Preparation and application of composite fresh-keeping pad capable of accurately regulating and controlling storage and fresh-keeping effects of picked cherries

Abstract

The invention discloses preparation and application of a multifunctional composite fresh-keeping pad capable of accurately regulating and controlling storage and fresh-keeping effects of picked cherries. The preparation method comprises the following steps: preparing fucoidin-nano-selenium by taking fucoidin as a template by adopting a chemical reduction method, and immobilizing chlorite in a nano-selenium shell; chitosan / tannic acid hydrophilic gel is prepared through a cross-linking method, nanoparticles are dispersed in a hydrogel base material, and the dry hydrogel is sprayed to the upper face and the lower face of the adhesive paper; and by taking polycaprolactone as a spinning agent, spraying electrospinning on the surface of the dry hydrogel by adopting an electrospinning technology, so as to prepare the multifunctional composite fresh-keeping pad with a sandwich structure. The layer-by-layer assembled composite fresh-keeping pad responds by adsorbing water vapor and carbon dioxide generated by respiration and transpiration of picked cherries, and the embedded chlorite is activated to generate chlorine dioxide gas which is released into a packaging environment, so that the storage quality and antibacterial activity of the picked cherries are remarkably improved; the substrate can be used as a preservation substrate for postharvest storage and e-commerce cold chain circulation of cherries.

Description

Technical Field

[0001] The present invention relates to the field of food preservation, and provides a preparation and application of a multifunctional composite fresh-keeping pad for precisely regulating the postharvest storage and fresh-keeping effect of cherries. Background Art

[0002] Cherry is a deciduous arbor fruit tree of the genus Prunus in the Rosaceae family. Its fruits are rich in nutrients, have thin skins, are soft, brightly colored, and sweet and sour, with high nutritional value and are deeply loved by consumers in recent years. Cherries belong to juicy berry fruits. After harvesting, due to high-intensity respiration and susceptibility to mechanical damage, they are extremely vulnerable to invasion by pathogens, leading to phenomena such as decay and spoilage, causing serious economic losses. At present, chlorine dioxide, as an efficient, safe, and broad-spectrum disinfectant and sterilizer, can destroy the lipid and protein structures in the cell membranes of pathogenic microorganisms through strong oxidation, thereby inhibiting the growth of microorganisms and slowing down the spoilage rate, achieving the purpose of maintaining food quality and extending the shelf life of food. However, the chlorine dioxide fresh-keeping treatment method has great drawbacks. The traditional chlorine dioxide gas fumigation method cannot control its release rate. Premature or too slow release of chlorine dioxide will affect some nutrients in fruits and vegetables, resulting in loss of storage quality and flavor of fruits and vegetables and reduction of their nutritional value. Secondly, commercial chlorine dioxide slow-release agents cannot precisely regulate the release according to the postharvest respiration characteristics of different fruits and are difficult to effectively exert the fresh-keeping effect. In addition, the single antibacterial property cannot meet the requirements of postharvest storage and fresh-keeping of fruits. Therefore, it is of great significance to construct a multifunctional packaging material with antibacterial activity using natural biological resources as the basic structural unit and the postharvest physiological changes of fruits and vegetables as the endogenous response characteristics.

[0003] Selenium is one of the essential trace elements for organisms and has powerful antioxidant and antibacterial activities. Selenium exists in three forms (organic selenium, inorganic selenium, and nano-selenium). Among them, nano-selenium particles have higher biological activity and utilization rate. They can be used either as a single selenium supplement or as an active delivery carrier to encapsulate active substances. Currently, they are widely used in the fields of biomedicine, animal nutrition, and food. Compared with traditional nano-material preparation technologies, nanofibers have become a hot technology for developing food active packaging films due to their simple operation and high preparation efficiency. Usually, a polymer with antibacterial efficacy is used as the spinning substrate, or an antibacterial agent is added to a spinnable polymer solution. Loading natural antibacterial agents through electrospinning technology can effectively improve the stability of antibacterial agents, achieve controlled release of antibacterial agents, and thus extend the shelf life of food. In addition, hydrogels have a unique three-dimensional structure material that can provide sufficient capacity to accommodate small molecules, polymers, and particles. Hydrogels can be dried in a certain way to replace the liquid phase in the hydrogel with gas to form a nano-porous solid aerogel material, which has the characteristics of moisture absorption, swelling, and shock absorption. Therefore, for the respiration rate model fitting of postharvest cherries, using the moisture absorption characteristics of hydrogels, taking the respiration and transpiration of postharvest cherries as the response source, selenium nanoparticles encapsulating chlorite are constructed, and then dispersed in the hydrogel matrix. The dry hydrogel is sprayed on the upper and lower surfaces of the sticker; polycaprolactone is used as the spinning solvent, and electrospun fibers are sprayed on the surface of the dry hydrogel by electrospinning technology, so as to prepare a multi-functional composite fresh-keeping pad with a sandwich structure, and finally achieve the controlled release of chlorine dioxide and the synergistic antibacterial and antioxidant characteristics.

