Method for preserving fruits

Abstract

This invention discloses a method for fruit storage and preservation, belonging to the field of fruit storage and preservation technology. Addressing the technical problems of post-harvest fruit's susceptibility to decay, rapid softening, and severe quality deterioration, this invention formulates a composite preservative solution containing γ-polyglutamic acid and nano-zinc oxide. The γ-polyglutamic acid has a molecular weight of 50,000-100,000, and the nano-zinc oxide has a particle size of 1-100 nm. The unripe fruit is immersed in or sprayed with the composite preservative solution for 5-15 minutes, ensuring the surface is covered. The solution is then dried at 20-30°C to form a composite coating. The coated fruit is then placed in a storage environment for preservation. This method utilizes the film-forming properties of γ-polyglutamic acid and the antibacterial properties of nano-zinc oxide to form a dense protective layer on the fruit surface, effectively inhibiting pathogen infection, delaying fruit softening, water loss, and nutrient depletion, and reducing the decay rate. This invention is mainly used for post-harvest preservation of fruits such as mangoes or tangerines, significantly extending shelf life, maintaining fruit quality, and is simple, green, and safe to operate.

Description

Technical Field

[0001] This invention relates to the field of fruit storage and preservation technology, specifically to a method for fruit storage and preservation. Background Technology

[0002] Fruits continue to undergo active physiological metabolism after harvest. Climax fruits (such as mangoes, bananas, kiwis, and tomatoes) experience a sudden increase in respiration intensity during post-harvest ripening, accompanied by a large release of ethylene, leading to rapid softening, color change, and flavor degradation. While citrus fruits (such as Wogan tangerines, Satsuma mandarins, and oranges) are not climax fruits, they still face problems such as water loss and wilting, rupture of oil cells in the peel, and pathogen infection after harvest. In particular, rot caused by Penicillium and Green mold can result in severe economic losses. Therefore, effectively delaying post-harvest senescence, inhibiting rot, and maintaining quality remains a long-standing technical challenge in the field of fruit storage and preservation.

[0003] Currently, coating preservation is one of the most commonly used methods in post-harvest fruit treatment. Coating preservation involves covering the fruit surface with a high-molecular-weight film, which acts as a barrier against oxygen and moisture, inhibits respiration and metabolism, and reduces pathogen infection. γ-Polyglutamic acid is a natural high-molecular-weight amino acid polymer with excellent film-forming properties, oxygen barrier properties, moisture retention, and biodegradability, and has been explored for application in the preservation of fruits such as lychee and apricot. Nano-zinc oxide is a metal oxide with a particle size ranging from 1 to 100 nm, possessing a large specific surface area and good antibacterial activity. It has been studied for antibacterial packaging and coating preservation of foods such as orange juice, apples, and tangerines. Combining γ-polyglutamic acid with nano-zinc oxide holds promise for utilizing their respective film-forming and antibacterial advantages to form a composite coating preservative.

[0004] However, when γ-polyglutamic acid and nano zinc oxide are directly blended to form a composite coating liquid for fruit preservation, the following technical challenges exist.

[0005] Firstly, the physical blending system of γ-polyglutamic acid and nano-zinc oxide has poor stability. γ-polyglutamic acid aqueous solution is weakly acidic to neutral, while nano-zinc oxide particles have a high density, making them prone to sedimentation and stratification after standing in the coating solution. The upper layer of the coating solution has a low concentration of nano-zinc oxide, resulting in insufficient antibacterial components in the coating; the lower layer has an excessively high concentration of nano-zinc oxide, which may cause localized irritation to the fruit peel after application. Sedimentation and stratification of the coating solution not only affect the uniformity of a single coating application but also lead to significant differences in the preservation effect of different batches of treated fruit, making it difficult to meet the requirements of short-term storage or continuous use of coating solutions in industrial production. In existing technologies, γ-polyglutamic acid preservatives typically improve stability through chemical complexation with copper ions, but this method introduces heavy metal ions, posing food safety risks and involving complex processes. Furthermore, the sedimentation problem in direct physical blending systems remains unresolved.

