Preservation method for spinach after harvesting

Abstract

The invention provides a postharvest fresh-keeping method of spinach, which comprises the following steps: picking spinach, selecting, washing with distilled water, airing, spraying melatonin solution, packaging with a polyethylene bag, and placing in a storage environment of 4 DEG C. After the spinach is cleaned and aired, melatonin is sprayed to the spinach for low-temperature storage at the temperature of 4 DEG C, deterioration of the spinach after picking can be effectively delayed, the sensory quality is maintained, the shelf life is prolonged, the economic value is improved, and the method has high commercial popularization value.

Description

Technical Field

[0001] The invention relates to a method for preserving spinach after harvest, in particular to preserving spinach by using exogenous melatonin, thereby better extending the shelf life of spinach. Background Art

[0002] Spinach is rich in carotenoids, vitamin C and minerals. In addition, spinach leaves have high water content and delicate tissues. After harvest, they have vigorous respiration and metabolism, which makes them very easy to lose water, wilt, rot and deteriorate, seriously affecting the sensory quality and nutritional value of spinach, and making it very difficult to store and preserve. Melatonin (MT) is an endogenous indoleamine in plants and a plant growth regulator. Studies have shown that MT can inhibit the oxidation of plants caused by internal and external factors and delay plant aging. This study aims to study the effect of MT on the preservation of post-harvest spinach and broaden the ideas for spinach preservation technology. . Summary of the invention

[0003] The invention aims at providing a method for preserving spinach by exogenous melatonin, which method can significantly delay the change of its quality.

[0004] A method for preserving spinach after harvest, characterized in that it comprises the following steps: The present invention is achieved through the following technical steps: Select spinach of uniform size, bright color, and free of rot and insect pests, wash it continuously in running distilled water to remove the dirt particles on the surface of the spinach, then remove it and drain it, and dry it at 10±1℃ and relative humidity of 45-55% for 30 min; Select 500g of spinach, spray it with 200mL of 0.10~0.50mg / mL melatonin solution, and air-dry it under low temperature for 20~60min.

[0005] The dried spinach was packed in polyethylene bags, 90 g per bag, and stored at 4 ℃.

[0006] Preferably, after spraying the melatonin solution, the solution is ventilated and air-dried for 60 minutes at 10±1° C. and a relative humidity of 45-55%.

[0007] Preferably, 500 g of selected spinach is sprayed with 200 mL of 0.30 mg / mL melatonin solution.

[0008] The present invention sprays melatonin on the spinach after washing and drying and stores it at a low temperature of 4°C, which can effectively delay the deterioration of the spinach after harvest, maintain the sensory quality, extend the shelf life, improve the economic value, and has strong commercial promotion value. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 Sensory photos of spinach treated with different concentrations of melatonin (A: control group, B: 0.10 mg / mL MT, C: 0.20 mg / mL MT, D: 0.30 mg / mL MT); Figure 2 Effects of different concentrations of melatonin on spinach color difference (A: control group, B: 0.10 mg / mL MT, C: 0.20 mg / mL MT, D: 0.30 mg / mL MT); Figure 3 Effects of different concentrations of melatonin treatment on vitamin C in spinach (A: control group, B: 0.10 mg / mL MT, C: 0.20 mg / mL MT, D: 0.30 mg / mL MT); Figure 4 Effects of different concentrations of melatonin on the weight loss rate of spinach (A: control group, B: 0.10 mg / mL MT, C: 0.20 mg / mL MT, D: 0.30 mg / mL MT); Figure 5 Effects of different concentrations of melatonin on the chlorophyll content of spinach (A: control group, B: 0.10 mg / mL MT, C: 0.20 mg / mL MT, D: 0.30 mg / mL MT); Figure 6 Effects of different concentrations of melatonin on the soluble solids content of spinach (A: control group, B: 0.10 mg / mL MT, C: 0.20 mg / mL MT, D: 0.30 mg / mL MT); Figure 7 Effects of different concentrations of melatonin on the soluble sugar content of spinach (A: control group, B: 0.10 mg / mL MT, C: 0.20 mg / mL MT, D: 0.30 mg / mL MT); Figure 8 Effects of different concentrations of melatonin on the antioxidant enzyme activities of spinach. (a) Effects of melatonin treatment on the activities of SOD, (b) POD, (c) CAT and (d) MDA content in spinach (A: control group, B: 0.10 mg / mL MT, C: 0.20 mg / mL MT, D: 0.30 mg / mL MT); Fig. 9Effects of different concentrations of melatonin on the free radical scavenging rate of spinach; Effects of melatonin on (a) DPPH free radical scavenging rate and (b) hydroxyl free radical scavenging rate of spinach (A: control group, B: 0.10 mg / mL MT, C: 0.20 mg / mL MT, D: 0.30 mg / mL MT). DETAILED DESCRIPTION