[0004] Research found that there is currently no relevant research or patent application for a chlorine dioxide / nano-selenium / nanofiber fresh-keeping pad designed to precisely regulate the fresh-keeping effect according to the postharvest respiration characteristics of cherries. Based on the above background, in the present invention, fucoidan is used as a template, and selenium nanoparticles are prepared by chemical reduction method, and chlorite is immobilized inside the nanoparticle shell; a chitosan / tannic acid hydrophilic gel is prepared by ionic cross-linking method, and the composite porous nanoparticles are dispersed in the hydrogel substrate. The dry hydrogel is sprayed on the upper and lower surfaces of the sticker; polycaprolactone is used as the spinning solvent, and electrospun fibers are sprayed on the surface of the dry hydrogel by electrospinning technology, so as to prepare a multi-functional composite fresh-keeping pad with a sandwich structure. The dry gel structure of this layer-by-layer assembled composite fresh-keeping pad responds by adsorbing water vapor and carbon dioxide generated by the respiration and transpiration of postharvest cherries, activating the encapsulated chlorite to generate chlorine dioxide gas and releasing it into the packaging storage environment, and synergistically antibacterial with selenium nanoparticles. Compared with traditional chlorine dioxide fumigation and commercial chlorine dioxide sustained-release agents, this composite fresh-keeping pad significantly inhibits the respiration intensity, color fission, and fruit decay rate of postharvest cherries, and maintains better fruit hardness, soluble solid content, and titratable acid content. In addition, the aerogel fresh-keeping pad has a tight network and good mechanical properties, reducing the mechanical damage of cherry fruits. Summary of the Invention

[0005] The object of the present invention is to provide a preparation method of a multifunctional composite fresh-keeping pad for precisely regulating the post-harvest storage and fresh-keeping effect of cherries, which is characterized by including: (1) Preparation of chlorite - fucoidan - nano selenium: (2) Preparation of chitosan / tannic acid composite hydrogel: (3) Preparation of the composite fresh-keeping pad: Among them, (1): Dissolve 0.5 - 1 g of fucoidan in 30 - 50 mL of glacial acetic acid (1.2%), stir magnetically in the dark; slowly add 40 - 60 mL of sodium selenite solution at 30 - 50 mmol / L and 1.2 - 1.5 g of chlorite, stir magnetically; slowly drip 30 - 50 mL of VC solution at 100 - 200 mmol / L, transfer to a reagent bottle after magnetic stirring, and store in the refrigerator; (2) Preparation of chitosan / tannic acid composite hydrogel: Dissolve 500 - 800 μL of fucoidan - nano selenium in a chitosan solution at 0.6 - 1 mg / mL, and then dropwise add a tannic acid solution at 1.2 - 1.5% to obtain a chitosan / tannic acid composite hydrogel.

[0006] (3) Preparation of the composite fresh-keeping pad: Freeze-dry 5 - 10 mL of the above composite hydrogel and spray it on both sides of the sticky paper, with a thickness of 0.2 - 0.5 mm; use polyvinyl alcohol as a solvent, and obtain a nanofiber membrane after high-voltage electrospinning at 10 kV, a spinning distance of 8 cm, and an injection rate of 0.3 mL / h, and spray it on the surface of the dry gel, with a thickness of 0.1 - 0.5 mm; After the composite fresh-keeping pad is made, the slow-release performance of chlorine dioxide is measured.

[0007] A typical example of the present invention: A preparation method of a multifunctional composite fresh-keeping pad for precisely regulating the post-harvest storage and fresh-keeping effect of cherries: (1) Preparation of chlorite - fucoidan - nano selenium: Take 1 g of fucoidan and dissolve it in 50 mL of glacial acetic acid (1.2%), stir magnetically in the dark; slowly add 60 mL of sodium selenite solution at 50 mmol / L and 1.5 g of chlorite, stir magnetically; slowly drip 50 mL of VC solution at 200 mmol / L, transfer to a reagent bottle after magnetic stirring, and store in the refrigerator; (2) Preparation of chitosan / tannic acid composite hydrogel: Dissolve 500 - 800 μL of fucoidan - nano selenium in a chitosan solution at 0.6 - 1 mg / mL, and then dropwise add a tannic acid solution at 1.2 - 1.5% to obtain a chitosan / tannic acid composite hydrogel.