[0006] Secondly, the γ-polyglutamic acid / nano zinc oxide coating solution has insufficient spreadability and adhesion on fruit surfaces. Many fruits have a natural waxy layer on their skin, exhibiting strong hydrophobicity. γ-polyglutamic acid molecules contain numerous carboxyl groups, making them highly hydrophilic. Since the coating solution uses water as a solvent, it is difficult to spread evenly on hydrophobic surfaces, often resulting in bead-like roll-offs. This leads to low coating coverage, leaving some fruit peels unprotected and becoming entry points for pathogens. Furthermore, the coating's adhesion to the fruit peel is weak after drying, making it prone to peeling and flaking during storage due to fruit volume changes or slight friction. This causes the coating's barrier and antibacterial functions to rapidly diminish over time. Different fruit varieties exhibit significant differences in their skin microstructure. For example, the skin of "Tainong No. 1" mango is relatively rough, the waxy layer of "Guifei Mango" is thicker, and citrus fruits like Wogan have densely packed oil cells, resulting in an even more uneven surface. Therefore, a single coating formulation is often insufficient to suit multiple varieties. Although some researchers have attempted to add chemical surfactants such as Tween 80 or plasticizers such as glycerin to the coating solution, a single additive is unlikely to solve the problems of spreading, crack resistance and adhesion at the same time, and chemical surfactants may affect the safety of fruit consumption.

[0007] Thirdly, for citrus fruits such as Wogan oranges, there is a specific problem of oil cell rupture leading to coating damage and off-odors. The peel of Wogan oranges and other citrus fruits is rich in oil cells containing terpene essential oils such as limonene and γ-terpinene. During post-harvest processing, transportation, and storage, these oil cells are prone to rupture due to mechanical damage or physiological changes. The released essential oils are highly soluble and can erode the coating layer, causing it to peel and fall off, thus losing its preservative function. Simultaneously, the volatilization of these essential oils produces an irritating odor, severely reducing the commercial value of the fruit. Most existing coating preservation technologies do not consider the problem of oil cell rupture in citrus fruits and lack specific solutions.

[0008] The aforementioned technical challenges have not been systematically addressed in existing studies using γ-polyglutamic acid or nano-zinc oxide alone. Furthermore, no research has been reported that simultaneously addresses the stability of the physical blend system, fruit surface spreading and adhesion, and the protection of citrus oil cells. Therefore, developing a fruit storage and preservation method that overcomes these deficiencies is of significant practical importance. Summary of the Invention

[0009] One objective of this invention is to address the issue that post-harvest fruits are highly reactive and prone to pathogen infection leading to rot, as well as rapid dehydration, wilting, discoloration, and softening. Existing chemical preservatives pose a risk of residue, and single coating materials (such as chitosan) have limited antibacterial and film-forming effects. While γ-polyglutamic acid possesses film-forming properties, it lacks active antibacterial ability when used alone; nano-zinc oxide, although possessing antibacterial properties, struggles to form a continuous protective film on the fruit surface. By combining these two materials and controlling their molecular weight and particle size range, a composite coating is formed on the fruit surface through immersion or spraying, solving the problem of incomplete functionality when used alone.

[0010] To achieve these objectives and other advantages of the present invention, the present invention provides a method for storing and preserving fruit, comprising the following steps: Step 1, preparing a composite preservative solution, wherein the composite preservative solution contains γ-polyglutamic acid and nano zinc oxide; wherein the molecular weight of the γ-polyglutamic acid is 50,000~100,000, and the particle size of the nano zinc oxide is 1~100 nm; Step 2, immersing or spraying the harvested unripe fruit in the composite preservative solution prepared in Step 1 for 5~15 min, so that the surface of the fruit is covered with the preservative solution; Step 3, removing the soaked fruit and drying it at 20~30℃ to form a γ-polyglutamic acid / nano zinc oxide composite coating on the surface of the fruit; Step 4, placing the coated fruit in a storage environment for preservation and storage.