[0010] In order to make the operation process and creative features of the present invention easy to understand and fully disclose a spinach preservation method, the present invention is further described below in conjunction with specific embodiments. Example

[0011] 1. Reagents and Instruments 1.1 Experimental Reagents

[0012] The main chemical reagents used are listed in Table 1.

[0013] Table 1 Chemical reagents used in the experiment Reagent name purity Origin Melatonin AR Aladdin Reagents (Shanghai) Co., Ltd. Anhydrous ethanol ≥99.7% Sinopharm Chemical Reagent Company Sodium chloride ≥99.7% Sinopharm Chemical Reagent Company Calcium carbonate powder ≥99.7% Sinopharm Chemical Reagent Company Quartz sand ≥99.7% Sinopharm Chemical Reagent Company SOD Kit - Nanjing Jiancheng Company MDA Kit - Nanjing Jiancheng Company 1.2 Experimental instruments The main experimental instruments used are shown in Table 2.

[0014] Table 2 Experimental instruments used in the experiment name model factory UV-Vis Spectrophotometer U-3900 Hitachi Ltd. Electronic balance FA1104-B Shanghai Yueping Scientific Instrument Co., Ltd. Colorimeter CR-400 / 410 Konica Minolta Holdings Co., Ltd. Digital constant temperature water bath JYD-90 Changzhou Henglong Instrument Manufacturing Co., Ltd. High temperature blast drying oven 914A Shanghai Yueping Scientific Instrument Co., Ltd. Select spinach of uniform size, bright color, and free of rot and insect pests, wash it continuously in running distilled water to remove the dirt particles on the surface of the spinach, then remove it and drain it, and dry it at 10±1℃ and relative humidity of 45-55% for 30 min;

[0015] (1) Spray 500 g of selected spinach with 200 mL of 0.10, 0.20, and 0.30 mg / mL melatonin solution, and air-dry for 60 min at 10±1°C and relative humidity of 45-55%. (2) Package the dried spinach in polyethylene bags, 90 g per bag, and store the packaged spinach at 4°C; (3) During the cold storage process, three spinach leaves were randomly selected from each group every 2 days to measure the indicators.

[0016] 2.2 Determination of spinach storage test indicators (1) Determination of color difference: The color of spinach leaves was evaluated using a colorimeter. The instrument was aligned with black and white standards and the experimental data of L*, a*, and b* were observed.

[0017] (2) Determination of weight loss: Use weighing method.

[0018] Weight loss rate (%) = (initial mass of spinach - mass of spinach after each storage period) / initial mass of spinach × 100%.

[0019] (3) Chlorophyll determination: Take 0.5 g of spinach leaves, add 2 mL of 95% ethanol, add a small amount of quartz sand and calcium carbonate, and grind into a homogenous slurry in a mortar. Filter and dilute to 50 mL with acetone solution. Measure the absorbance at 652 nm and calculate according to the following formula.

[0020] Chlorophyll content (mg / g) = (2-1) Among them, D 652 represents the absorbance of sample y at a wavelength of 652 nm; V represents the volume of sample extract (mL); W represents the weight of fresh spinach (g).

[0021] (4) Determination of vitamin C content: The 2,6-dichloroindophenol titration method was used. Samples were taken on the day of harvest and on the 2nd, 4th, 6th, 8th, 10th and 12th days for determination.

[0022] (5) Determination of TSS (soluble solids) content The soluble solids content was determined using a handheld RA-250WE digital refractometer. Different parts of spinach were taken, wrapped with three layers of gauze and filtered to squeeze the pulp juice into a 100 mL beaker. After mixing, a certain volume of the mixed solution was placed on the refractometer detection mirror, and then the value was read. Repeat three times and take the average value.

[0023] Soluble sugar, MDA and SOD were measured using kits.

[0024] 2.3 Determination of antioxidant enzyme activity The CAT enzyme activity assay was performed according to the paper of Periyar with some modifications. The reaction mixture consisting of sodium phosphate buffer, H2O2 and spinach extract (50 μL) was analyzed, after which the decomposition of hydrogen peroxide was observed at 240 nm. The enzyme activity was reported as units per milligram of protein (U / mg). One unit represents the amount of catalase required to convert U / mg.