[0008] (3) Preparation of the composite fresh-keeping pad: 10 mL of the above-mentioned composite hydrogel was freeze-dried and then sprayed on both sides of the sticker paper, with a thickness of 0.5 mm; Using polyvinyl alcohol as the solvent, a nanofiber membrane was obtained after high-voltage electrospinning at 10 kV, with a spinning length of 8 cm and an injection rate of 0.3 mL / h, and then sprayed on the surface of the dry gel, with a thickness of 0.5 mm; The specific preparation method of the nanofiber membrane using polyvinyl alcohol as the solvent, after high-voltage electrospinning at 10 kV, with a spinning length of 8 cm and an injection rate of 0.3 mL / h is as follows: Weigh 1.65 g of polycaprolactone and dissolve it in 7 mL of dichloromethane solution, magnetically stir at 600 r / min in the dark for 8 h to prepare the electrospinning solution. Fix a micro-injection pump with a 10 mL syringe for electrospinning: the injection flow rate is 3 mL / h, the voltage is (10 ± 0.5) kV, and the needle is 20 cm away from the receiving plate. Dry in a vacuum drying oven for 48 h.

[0009] (4) After the composite fresh-keeping pad is made, the chlorine dioxide slow-release performance is measured.

[0010] The present invention mainly solves the problem of the single effect of chlorine dioxide in the post-harvest storage and preservation of cherries. It focuses on the preparation of the multifunctional composite fresh-keeping pad and its application in improving the synergistic antibacterial, antioxidant, and shock-absorbing protection of post-harvest cherries. The main difficulty lies in embedding chlorite in fucoidan-nano selenium particles, designing a dry gel-nanofiber-based composite pad with a moisture-absorbing function, and releasing chlorine dioxide gas through the response of post-harvest respiration and transpiration of cherries, and improving the preservation effect and shock-absorbing protection through the synergistic effect with nano selenium.

[0011] The positive effects of the composite fresh-keeping pad for precisely regulating the post-harvest storage and preservation effect of cherries disclosed in the present invention compared with the prior art are as follows: (1) Embed chlorite and release it through the action of post-harvest carbon dioxide and water vapor of the fruit, and design a multifunctional composite fresh-keeping pad with synergistic antioxidant, cherry preservation, and shock-absorbing effects.

[0012] (2) Compared with traditional chlorine dioxide fumigation, commercial chlorine dioxide slow-release agents, and commercial chlorine dioxide quick-release agents, it has a better storage and preservation effect on post-harvest cherries and a better preservation effect during e-commerce cold chain transportation.

[0013] The present invention further discloses the application of the multifunctional composite fresh-keeping pad for precisely regulating the post-harvest storage and preservation effect of cherries in improving the storage quality and synergistic antibacterial and antioxidant activities of post-harvest cherries. To confirm its function, the following experiments were mainly carried out: I. It has the ability to reduce the post-harvest respiratory intensity of cherries; II. It has the ability to maintain the post-harvest storage quality of cherries; III. Capable of maintaining good color and taste of cherries after harvest; IV. Capable of reducing mechanical damage of cherries during post - harvest transportation. Description of the Drawings

[0014] Figure 1 Design of the multifunctional composite fresh - keeping pad; (A) Schematic diagram of the rational design structure of the multifunctional composite fresh - keeping pad; (B) Physical picture of the multifunctional composite fresh - keeping pad; Figure 2 Characterization of the physical and chemical structure of the multifunctional composite fresh - keeping pad; (A) Macroscopic morphology of chlorite / fucoidan - selenium nanoparticles; (B) STEM - EDS map of chlorite / fucoidan - selenium nanoparticles; (C) Fourier transform infrared spectroscopy map; (D) Transmission electron microscope image of chlorite / fucoidan - selenium nanoparticles; (E) Scanning electron microscope image of chitosan / tannic acid - hydrogel; (F) Scanning electron microscope image of composite dry gel / spinning; Figure 3 Determination of chlorine dioxide release performance Figure 4 Effect of the multifunctional composite fresh - keeping pad on the storage quality of post - harvest cherries; (A) Respiration intensity; (B) Fruit rot rate; (C) Fruit weight loss rate; Figure 5 Effect picture of different treatment groups on the storage quality of post - harvest cherries; Figure 6 Effect of the multifunctional composite fresh - keeping pad on the antioxidant capacity of post - harvest cherries; (A) DPPH free radical scavenging rate; (B) Flavonoid content; (C) Anthocyanin content; Figure 7 Sensory score of cherries at 35 d of storage; Figure 8 Simulation of transportation vibration stress test; (A) Simulation of a dynamic transportation test vehicle; (B) Cherry fruit hardness; (C) Cherry fruit damage rate; (D) Cherry fruit softening rate; Note: Different lowercase letters indicate significant differences between groups ( P <0.05). Detailed Implementation Modes