[0011] Preferably, in the composite preservative solution described in step 1, the mass-volume concentration of γ-polyglutamic acid is 0.1%~5%, and the mass-volume concentration of nano zinc oxide is 0.01%~1%.

[0012] Preferably, the drying in step 3 is natural air drying for 1 to 2 hours, until a transparent, non-sticky γ-polyglutamic acid / nano zinc oxide composite coating is formed on the surface of the fruit.

[0013] Preferably, the composite preservative solution in step 1 is prepared by mixing γ-polyglutamic acid and nano zinc oxide in water.

[0014] Preferably, before step 2, a pretreatment step is included for the harvested fruit: washing the surface of the fruit with clean water to remove impurities, absorbing the surface moisture with absorbent paper, and then air-drying it naturally in a ventilated place.

[0015] Preferably, the preparation method of the composite preservative solution in step 1 includes the following sequential operations: adding nano-zinc oxide to pure water accounting for 10%~30% of the total volume of the composite preservative solution, and ultrasonically dispersing it for 5~30 min at a frequency of 40~100 kHz and a power of 100~300 W to obtain a primary dispersion of nano-zinc oxide; dissolving γ-polyglutamic acid in the remaining pure water, and stirring it in a water bath at 20~40℃ at 300~600 rpm for 20~60 min until completely dissolved to obtain a γ-polyglutamic acid solution; slowly adding the obtained primary dispersion of nano-zinc oxide to the obtained γ-polyglutamic acid solution at a stirring speed of 300~800 rpm and a dropping rate of 10~30 mL / min, and continuing to stir for 10~30 min after the addition is complete; adding a food-grade pH adjuster to the obtained mixture to adjust the pH value to 7.5~8.5, and continuing to stir for 5~15 min; and then ultrasonically dispersing it for 20~50 min at a temperature of 20~50℃. The homogenized solution is homogenized 1-3 times under MPa pressure using a high-pressure homogenizer, and cooled to 20-30℃ after each homogenization. The resulting homogenized solution is then allowed to stand at room temperature for 30-60 min to defoam, thus obtaining the composite preservation solution.

[0016] Preferably, step 2 includes a pretreatment step for the fruit surface, and a plasticizer stabilizer is added to the composite preservative solution in step 1. The drying in step 3 is a gradient humidity-controlled drying process. Specifically, the operations are performed in the following sequence: Step 201, preparing a surface wetting modification solution containing 0.1%~1% cocamidopropyl betaine by volume, with an ethanol-water solution at a volume ratio of 5%~15%; Step 202, spraying the prepared surface wetting modification solution onto the fruit surface using a spray method, with a spray volume of 5~20 mL per kilogram of fruit, and allowing it to stand for 1~3 minutes; Step 203, pre-drying the treated fruit in an environment of 20~30℃ and 40%~60% relative humidity for 3~5 minutes; Step 204, preparing a composite preservative solution and adding a plasticizer stabilizer to the composite preservative solution, wherein the amount of plasticizer stabilizer added is: 1~3 g of glycerin and 0.5~1.5 g of sorbitol per liter of composite preservative solution. g, cocamidopropyl betaine 0.1~0.5 g; Step 205, immerse the fruit treated in step 203 in the composite preservative solution obtained in step 204 for 5~15 min, so that the surface of the fruit is covered with the preservative solution; Step 206, take out the fruit soaked in step 205 and place it in a gradient humidity-controlled drying chamber for staged drying: First stage: temperature 25~35℃, relative humidity 30%~40%, drying for 5~10 min; Second stage: temperature 20~30℃, relative humidity 50%~60%, drying for 15~25 min; Third stage: temperature 18~25℃, relative humidity 65%~75%, drying for 30~60 min, forming a composite coating on the surface of the fruit.

