present invention uses the TC2039A6 vitamin C detection kit (phenanthrene colorimetric method) to detect the vitamin C content in the sample. The specific method is as follows: 15 mL of 5× tissue homogenate and 60 mL of distilled water are added to a centrifuge tube to obtain 1× tissue homogenate (75 mL); 3 g of lettuce from different treatment groups are prepared, ground, and a small amount of 1× tissue homogenate is added twice. The mixture is poured into a 50 mL centrifuge tube, and the volume is increased to 22 mL. The mixture is mixed, centrifuged at 4000 rpm for 5 min, and the supernatant is the test solution. 25 mg of vitamin C standard was dissolved in 1 mL of 1× tissue homogenate to prepare 25 mg / mL vitamin C standard solution 1. 0.02 mL of the above vitamin C standard solution 1 was taken and mixed with 9.98 mL of 1× tissue homogenate to prepare 50 μg / mL vitamin C standard solution 2. Vitamin C sample addition: Blank tubes, standard tubes, and test tubes were set up according to the table below. The solutions should be added sequentially, taking care to avoid generating air bubbles. If the vitamin C content in the sample is too high, the sample volume can be reduced or appropriately diluted before measurement. Two parallel tubes were used for sample detection, and the average value was calculated. The reagents added to different tubes are shown in Table 1. The reagents in each tube were mixed thoroughly, and the reaction was carried out in a 30℃ incubator for 60 min. The blank tube was used to zero the sample, and the absorbance of the 534 nm series of standard tubes and test tubes was measured using a spectrophotometer.

[0059] Table 1

[0060] (2) The content of chlorophyll a and chlorophyll b in lettuce was determined by adjusting the wavelength of the spectrophotometer using a carotenoid detection kit to characterize the chlorophyll content in the sample. The specific method was as follows: 0.1g of lettuce was crushed and mixed with 50mg of extraction powder and 1mL of CAb (Carotenoid Assay buffer). The mixture was transferred to a 10mL centrifuge tube, and the homogenizer was rinsed with a small amount of CAb. Finally, the mixture, along with the residue, was poured into the centrifuge tube, and CAb was added to bring the total volume to 10mL. The mixture was mixed and placed in the dark for 2 hours until the bottom tissue was nearly white. The filtrate was then filtered, and the crude carotenoid extract was collected. The crude carotenoid extract was placed in a 96-well plate, with 200μL added to each well. The plate was zeroed with CAb, and the absorbance of chlorophyll a was measured at 665nm, and the absorbance of chlorophyll b was measured at 649nm. The contents of chlorophyll a and chlorophyll b were calculated.

[0061] (3) The water-insoluble cellulose content in the sample was determined using the G0715F-cellulose content kit.

[0062] The senescence process, cell membrane integrity, preservation treatment effect, and storage period of lettuce were characterized by detecting changes in vitamin C, chlorophyll, and cellulose. The specific measurement method is as follows: Sample preparation: ① Take 0.1g of lettuce to be tested, add 1.5mL of 80% ethanol, grind into a homogenate, incubate in a 50℃ water bath for 20min, remove and cool under running water, centrifuge at 12000rpm and 25℃ for 10min, discard the supernatant, and retain the precipitate. ② Add 1mL of 80% ethanol to the precipitate, vortex and mix for 2min, incubate in a 50℃ water bath for 20min, remove and cool under running water, incubate at 12000rpm and 25℃ for 10min, discard the supernatant, and retain the precipitate. ③ Add 1mL of the extraction solution (perchloric acid aqueous solution, for starch removal) from the kit to the above precipitate, incubate in a 90℃ water bath for 15min, centrifuge at 12000rpm and room temperature for 10min, discard the supernatant, retain the precipitate, add 1mL of acetone to the precipitate, vortex and mix, centrifuge at 12000rpm and room temperature for 10min, discard the supernatant, retain the precipitate, open the EP tube and incubate at 90℃ for 20min to dry the precipitate. ④ Add 0.2 mL of reagent one to the precipitate, incubate in a 30°C water bath for 1 hour, then pour into a 10 mL centrifuge tube. Rinse the 2 mL EP tube several times with 5.6 mL of distilled water and collect the liquid into the 10 mL centrifuge tube. Mix well and seal the tube. Incubate at 110°C for 1 hour, remove and cool, mix well, and take 1 mL of the mixture into a 2 mL EP tube. Centrifuge at 8000 rpm at room temperature for 5 min, and take the supernatant for testing.

[0063] Instrumental Testing: ① Preheat the visible spectrophotometer for 30 minutes, set the wavelength to 620 nm, and zero the instrument with distilled water. ② Dilute the sample using appropriate gradients (20 times, i.e., 1 part supernatant + 19 parts distilled water) to determine the appropriate dilution factor D for this experiment. ③ According to Table 2, add different reagents sequentially to the EP tube, mix well, and incubate in a boiling water bath for 5 minutes (to prevent moisture loss, seal with film). After cooling, transfer all the liquid to a 1 mL glass cuvette and read the absorbance value A at 620 nm. ΔA = A 测定管 -A 空白管 .