[0015] The following examples facilitate a better understanding of the present invention, but do not limit the present invention. The experimental methods in the following examples are all conventional methods unless otherwise specified. The test materials used in the following examples are all obtained from regular biochemical reagent stores unless otherwise specified. In the following examples, quantitative tests are all set with three repeated experiments, and the results are averaged. Example

[0016] I. Preparation of chlorite - fucoidan - nano - selenium Prepared by the method of in-situ reduction of Na2SeO3 with vitamin C (VC). The specific steps are as follows: 0.5 g of fucoidan is dissolved in 30 mL of glacial acetic acid (1.2%), and magnetically stirred in the dark at 800 r / min for 8 h; 40 mL of 30 mmol / L sodium selenite solution and 1.2 g of chlorite are slowly added, and magnetically stirred at 800 r / min for 30 min; 30 mL of 100 mmol / L VC solution is slowly dropped in, magnetically stirred at 800 r / min for 30 min, then transferred to a reagent bottle and stored in a 4 °C refrigerator.

[0017] II. Preparation of the multifunctional composite preservation pad Using tannic acid as a cross-linking agent, chitosan with a degree of deacetylation of 98% and a molecular weight of 3 kDa is selected as the basic framework, and chitosan / tannic acid hydrogel is prepared by the ion cross-linking method. First, 500 μL of fucoidan-nano selenium is dissolved in 0.6 mg / mL tannic acid solution, and then 1.2% calcium chloride solution is added. After freeze-drying 5 mL of the above composite hydrogel for 24 h, it is sprayed on both sides of the sticky paper with a thickness of 0.2 mm. Using polyvinyl alcohol as a solvent, a nanofiber membrane is obtained by high-voltage electrospinning at 10 kV, a spinning distance of 8 cm, and an injection rate of 0.3 mL / h, and it is sprayed on the surface of the dry gel with a thickness of 0.1 mm.

[0018] III. Observation by transmission electron microscope Take 3 μL of the sample solution to be tested, drop it on a copper mesh with a carbon support film, dry it and then carry out subsequent experiments. The detection voltage is 30 kV.

[0019] IV. Fourier transform infrared spectroscopy analysis The freeze-dried sample of fucoidan-nano selenium is ground and mixed with KBr (the ratio is about 1:50), then pressed into a tablet, baked under an infrared lamp for 2 minutes and then sampled to collect the KBr background; the wavenumber range: 800 - 4000 cm -1 , resolution: 4.2 cm -1 , scanning times: 35.

[0020] V. Observation by scanning electron microscope The sample is dehydrated with gradient ethanol (30%, 60%, 90%, 100%) for 10 min each time, freeze-dried, then fixed on the sample stage with conductive glue, and sputtered with noble metal gold with a thickness of about 10 nm under vacuum conditions. The processed sample is placed on the carrier stage for observation by scanning electron microscope.

[0021] VI. Determination of the effective concentration of chlorine dioxide Place the treated samples with the same chlorine dioxide concentration in a sterilized non-woven breathable packaging bag of 6 cm × 8 cm and seal it. Then place it in a sealed bag of 12 cm × 18 cm and seal it. Randomly select 5 samples from each group every day, and use a chlorine dioxide gas detector to detect the gas concentration in the sealed bag, and observe for a total of 24 days. Chlorine dioxide fumigation group: 2.0 mg / L; Chlorine dioxide sustained-release agent: 1 g; Chlorine dioxide quick-release agent: 1 g.

[0022] VII. Determination of Fruit Respiration Intensity Randomly weigh 200 g of cherry fruits at different storage time points, put them into a sealed plastic box with a lid and suffocate for 2 h, and measure with a CO2 / O2 gas analyzer, with the unit of mg / (kg·h).

[0023] VIII. Determination of Fruit Rot Rate Select cherry samples with obvious breakage and juice leakage or rot on the fruit surface at different storage time points. Rot index (%) = (number of rotten fruits / total number of fruits) × 100%.

[0024] IX. Determination of Fruit Weight Loss Rate The weight loss rate (S, %) is measured by the weighing method, and the calculation formula is as follows.

[0025]

[0026] Where: m is the initial mass of cherries, g; m n is the mass at the nth day of storage, g.