[0025] POD enzyme activity assay: Spinach extract was mixed with a buffer solution containing sodium phosphate solution and guaiacol and incubated at 20°C for 5 min. Subsequently, H2O2 was added and the increase in absorbance at 460 nm was measured within 120 seconds. One unit of enzyme activity was expressed as the number of enzymes that caused a change in absorbance per milligram of protein (U / mg). For each treatment, three enzyme analyses were performed for each sample.

[0026] 2.4 Determination of free radical scavenging rate Weigh 0.1984 g DPPH and dissolve it in anhydrous ethanol to 50 mL. Take 2 mL of DPPH and dilute it to 100 mL with anhydrous ethanol. After reacting in the dark for 30 min, measure the absorbance at 517 nm. The calculation method is shown in Table 3: Table 3 Calculation method of DPPH free radical scavenging rate Absorbance Reaction solution <![CDATA[A0]]> 2 mL DPPH solution + 2 mL 70% ethanol <![CDATA[A1]]> 2 mL DPPH solution + 2 mL extract <![CDATA[A2]]> 2 mL 70% ethanol + 2 mL extract Calculation method: DPPH free radical scavenging rate (%) = ((A0-A1+A2) / A0)*100%; OH·free radical scavenging rate (%) = (1-(A1-A0))*100%.

[0027] 3 Results and analysis 3.1 Sensory analysis of spinach

[0028] like Figure 1 As shown in the figure, with the increase of storage time, the leaves of fresh spinach gradually turned yellow and the top of the leaves gradually rotted. The leaves of the control spinach had the highest degree of rot, and the best results were obtained with 0.20 mg / mL melatonin treatment. During storage, spinach leaves tend to turn yellow, which is mainly caused by chlorophyll degradation. During storage, the sensory quality of spinach gradually deteriorated and rotted. At the beginning of storage, the spinach in each group was emerald green, with full leaves and beautiful shapes, and had the inherent aroma of spinach. This shows that 0.20 mg / mL melatonin treatment can maintain the appearance, shape and smell of post-harvest spinach and improve its commercial value. This may be because melatonin has a significant antioxidant effect, which can reduce the accumulation of reactive oxygen and membrane lipid peroxidation, thereby improving the sensory quality of spinach during storage. 3.2 Sensory and color analysis of spinach

[0029] The ΔE value reflects the change in L* and hue angle of spinach. The smaller the ΔE value, the more stable the color change of spinach. With the beginning of storage, the biochemical processes carried out by fruits and vegetables lead to the production of enzymes, which affect the color of vegetables. The smaller color difference of spinach treated with melatonin indicates that melatonin successfully slowed down the biochemical processes, resulting in a reduction in enzymatic production. Figure 2 As shown in the figure, the ΔE values ​​of the 0.20 mg / mL and 0.30 mg / mL MT treatment groups increased more slowly compared with the blank sample. During storage, spinach leaves tend to turn yellow, which is mainly caused by chlorophyll degradation. The ΔE value of spinach treated with melatonin was lower, indicating that melatonin successfully maintained the color of spinach and delayed its change from green to yellow. The results showed that the color difference of the control group and the treatment group showed an increasing trend (Figure 2). As the storage time increased, the color difference of spinach treated with 0.20 mg / mL melatonin was the smallest.

[0030] 3.3 Analysis of Vitamin C Content from Figure 3 It can be seen that the vitamin C content of spinach in the 0.10, 0.20 and 0.30 mg / mL melatonin treatment groups was significantly higher than that in the control group. In addition, the slowest decrease in vitamin C content was observed under the condition of 0.20 mg / mL melatonin. Vitamin C is one of the parameters of spinach nutritional quality. Spinach treated with three concentrations of melatonin showed significantly higher vitamin C content than the control. The results of this experiment may be attributed to the difference in the effect of melatonin on the vitamin C metabolism of spinach. Studies have shown that melatonin can improve the scavenging rate and reducing capacity of superoxide anion radicals, hydroxyl radicals, and DPPH radicals in spinach. Melatonin can promote the biosynthesis process of amino acids in fruits and vegetables, thereby producing more nutrients. Spraying melatonin after harvest can maintain the vitamin C content of spinach. Zahed et al. sprayed melatonin weekly to effectively maintain the soluble solids, total acidity and vitamin C content in strawberry fruits; Miao et al. treated broccoli with melatonin to maintain the vitamin C content. Similar results were obtained in the present study, where melatonin treatment maintained high levels of soluble sugars, soluble solids, and vitamin C, resulting in better nutritional quality of spinach.