[0017] Preferably, after step 206, a crosslinking agent treatment step is further included, specifically comprising the following operations performed in sequence: Step 207, preparing a crosslinking agent treatment solution, the crosslinking agent treatment solution containing a calcium salt with a mass-volume concentration of 0.1%~1%, wherein the calcium salt is selected from at least one of calcium lactate and calcium gluconate, and the solvent is an ethanol aqueous solution with a volume ratio of 10%~30%; Step 208, after the three-stage drying in step 206 is completed, uniformly spraying the crosslinking agent treatment solution prepared in step 207 onto the coating surface by atomization spraying, the spraying amount is 10~30 mL per kilogram of fruit, the spraying pressure is 0.1~0.3 MPa, and the nozzle orifice diameter is 0.5~1.0 mm; Step 209, placing the fruit treated in step 208 in an environment with a temperature of 25~35℃ and a relative humidity of 35%~45% and drying for 15~30 minutes. min; Step 210, place the fruit treated in step 209 in an environment with room temperature and relative humidity of 50%~65% for 24~48 hours to equilibrate.

[0018] Preferably, the γ-polyglutamic acid in step 1 is composed of a high molecular weight component and a low molecular weight component in a mass ratio of 1:0.5~2, wherein the molecular weight of the high molecular weight component is 100,000 and the molecular weight of the low molecular weight component is 50,000; and the composite preservative liquid in step 1 also contains a biodegradable pore-forming agent ammonium carbonate with a mass-volume concentration of 0.05%~0.5%.

[0019] Preferably, when the fruit is Wogan orange, the compound preservative solution in step 1 further contains β-cyclodextrin with a mass-volume concentration of 0.2%~1.0% and tea polyphenols with a mass-volume concentration of 0.05%~0.2%; and before step 2, the Wogan orange is placed in hot air at 55~60°C for 0.5~1.5 minutes, cooled, and then soaked or sprayed with a coating in step 2.

[0020] The present invention has at least the following beneficial effects: First, this invention combines γ-polyglutamic acid with nano-zinc oxide to create a composite preservative solution, which forms a uniform coating on the fruit surface through soaking or spraying. γ-polyglutamic acid has excellent film-forming and moisturizing properties, forming a dense physical barrier on the fruit peel surface, reducing water evaporation and oxygen contact, and inhibiting respiratory metabolism; nano-zinc oxide has antibacterial activity, inhibiting the growth of pathogenic microorganisms on the fruit surface and at wound sites. The synergistic effect of the two effectively reduces the spoilage rate and disease index during storage.

[0021] Secondly, this invention ensures the film-forming quality and antibacterial effect of the coating by limiting the range of molecular weight of γ-polyglutamic acid and the particle size of nano-zinc oxide, as well as optimizing the concentration. γ-polyglutamic acid with a molecular weight of 50,000 to 100,000 has good film-forming properties and moderate solution viscosity, facilitating coating; nano-zinc oxide with a particle size of 1 to 100 nm has a high specific surface area and antibacterial activity. A mass-volume concentration of 0.1% to 5% of γ-polyglutamic acid and 0.01% to 1% of nano-zinc oxide can ensure the integrity of the coating while avoiding excessively thick coatings or increased costs due to excessive concentrations. In addition, this invention adds a pretreatment step of rinsing with clean water, absorbing moisture, and air-drying before coating, removing impurities and excess moisture from the fruit surface and providing a good substrate for uniform coating adhesion.

[0022] Third, this invention provides an optimized preparation method to address the problem of easy sedimentation and stratification of nano-zinc oxide in physically blended systems. Through a series of steps including ultrasonic pre-dispersion, gradient dropwise addition, pH adjustment to 7.5-8.5, high-pressure homogenization, and static degassing, the electrostatic adsorption of γ-polyglutamic acid carboxyl groups onto the surface of nano-zinc oxide is utilized, ensuring uniform dispersion and stable presence of nano-zinc oxide in the coating solution. The composite preservative solution prepared using this method showed a sedimentation rate of only 2.1% after 48 hours of standing, with an average nano-zinc oxide particle size of 95 nm and a batch-to-batch coefficient of variation of less than 6.8%, significantly superior to the control example with simple stirring. This ensures the uniformity of the coating solution's composition during storage and use, providing a technological basis for continuous industrial production.