[0027] X. Determination of DPPH Free Radical Scavenging Ability Use the DPPH free radical scavenging ability kit from Nanjing Jiancheng Bioengineering Research Institute for determination. The unit of DPPH free radical scavenging ability is mg Trolox / kg. The DPPH free radical scavenging ability of fruits is calculated based on the fresh weight of the fruits.

[0028] XI. Determination of Soluble Solids and Titratable Acid Contents Randomly take 10 cherries from each treatment, squeeze the juice and filter it through 4 layers of gauze. Take 200 μL of the filtrate, dilute it 50 times with deionized water, and measure it with a PLA-1 refractometer. Repeat 3 times and calculate the average value, with the unit expressed as %.

[0029] XII. Determination of Color Difference Use a color difference meter to record L * 、 a * 、 b * values and take the average value.

[0030] 13. Sensory Evaluation The cherry sensory evaluation form includes five evaluation indicators and detailed scoring criteria: appearance, aroma, juiciness, flavor, and firmness. Each indicator has a maximum score of 10 and is divided into four levels. A trained sensory evaluation panel of nine students assigns scores based on this scoring system. The higher the score, the better the quality of the cherry. Cherries with a score below 30 lose commercial value, and those with a score below 25 lose edible value. See Table 1 for details.

[0031]

[0032] 14. Simulated transport vibration stress test E-commerce packaging methods were divided into two groups: a control group (CK) using corrugated cardboard with a separator, and a composite fresh-keeping mat group using a corrugated cardboard with a composite fresh-keeping mat. The selected cherries were boxed in a laboratory at room temperature (25°C), with 12 cherries per box placed in a single layer. They were then preconditioned in a simulated laboratory for 24 hours (temperature: 25°C, relative humidity: 50%). They then underwent an ISTA 6A simulated transport test (highway trailer for 30 minutes). Following this, a vibration stress test was conducted, with a frequency of 5 Hz, a duration of 8 hours, a temperature of 4°C, and a relative humidity of approximately 95%. Fifteen cherries were randomly selected to measure changes in quality.

[0033] 1. Rational design and physical and chemical structure characterization of multifunctional composite fresh-keeping pad like Figure 1 As shown in the figure, the multifunctional composite fresh-keeping pad is similar to a "sandwich" structure, with the middle layer being sticky paper. The upper and lower outer layers of the sticky paper are sprayed with composite dry gels with antibacterial, antioxidant, hygroscopic and swelling functions. The composite dry gel is chlorite / fucoidan-nanoselenium / chitosan hydrogel, and the outermost layer is covered with an electrospun membrane. Figure 2 As shown in A, the synthesized chlorite / fucoidan-nanoselenium particle solution is clear and exhibits a typical orange-red color due to surface plasmon resonance excitation of selenium atoms. Transmission electron microscopy results show ( Figure 2 B) This system is a typical "shell-core" structure with a particle size of about 80 nm. In order to verify that the nanoparticles are selenium-containing nanoparticles, STEM-EDS mapping scans were performed on the sample. Selenium atoms emitted strong signals, indicating that the nanoparticles are indeed selenium-containing nanoparticles. In addition, Figure 2 As shown in C, the results of Fourier infrared spectrometer show that there are -OH bending vibration and CO stretching vibration on the fucoidan molecular chain in the wave number range of 1050-1500, which proves the successful incorporation of fucoidan in this system; there is -NH bending vibration absorption in the wave number range of 1500-1700, which proves that fucoidan NH3 +Protonation plays a role in stabilizing nano-selenium, and the whole system is positively charged; there is a hydroxyl absorption peak in the wavenumber range of 3000 - 3650, which proves the existence of hydrogen bond binding between the molecules of this system. Further, the chlorite / fucoidan-nano-selenium particles are incorporated into the prepared chitosan / tannic acid hydrogel. The scanning electron microscope results show ( Figure 2 D) that the composite xerogel presents a dense porous structure with a pore size of about 10 nm. Finally, an electrospun nanofiber membrane is sprayed on the composite xerogel. The scanning electron microscope results show ( Figure 2 F) that a fibrous structure with uniform thickness and density is successfully prepared.

[0034] Physicochemical structure characterization of the composite fresh-keeping pad. (A) Macroscopic morphology of chlorite / fucoidan-selenium nanoparticles; (B) STEM-EDS diagram of chlorite / fucoidan-selenium nanoparticles; (C) Fourier transform infrared spectroscopy diagram; (D) Transmission electron microscope diagram of chlorite / fucoidan-selenium nanoparticles; (E) Scanning electron microscope diagram of chitosan / tannic acid-hydrogel; (F) Scanning electron microscope diagram of composite xerogel / spun fiber.