[0031] 3.4 Analysis of weight loss rate The storage quality of spinach can be reflected by the weight loss rate. Figure 4 As shown in the figure, the weight loss rate of spinach increased with the extension of storage time. However, the weight loss rate of spinach in the control group during storage was significantly higher than that in the melatonin-treated group. In addition, the weight loss rate of spinach in the 0.20 mg / mL melatonin-treated group was lower than that in the 0.10 and 0.30 mg / mL melatonin-treated groups. This may be due to the fact that melatonin treatment can reduce the decay rate of fruits and vegetables. The weight loss of spinach will increase during storage due to high respiration and evaporation rates and rapid utilization of stored metabolites by cellular metabolic activities. Studies have shown that melatonin treatment can significantly reduce the weight loss of sweet cherry fruit, delay senescence, and improve its resistance to abiotic stress. 3.5 Analysis of chlorophyll content

[0032] During storage, spinach leaves tend to turn yellow, which is mainly due to the degradation of chlorophyll. The color change of green leafy vegetables is closely related to the yellowing rate and chlorophyll content. Figure 5As shown in the results, the chlorophyll content of both the control group and the treatment group showed a downward trend. On the 12th day, the chlorophyll content of spinach leaves in the control group was 0.17 mg / g, while the chlorophyll content of spinach treated with 0.10, 0.20 and 0.30 mg / mL melatonin was 0.33, 0.53 and 0.47 mg / g. Compared with the control group, the 0.20 mg / mL melatonin treatment maintained a relatively good color, delayed the occurrence of yellowing and chlorophyll content, maintained a good appearance of spinach, and significantly delayed the senescence of spinach. This shows that melatonin treatment can effectively delay the yellowing rate of leaves, which is consistent with the results of Lu Ruixue et al. Melatonin delays the decline of chlorophyll content. At the same time, melatonin also inhibits the activity of chlorophyll decomposition enzymes such as chlorophyllase (Chlase, CLH) and red chlorophyll catabolitereductase (Red chlorophyll catabolitereductase, RCCR). 3.6 Analysis of soluble solids content

[0033] Soluble sugar and soluble solid content are the main factors in determining the quality of post-harvest vegetables. Spinach contains 70% to 90% water, is fresh and delicious, and contains vitamin C, minerals, sugars, organic acids, crude fiber and other nutrients. Figure 6 As shown in the figure, the soluble solids content of spinach gradually decreased in both the control group and the treatment group. After the fourth day of storage, the soluble solids content of the treatment group was significantly higher than that of the control group. This may be because melatonin treatment can reduce the decay rate of fruits and vegetables, inhibit ROS accumulation, and delay the aging of fruits and vegetables, thereby maintaining the soluble solids content of spinach after harvest.

[0034] 3.7 Analysis of soluble sugar content Similarly, the soluble sugar content of spinach decreases with storage time. Soluble sugar is one of the more abundant components of spinach. Figure 7 As shown in the data, during the 12-day storage period, the soluble sugar content of spinach in the control and melatonin treatment groups showed a downward trend, and the 0.20 and 0.30 mg / mL melatonin treatments significantly delayed the decline in the soluble sugar content of spinach. In the later stage of storage, due to the respiratory metabolism of fruits and vegetables and the influence of microorganisms, sugar, acid and other nutrients will decrease. Melatonin treatment can affect the respiratory metabolism of spinach and effectively inhibit the production of ROS, thereby delaying the decline in soluble sugar content.

[0035] 3.8 Analysis of antioxidant enzyme activity SOD can remove superoxide anions, produce H2O2 and O2, maintain the balance of active oxygen metabolism, and play a role in cell protection. Figure 8In a, during storage, the overall trend of SOD activity in all four spinach groups increased, which promoted the increase of SOD activity in the pre-storage treatment group and delayed the decrease of SOD activity in the post-storage treatment group. At the end of storage, the activity of the 0.20 mg / mL melatonin treatment group was the highest. This may be because melatonin can reduce oxidative damage, maintain the intact cell membrane structure and function, and delay the post-harvest decay of spinach. This shows that melatonin plays an important role in promoting the post-harvest preservation of spinach by improving the antioxidant activity of spinach.