[0023] Fourth, this invention addresses the problems of poor spreadability, easy cracking, and weak adhesion of coatings on the hydrophobic surfaces of fruits such as mangoes by providing a technical solution of surface wetting modification and gradient moisture-controlled drying. An ethanol-water solution containing cocamidopropyl betaine is sprayed before coating to make the mango surface hydrophilic; glycerol, sorbitol, and cocamidopropyl betaine are added to the composite preservative solution as plasticizers and stabilizers to improve the flexibility and crack resistance of the coating; a three-stage gradient moisture-controlled drying process (high temperature and low humidity for rapid setting, medium temperature and medium humidity for stress release, and low temperature and high humidity for slow drying) avoids coating damage caused by excessively rapid moisture evaporation.

[0024] Fifth, this invention addresses the issues of easy loss of nano-zinc oxide and softening of plasticizers in coatings by providing a post-treatment solution for ionic crosslinking. After the coating is dried, an ethanol-water solution containing calcium salts (such as calcium lactate) is sprayed, followed by post-drying and equilibration treatment. 2+A reversible ionic crosslinking network is formed with the carboxyl groups on the γ-polyglutamic acid molecular chain, simultaneously anchoring nano-zinc oxide within the network and inhibiting the dissolution of nano-zinc oxide and the migration of plasticizers. Experiments show that after ionic crosslinking treatment, the swelling rate of the coating decreased from 285% to 120%, the retention rate of nano-zinc oxide increased from 52% to 88%, the weight gain after 7 days in a high-humidity environment (RH 85%) decreased from 32.5% to 9.8%, and the peel strength increased from 8.5 N / m to 28.5 N / m.

[0025] Sixth, this invention addresses the difficulty in balancing the density and permeability of single-molecular-weight γ-polyglutamic acid coatings by providing a molecular weight blending and pore-forming agent solution. High molecular weight (100,000) and low molecular weight (50,000) γ-polyglutamic acid are blended in a 1:1 ratio, with ammonium carbonate added as a biodegradable pore-forming agent. The high molecular weight component forms a dense framework, the low molecular weight component generates molecular chain end defects during film formation, and the ammonium carbonate decomposes during drying to release CO2 and form micropores. Together, these three components construct a multi-level permeable structure. Experiments show that after adopting this solution, anaerobic respiration products decreased from 245 mg / kg to 48 mg / kg, the decay rate decreased from 16.7% to 8.0%, and the coating integrity remained good with a mass loss rate of only 8.5%. This solution does not change the original soaking and natural air-drying process, is simple to operate, and has low cost.

[0026] Seventh, this invention addresses the technical challenges of Wogan oranges during storage, including easy dehydration, mold growth, and coating damage and off-odors caused by the rupture of oil cells in the peel. It provides a preservation solution combining hot air treatment and composite coating. Before coating, hot air treatment at 55-60℃ for 0.5-1.5 minutes can passivate latent Penicillium and Pseudomonas spores on the peel surface, induce the fruit to produce heat shock proteins, and enhance its disease resistance. β-cyclodextrin and tea polyphenols are added to the composite preservation solution. β-cyclodextrin, through inclusion complexation, captures terpene essential oil molecules such as limonene released from the oil cells, preventing essential oils from eroding the coating and inhibiting odor volatilization. Tea polyphenols, as a natural antioxidant, synergistically inhibit membrane lipid peroxidation with nano-zinc oxide.

[0027] Other advantages, objectives and features of the present invention will become apparent in part from the following description, and in part from those skilled in the art through study and practice of the invention. Detailed Implementation

[0028] The present invention will be further described in detail below with reference to embodiments, so that those skilled in the art can implement it based on the description.