[0035] II. Release performance of chlorine dioxide Taking traditional chlorine dioxide fumigation, commercial chlorine dioxide slow-release agents, and commercial chlorine dioxide quick-release agents as controls, the concentration of chlorine dioxide in the packages of postharvest cherries was measured. As Figure 3 shown, during the entire storage period, with the extension of storage time, the effective concentration of chlorine dioxide in each group of packages first increased and then gradually decreased as a whole; at 2 d in the early stage of storage, the effective concentrations of chlorine dioxide in the chlorine dioxide fumigation, chlorine dioxide quick-release agent, and composite fresh-keeping pad treatment groups all reached the maximum value, while the rate of chlorine dioxide release in the chlorine dioxide slow-release agent treatment group was slower; in addition, compared with the commercial chlorine dioxide quick-release agent, with the extension of storage time, the treatment with the composite fresh-keeping pad of the present invention can significantly increase the chlorine dioxide concentration in the package, and the effect is equivalent to that of the commercial chlorine dioxide slow-release agent treatment. This shows that the chlorine dioxide release in the composite fresh-keeping pad treatment group of the present invention is more stable, can ensure that the effective concentration of chlorine dioxide in the package remains at a relatively stable level, and plays a positive role in the preservation of cherries in the package.

[0036] III. Effects of the multifunctional composite fresh-keeping pad on the storage quality of postharvest cherries Figure 4 and Figure 5 show the effects of the multifunctional composite fresh-keeping pad on the storage quality of postharvest cherries. The respiration of fruits and vegetables determines the speed of the metabolic and senescence processes of fruit tissues. Fruits and vegetables consume various nutrients during respiration. Reducing their respiration intensity can extend the shelf life of fruits and vegetables. As Figure 4As shown in Figure A, on the 7th day of storage, the CK group showed a respiratory peak. The commercial slow-release chlorine dioxide treatment group delayed the respiratory peak of cherry fruits to the 14th day, while the traditional chlorine dioxide fumigation, commercial fast-release chlorine dioxide, and the treatment group with the composite fresh-keeping pad of this patent could delay the respiratory peak of cherry fruits to the 21st day. At this time, the respiratory intensity of cherry fruits in the composite fresh-keeping pad treatment group was 51.46 mg / (kg·h). The results showed that the treatment with the composite fresh-keeping pad of this patent could effectively inhibit the respiration of cherries, delay fruit senescence, and thus maintain the storage quality of cherries. Secondly, hardness can reflect the degree of fruit softening and is one of the important indicators for judging fruit maturity and storage quality.

[0037] The decay rate and weight loss rate are important indicators for measuring the storage quality of fruits and vegetables. During storage, the gradual loss of water will cause the fruit to wilt in shape and lose its plump texture, and even lose its commercial value. As Figure 4 As shown in Figures B and C, with the extension of storage time, the weight loss rate and decay rate of cherry fruits continued to increase, but the chlorine dioxide treatment inhibited the weight loss and decay of cherry fruits. On the 35th day of storage, the order of the weight loss rate of cherry fruits was CK group < commercial fast-release chlorine dioxide treatment group < traditional chlorine dioxide fumigation treatment group < commercial slow-release chlorine dioxide treatment group < treatment group with the composite fresh-keeping pad of this patent; in addition, on the 21st day of storage, the decay rate of cherry fruits began to increase significantly, and the increase rate of fruits in the CK group was significantly faster than that in the chlorine dioxide treatment group. On the 35th day of storage, the decay rate of fruits in the CK group was 58.20%, which was 37.63% higher than that in the composite fresh-keeping pad treatment group; this was because the chlorine dioxide treatment destroyed the decomposition of microbial amino acids inside cherry fruits, controlled the synthesis of their proteins, killed pathogenic bacteria, and reduced the fruit decay rate. In this experiment, it was found that the weight loss rate and decay rate of cherries in the commercial fast-release chlorine dioxide treatment group were higher than those in other treatment groups. This was because the too-high concentration of chlorine dioxide at the initial stage of storage increased the respiratory intensity and transpiration of fruits due to its strong oxidizing property, causing the water loss of cherries to be faster than that in other treatment groups. Therefore, precisely regulating the respiration of postharvest cherries is important for reducing the fruit weight loss rate and actual weight loss rate.