[0064] Table 2

[0065] II. Results of Index Measurement Fresh lettuce was washed and cut into 3cm×3cm pieces, with 3 parallel samples per group. Each leaf was inoculated with 1mL of Escherichia coli ATCC25922 suspension and co-cultured in a 37℃ biochemical incubator for 24h to obtain lettuce artificially contaminated with Escherichia coli.

[0066] For samples treated in the control group, SAEW group, LTP group, and SAEW+LTP group, the texture and color difference were determined according to the determination methods described above.

[0067] In the control group, lettuce was artificially contaminated with E. coli without any preservation treatment. The surface moisture of the lettuce was dried, and it was placed in a physiological saline test tube and sonicated. The test solution was serially diluted and inoculated into eosin methylene blue medium. After incubation at 37°C for 24 hours, the bacteria were counted. In the SAEW group, 5g of lettuce artificially contaminated with E. coli was placed in a glass petri dish containing 30mL of slightly acidic electrolyzed water (pH 5.5, ACC 30mg / L) and treated for 15min. After treatment, the lettuce surface moisture was dried, and it was placed in a physiological saline test tube and sonicated. The test solution was then serially diluted, inoculated into eosin methylene blue medium, and incubated at 37℃ for 24h before counting. In the LTP group, 5g of lettuce artificially contaminated with E. coli was placed in a glass petri dish containing 30mL of physiological saline. The plasma output power was adjusted to 45W, the electrode spacing to 1mm, and the treatment time to 3min. Then, the plasma discharge was stopped, and the lettuce was allowed to stand in the system for 12min. After treatment, the surface moisture of the lettuce was dried, and it was placed in a physiological saline test tube and sonicated. The test solution was then serially diluted, inoculated into eosin methylene blue medium, and incubated at 37℃ for 24h before counting.

[0068] In the S+L group, 5g of lettuce artificially contaminated with E. coli was placed in a glass petri dish containing 30mL of slightly acidic electrolyzed water (pH 5.5, ACC 30mg / L). The plasma output power was adjusted to 45W, the electrode spacing to 1mm, and the treatment time to 3min. Then, the plasma discharge was stopped, and the mixture was allowed to stand in the system for 12min. After treatment, the surface moisture of the lettuce was dried, and it was placed in a physiological saline test tube and sonicated. The test solution was then serially diluted and inoculated into eosin methylene blue medium and incubated at 37℃ for 24h before counting.

[0069] After treatment with different preservation methods, the texture and color difference of different samples were measured according to the determination methods described above. The measurement results are shown in Table 3-4.

[0070] Table 3

[0071] Note: Different lowercase letters in the same column indicate significant differences (P ≤ 0.05).

[0072] Table 4

[0073] Note: Different lowercase letters in the same column indicate significant differences (P≤0.05).

[0074] As shown in Table 3-4, compared with the control group that was not sterilized, the group treated with low-temperature plasma and slightly acidic electrolyzed water (S+L group) showed no significant difference in hardness, elasticity, chewiness and color difference. This indicates that the quality of lettuce did not change much after treatment, and the preservation method provided by this invention can well maintain the original quality of vegetables.

[0075] Example 3 Taking the leafy vegetable preservation method provided in Example 1 as an example, the preservation effect of the leafy vegetable preservation method provided in this embodiment of the invention was evaluated. The preservation effects of Examples 2-3 are basically equivalent to those of Example 1.

[0076] After treating lettuce with the preservation method for leafy vegetables provided in Example 1, samples were taken every two days to measure changes in total bacterial count, appearance, texture, color difference, weight loss, chlorophyll content, vitamin C content, and cellulose content. In this example, fresh lettuce was washed and dried to serve as the control group; the treatment group in Example 1 was designated as the SAEW+LTP group.

[0077] The details are as follows: 1. Determination of total natural bacterial count Take out lettuce, cut it into small pieces according to the group, and place it into centrifuge tubes. Add physiological saline, mix well by sonication, and centrifuge. Collect the supernatant and perform serial dilutions of different groups of lettuce according to the storage period. Spread the diluted bacterial suspension on pre-prepared culture medium and incubate at 37°C for 24 hours, then count the bacteria. The results of the determination of the total number of natural colonies in different groups of samples during the storage period are as follows: Figure 5 As shown.

[0078] Depend on Figure 5 It can be seen that the number of bacterial colonies in lettuce continuously increases during storage, but the growth of bacterial colonies in the treated lettuce is slower and consistently lower than that in the control group. This indicates that the lettuce treated with the preservation method provided in Example 1 has a certain inhibitory effect on bacterial growth.