[0029] Example 1 (1) Raw material preparation Mangoes of the "Katek" variety, at their green-ripe stage, were selected, requiring uniform maturity, freedom from pests, diseases, and mechanical damage, with a fruit diameter > 4 cm. The γ-polyglutamic acid used was purchased from Shanghai Maclean Biochemical Technology Co., Ltd., with a weight-average molecular weight (Mw) of 80,000. Nano zinc oxide, with a purity ≥ 99% and a particle size of 1–100 nm, was also purchased from Shanghai Maclean Biochemical Technology Co., Ltd.

[0030] (2) Pretreatment After harvesting, gently wash the mangoes with clean water to remove surface impurities, use absorbent paper to absorb excess moisture, and then place them in a ventilated area to air dry naturally for later use.

[0031] (3) Preparation of compound preservation liquid Weigh out γ-polyglutamic acid and nano-zinc oxide, add them to deionized water, and stir with a magnetic stirrer at 300 rpm for 20 min until completely dispersed and homogeneous to prepare a composite preservative solution. The γ-polyglutamic acid concentration is 2.5% (w / v), and the nano-zinc oxide concentration is 0.05% (w / v). This composite preservative solution is composed only of γ-polyglutamic acid, nano-zinc oxide, and water, and contains no chemical cross-linking agents or complexing agents.

[0032] (4) Soaking treatment Immerse the pre-treated green-ripe mangoes completely in the above-mentioned compound preservation solution for 10 minutes, gently turning them over during the process to ensure that the mangoes are evenly covered with the preservation solution.

[0033] (5) Air drying to form a film Remove the soaked mangoes and air dry them naturally at 25°C for 1.5 hours until a transparent, non-sticky γ-polyglutamic acid / nano zinc oxide composite coating forms on the mango surface.

[0034] (6) Storage The coated mangoes were placed in cardboard boxes and stored for 15 days at room temperature of 25°C and relative humidity of approximately 60%. Relevant indicators were observed and measured regularly.

[0035] To verify the effect, the following comparison ratio was set: Comparative Example 1 (Blank Control): Except for no coating treatment, the other operations were the same as in Example 1. That is, Kettering mangoes of the same variety and maturity were selected, washed with clean water, dried naturally, and then stored directly under the same conditions for 15 days.

[0036] Comparative Example 2 (Single γ-polyglutamic acid coating): Except that no nano zinc oxide was added to the composite preservative solution, the other operations were the same as in Example 1. That is, a γ-polyglutamic acid aqueous solution with a mass-volume concentration of 2.5% was prepared, the mango was soaked for 10 min, and then air-dried to form a film. It was stored under the same conditions.

[0037] Comparative Example 3 (Chitosan / Nano Zinc Oxide Composite Coating): Referring to existing technologies (e.g., Jiang Min, Ye Xialan, Qiu Qiuhong. Research on Chitosan-Nano Zinc Oxide Composite Coating for Preserving Satsuma Mandarins [J]. Food Industry Technology, 2012, (01):348-351.), a chitosan / nano zinc oxide composite coating solution was prepared: chitosan was dissolved in a 1% acetic acid solution, and nano zinc oxide (particle size 1~100 nm, concentration 0.05%) was added and stirred evenly. Mangoes were soaked for 10 min, air-dried naturally to form a film, and stored under the same conditions.

[0038] Effect Experiment Mangoes treated in each group were stored at 25℃. Samples were taken on days 0, 7, 9, 11, 13, and 15 of storage to determine the following indicators. Each treatment group had three replicates, with six mangoes per replicate. The following indicators were measured: Apparent changes: Recorded by smartphone under fixed lighting and distance.

[0039] Disease index: Calculated based on lesion area. Mangoes in each treatment group were graded according to the proportion of lesion area to the fruit surface area: Grade 0 (no lesions), Grade 1 (lesion area < 1 / 10), Grade 2 (lesion area 1 / 10–1 / 5), Grade 3 (lesion area 1 / 5–1 / 2), Grade 4 (lesion area > 1 / 2). Six mangoes were used for each treatment, and the experiment was repeated three times. The disease index was calculated using the following formula.