[0038] IV. Effects of the multifunctional composite fresh-keeping pad on the antioxidant capacity of postharvest cherries As Figure 6 As shown in Figure A, during the entire storage period, the DPPH free radical scavenging rate of postharvest cherries showed a downward trend; compared with the CK, chlorine dioxide fumigation, chlorine dioxide fast-release agent, and chlorine dioxide slow-release agent treatment groups, the composite fresh-keeping pad treatment significantly inhibited the decline of the DPPH free radical scavenging rate of cherry fruits. In addition, flavonoids are important nutrients in cherries and have antioxidant, anti-inflammatory, antibacterial, tumor cell proliferation inhibition, and immunomodulatory functions, and have preventive or therapeutic effects on cancer, cardiovascular, and neurodegenerative diseases. As Figure 6As shown in Figure B, it is the effect of chlorine dioxide treatment in different ways on the flavonoid content of cherries. The flavonoid content of cherry fruits generally shows a gradually decreasing trend. At the end of storage, the flavonoid content of cherries in the composite fresh-keeping pad treatment group is significantly higher than that of other treatment groups. In addition, cherry fruits present a delicious red color due to the large amount of anthocyanins contained inside, and have strong antioxidant activity; the physiological functions of anti-aging and anti-mutation, and the higher its content indicates that the fruit has good color and higher antioxidant capacity. Figure 6 Figure C shows the effect of different chlorine dioxide treatments on the anthocyanin content of cherries. During the entire storage period, the anthocyanin content of cherry fruits is similar to the flavonoid content, and generally shows a gradually decreasing trend. On the 21st day of storage, the anthocyanin content in the composite fresh-keeping pad treatment group is 0.383 mg / g, which is 47.30% higher than that of the CK group. At the end of storage, the order of anthocyanin content is CK group < commercial chlorine dioxide quick-release agent treatment group < traditional chlorine dioxide fumigation treatment group < commercial chlorine dioxide slow-release agent treatment group < composite fresh-keeping pad treatment group of this patent. The results show that postharvest chlorine dioxide treatment can delay the decline of anthocyanin content in cherry fruits, and the composite fresh-keeping pad treatment group has the most obvious effect on delaying the decline of anthocyanin content. To sum up, this may be due to the synergistic effect of fucoidan-nano selenium and chlorine dioxide, which enhances the antioxidant enzyme defense system of cherry fruits.

[0039] V. Effect of the multifunctional composite fresh-keeping pad on the sensory quality of postharvest cherries Sensory evaluation not only reflects the enjoyment and edibility of food, but also comprehensively reflects the safety of food. Therefore, sensory evaluation is often the most intuitive index to describe and judge the quality of products. Figure 7 Figure shows the sensory scores of cherries at 35 days of storage. Sensory evaluation was carried out on 5 indexes of the appearance, smell, juice, taste and hardness of cherries. The results show that after 35 days of storage, the decline of the sensory quality of cherries in each treatment group is alleviated, and the shelf life of cherry fruits is extended. At this time, the total score of the CK group is less than 25 points, and the cherries lose their edible value, while the total score of the composite fresh-keeping pad treatment group is 35 points. Compared with other experimental groups, it has better sensory quality and is the best in maintaining the storage quality of cherries. Secondly, the color of the fruit can effectively reflect its appearance quality, freshness and maturity, and then reflect the commercial value of the fruit. Fruits with bright colors usually have better quality and nutritional value. a * The negative and positive values of L* correspond to the green and red colors of the fruit. b * The negative and positive values of a* correspond to the blue and yellow colors of the fruit. L * The value of b* corresponds to the brightness and darkness of the fruit. Table 2 shows the color change of cherries during the entire storage period. During the entire storage process, the L * value of L* of cherry fruits gradually decreases during storage, and theL * The values were all higher than those of the CK group. Therefore, chlorine dioxide treatment could delay the darkening of the fruit surface color. This might be because the appropriate chlorine dioxide concentration inhibited the oxidative senescence of cherries. Among them, the L * value of the commercial chlorine dioxide quick-release agent treatment group decreased faster than that of the composite fresh-keeping pad treatment group, probably because too high a concentration of chlorine dioxide accelerated the fruit senescence rate. During the whole storage period, the a * values of cherry fruits all showed an upward trend, and the a * values of all treatment groups were lower than those of the CK group. This indicated that chlorine dioxide treatment could alleviate the loss of green color of cherry pericarp, and at the 35th day, the a * value of the composite fresh-keeping pad treatment group was 4.33% lower than that of the CK group. During the whole storage period, the b * values of cherry fruits gradually decreased as a whole, but the b * values of all treatment groups were higher than those of the CK group, and the composite fresh-keeping pad treatment group had a more significant inhibitory effect on the decrease of the b * value of cherry fruits. This might be because chlorine dioxide had strong oxidizing property, which could prevent ethylene production and decompose the formed ethylene, delaying the yellowing of cherries. To sum up, the colors of cherries in the chlorine dioxide treatment group and the CK group changed during storage, the red color increased significantly, the yellow color and brightness decreased significantly, and the composite fresh-keeping pad treatment group had the most significant delaying effect on the color change of cherries.