[0036] The POD activity of spinach increased first and then decreased during storage, and there was a significant difference between the 0.20 mg / mL melatonin treatment group and the control group ( Figure 8 b). CAT catalyzes the decomposition of H2O2 into water and molecular oxygen. The trends of CAT activity in the control and treatment groups were similar, both showing a gradual increase, while the highest CAT activity was observed on day 12 in the 0.20 mg / mL melatonin treatment group ( Figure 8 c) If Figure 8 As shown in Figure d, the MDA content of spinach continued to increase during the entire storage period, and the 0.10, 0.20, and 0.30 mg / mL melatonin treatment groups were lower than the control group. The effect of the 0.20 mg / mL melatonin treatment group was the most significant during the entire storage period, indicating that the 0.20 mg / mL melatonin treatment group effectively controlled the degree of cell membrane damage, thereby delaying the decay of spinach.

[0037] POD can catalyze H2O2 to oxidize phenolic substances to produce quinone compounds. Melatonin-treated plants can reduce plant oxidative damage under stress by increasing the activity of POD. Corn treated with melatonin can remove excess H2O2 through the melatonin scavenging cascade reaction, promote the expression and activity of antioxidant enzyme genes such as POD and CAT, and significantly reduce the level of reactive oxygen.

[0038] 3.9 Analysis of free radical scavenging rate Reactive oxygen free radicals are superoxide anions produced by the electron transport chain, which can damage the composition and activity of nucleic acids, leading to cell deformity, mutation and apoptosis. The generation and accumulation of ROS is one of the main reasons for the browning, rotting and deterioration of fruits and vegetables. In order to slow down the occurrence of the above situation, enzymes such as SOD, POD and CAT in cells are needed to play a scavenging role. Adding exogenous melatonin can improve its reactive oxygen scavenging ability and can be used to preserve fruits. During the storage period, if Fig. 9 As shown in the figure, the DPPH free radical scavenging rate of spinach showed a downward trend, while the hydroxyl free radical scavenging rate of spinach showed a trend of first increasing and then decreasing. The DPPH free radical scavenging rate and hydroxyl free radical scavenging rate of the 0.20 mg / mL and 0.30 mg / mL melatonin treatment groups were significantly higher than those of the control group.

[0039] in conclusion Spinach was sprayed with melatonin and the contents of vitamin C, soluble sugar, soluble solid chlorophyll, color difference, malondialdehyde, chlorophyll, SOD, CAT, and POD activities, as well as the changes in free radical scavenging rate were determined to determine the best melatonin concentration for spinach preservation. The results showed that melatonin, as an antioxidant, could effectively extend the shelf life of spinach from 4 days to 12 days. Spinach treated with melatonin had lower weight loss rate and color difference, higher soluble sugar, soluble solid, vitamin C, chlorophyll content and antioxidant enzyme activity, and could improve the scavenging rate of DPPH free radicals and hydroxyl free radicals in spinach, especially spinach treated with 0.20 mg / mL melatonin had the best effect. Therefore, melatonin treatment may be an effective method to extend the shelf life of spinach and improve its quality.

[0040] The above embodiments are merely illustrative of the design principles and uses of the present invention, and are not intended to limit the present invention. Anyone familiar with the technology may modify or change the above embodiments without violating the spirit and scope of the present invention. Therefore, all equivalent modifications or changes made by those with ordinary knowledge in the technical field without departing from the spirit and technical ideas disclosed by the present invention should still be covered by the claims of the present invention.

Claims

1. A method for preserving spinach after harvest, comprising the following steps: The present invention is achieved through the following technical steps: (1) Select spinach of uniform size, bright color, and free of rot and insect pests, wash it continuously in running distilled water to remove dirt particles on the surface of the spinach, then remove it and drain it, and dry it at 10±1℃ and relative humidity of 45~55% for 30 min; (2) Spray 500 g of selected spinach with 200 mL of 0.10-0.50 mg / mL melatonin solution and air dry it under low temperature for 60 min; (3) Package the dried spinach in polyethylene bags, 90 g per bag, and store the packaged spinach at 4°C.

2. A method for preserving spinach after harvest according to claim 1, characterized in that: After spraying the melatonin solution, place the spinach in an air-dried environment at 10±1℃ and relative humidity of 45-55% for 60 minutes.

3. A method for preserving spinach after harvest according to claim 1, characterized in that: 500 g of selected spinach were sprayed with 200 mL of 0.30 mg / mL melatonin solution.

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