[0079] 2. Changes in appearance On days 0, 2, 4, 6, and 8 of storage, lettuce from different groups was removed, photographed, and the changes in appearance of the lettuce during the storage period were recorded. The changes in appearance of different groups of lettuce during the storage period are shown below. Figure 6 As shown.

[0080] Depend on Figure 6 As time went on, the lettuce in the control group wilted noticeably, with yellowing edges and a dull color. While the lettuce in the SAEW+LTP group also showed changes, its freshness was significantly better than that of the control group.

[0081] 3. Changes in texture, color difference, and weight loss rate Using the method described in Example 2, three leaves were selected from each sample group for repeated measurements, and the average value was taken. The results of the textural changes of lettuce during the storage period are as follows: Figure 7 As shown in the figure. Hardness refers to hardness, Fracturability to brittleness, and Springiness to elasticity.

[0082] Depend on Figure 7 It can be seen that the hardness, crispness and elasticity of the lettuce in the control group and the SAEW+LTP group gradually decreased during the storage period. However, the rate of change of each data of the lettuce in the SAEW+LTP group was smaller than that in the control group, indicating that the lettuce treated with the preservation method of leafy vegetables provided in the embodiments of the present invention can better maintain the original taste of the lettuce.

[0083] Using the method described in Example 2, three uniform sites were selected from the middle leaves of the sample to measure the total color difference (ΔE), brightness (L), redness (a), and yellowness (b), and the average value was taken. The results of the color difference measurement of lettuce during the storage period are as follows: Figure 8 As shown.

[0084] Depend on Figure 8 It can be seen that the total color difference ΔE of lettuce in both the control group and the SAEW+LTP group continuously increased during storage, and the difference from the standard color became increasingly larger. Although lettuce in the SAEW+LTP group also showed changes, these changes were consistently smaller than those in the control group. Figure 6 It can be seen that the preservation method provided in the embodiments of the present invention can maintain the original color of vegetables to a large extent.

[0085] Weigh the lettuce to be stored and record the weight as M0. Weigh different groups of lettuce every two days and record the resulting weight as M. n The weight loss rate of lettuce during storage was calculated and statistically analyzed. The results are as follows: Figure 9 As shown.

[0086] Depend on Figure 9 It can be seen that the weight of lettuce continuously decreases during storage, and the weight loss of lettuce in the SAEW+LTP group is slower than that in the control group. From day 4 to day 8 of storage, the weight loss rate of the SAEW+LTP group was significantly lower than that of the control group. This indicates that the preservation method provided in this embodiment of the invention can effectively inhibit the moisture loss of vegetables, thereby reducing their weight loss.

[0087] 4. Changes in nutritional composition Using the method described in Example 2, the contents of chlorophyll, vitamin C, and cellulose in different groups of samples were determined during the storage period. The results of the chlorophyll content determination in different groups of samples during the storage period are as follows: Figure 10As shown, the results of vitamin C content determination for different groups of samples during storage are as follows: Figure 11 As shown, the cellulose content determination results of different groups of samples during the storage period are as follows: Figure 12 As shown.

[0088] Depend on Figure 10-12 It can be seen that, compared with the control group, the SAEW+LTP group showed a slower rate of decline in chlorophyll, vitamin C, and cellulose content, resulting in less nutrient loss. This indicates that the preservation method provided by this invention has a better preservation effect on lettuce and can effectively extend the storage period of lettuce.

[0089] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions or improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preserving leafy vegetables, characterized in that: Leafy vegetables are subjected to low-temperature plasma treatment in slightly acidic electrolyzed water.

2. The method for preserving leafy vegetables as described in claim 1, characterized in that: The slightly acidic electrolyzed water has a pH of 5.2-5.8 and an effective chlorine concentration of 15 mg / L-40 mg / L.

3. The method for preserving leafy vegetables as described in claim 1, characterized in that: The low-temperature plasma treatment has a power of 40W-60W, an electrode spacing of 0.7mm-5mm, and a treatment time of 3min-6min.

4. The method for preserving leafy vegetables as described in claim 3, characterized in that: After treating leafy vegetables with low-temperature plasma in slightly acidic electrolyzed water for 3-6 minutes, the plasma discharge was stopped, and the vegetables were left to stand in the system for 7-16 minutes.

5. The method for preserving leafy vegetables as described in claim 1, characterized in that: The leafy vegetables include at least one of lettuce, romaine lettuce, lettuce leaves, endive, spinach, water spinach, or garland chrysanthemum.

6. The method for preserving leafy vegetables as described in claim 1, characterized in that: The mass ratio of the leafy vegetables to slightly acidic electrolyzed water is 1:4 to 1:

8.

7. The method for preserving leafy vegetables as described in any one of claims 1-6, characterized in that: The low-temperature plasma treatment employs a dielectric barrier discharge mode.

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