[0040] The content of soluble solids (TSS) can directly affect the taste of cherries and, to a certain extent, also reflects the content of nutritional components in the fruits. Table 1 shows the effects of different treatments on the TSS content of cherries. During the entire storage period, the TSS content of cherry fruits in each treatment group showed a gradually decreasing trend as a whole. This is because cherry fruits carry out respiration to consume reducing sugars, and soluble solids are continuously consumed, thus causing the soluble solid content of cherries to gradually decrease. From 21 to 35 days, the TSS content of the composite fresh-keeping pad treatment group was higher than that of the CK group throughout the storage period. In addition, fruit acidity determines its flavor, and the change in the content of titratable acid can reflect the degree of consumption of nutrients in cherries. During storage, organic acids are continuously consumed as substrates for metabolic activities, and their content can determine the flavor quality of cherries. The content of titratable acid (TA) in cherry fruits is negatively correlated with the storage time. Compared with the CK group, the decline in the content of titratable acid in cherry fruits of each treatment group was slow. At the end of storage, the content of titratable acid in the composite fresh-keeping pad treatment group was still significantly higher than that of the CK group, 15.42% higher than that of the CK group. It has a positive effect on maintaining the TA content and can better maintain the flavor of cherry fruits.

[0041] Table 2 Effects of Different Chlorine Dioxide Treatments on the Color of Postharvest Cherries

[0042] Table 3 Effects of Different Chlorine Dioxide Treatments on the Contents of Soluble Solids and Titratable Acids in Postharvest Cherries

[0043] VI. Effects of the Composite Fresh-keeping Pad on the Quality of Treated Cherries Based on Transport Vibration Simulation An appropriate buffer packaging method can reduce mechanical damage. In this experiment, a simulated transport bump test bench was used to accurately simulate the bumpy conditions during automobile transportation to evaluate the ability of the composite fresh-keeping pad to package cherry fruits to withstand environmental vibrations. As Figure 8 shown, compared with the CK group, after vibration stress, the decrease in hardness, damage rate, and softening rate of cherry fruits in the composite fresh-keeping pad group were all significantly reduced.

[0044] This shows that using a corrugated cardboard box + composite fresh-keeping pad as the buffer packaging form can better protect the integrity of fruits during e-commerce logistics transportation, reduce mechanical damage, and maintain good shelf quality.

Claims

1. A preparation method of a multifunctional composite fresh-keeping pad for precisely regulating the postharvest storage and fresh-keeping effect of cherries, characterized in that It includes: (1) Preparation of chlorite - fucoidan - nano selenium: (2) Preparation of chitosan / tannic acid composite hydrogel: (3) Preparation of composite fresh - keeping pad: Among them, for (1): Dissolve 0.5 - 1 g of fucoidan in 30 - 50 mL of glacial acetic acid (1.2%), stir magnetically in the dark; slowly add 40 - 60 mL of sodium selenite solution at 30 - 50 mmol / L and 1.2 - 1.5 g of chlorite, stir magnetically; slowly drip 30 - 50 mL of VC solution at 100 - 200 mmol / L, transfer to a reagent bottle after magnetic stirring, and store in the refrigerator; (2) Preparation of chitosan / tannic acid composite hydrogel: Dissolve 500 - 800 μL of fucoidan - nano selenium in a chitosan solution at 0.6 - 1 mg / mL, and then dropwise add a tannic acid solution at 1.2 - 1.5%, to obtain a chitosan / tannic acid composite hydrogel; (3) Preparation of composite fresh - keeping pad: Freeze - dry 5 - 10 mL of the above - mentioned composite hydrogel and spray it on both sides of the sticky paper, with a thickness of 0.2 - 0.5 mm; use polyvinyl alcohol as a solvent, and obtain a nanofiber membrane after high - voltage electrospinning at 10 kV, spinning for 8 cm, and injecting at 0.3 mL / h, and spray it on the surface of the dry gel, with a thickness of 0.1 - 0.5 mm; (4) After the composite fresh - keeping pad is made, the slow - release performance of chlorine dioxide is measured.

2. Application of the multifunctional composite fresh - keeping pad for precisely regulating the post - harvest storage and preservation effect of cherries as claimed in claim 1 in improving the post - harvest storage quality and synergistic antibacterial activity of cherries.

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