[0040] .

[0041] Rot rate: The number of rotten fruits was counted on an individual fruit basis. Based on the observation of the rotten appearance of mangoes in each treatment group, with 6 mangoes in each treatment group and the experiment repeated 3 times, the rot rate was calculated on an individual fruit basis using the following formula: .

[0042] Weight loss rate: The weight loss rate was calculated using a weighing method. The weight of mangoes in each treatment group was measured and recorded before and after storage. Three mangoes were treated in each treatment group, and the experiment was repeated three times. .

[0043] Peel color difference: L*, a*, and b* values ​​were measured using a colorimeter. Three mangoes were used in each treatment group, and the experiment was repeated three times.

[0044] Hardness: The hardness of the peel and flesh was measured using a texture analyzer. The mangoes were divided into two treatment groups, with three mangoes treated in each group per test, and the experiment was repeated three times. A 2mm diameter stainless steel cylindrical probe of type "p / 2" was selected, with a contact area of ​​3.14mm². The pre-test velocity was 5.00mm / s, the mid-test velocity was 1.00mm / s, and the post-test velocity was 1.00mm / s. The test target mode was set to "displacement," the displacement value was set to 10mm, and the unit was set to "g".

[0045] Titratable acid (citric acid equivalent): Acid-base titration method. Mangoes were divided into two treatment groups, with 3 mangoes treated in each group each time, and the experiment was repeated 3 times. 2g of mango pulp from the middle of the fruit was weighed, added to 5mL of distilled water, ground, and then diluted to 20mL. The mixture was centrifuged at 8000r / min for 10min, and 10mL of the supernatant was collected. Two drops of 1% phenolphthalein reagent were added, and the solution was titrated with 0.01mol / L sodium hydroxide until a pale pink color remained unchanged for 15s, which was the endpoint. The amount of sodium hydroxide used was recorded and calculated using the following formula: .

[0046] Soluble solids: Measured using a handheld refractometer. Mangoes were divided into two treatment groups, with 3 mangoes treated in each group per cycle, and the experiment was repeated 3 times. 20.0g of mango pulp was accurately weighed and juiced. After high-speed centrifugation at 12000r / min for 10min, the soluble solids content of the supernatant was measured using a handheld refractometer.

[0047] Malondialdehyde (MDA): Thiobarbituric acid method. Mangoes were divided into two treatment groups, with 3 mangoes treated in each group per test, and the experiment was repeated in triplicate. 3.0 g of fruit and vegetable sample was weighed, 15.0 mL of 10% TCA was added, and the mixture was homogenized. The homogenate was then centrifuged at 4℃ and 10000×g for 20 min. The supernatant was collected and stored at low temperature for later use. 6.0 mL of the supernatant (6.0 mL of 10% TCA solution was added to the control blank tube instead of the extraction solution) was taken, and 6.0 mL of 0.67% TBA was added. The mixture was boiled in a boiling water bath for 20 min, cooled, and then centrifuged again at 4℃ and 10000×g for 20 min. Finally, the absorbance of the supernatant was measured at wavelengths of 450 nm, 532 nm, and 600 nm. The results are expressed in μmol / L based on fresh weight, and the calculation method is as follows: C (μmol / L) = 6.45 × (OD532 - OD600) - 0.56 × OD450; MDA content (μmol / g) = [(6.45×(OD532-OD600)-0.56×OD450) ×V] / (Vs × m×1000).

[0048] In the formula, C is the MDA concentration in the reaction mixture (μmol / L), V is the total volume of the sample extract (mL), Vs is the volume of the sample extract taken during the determination (mL), and m is the sample mass (g). The calculation results are expressed as μmol / kg fresh weight.

[0049] Total polyphenols: Folin-Ciocalteu method.

[0050] The experimental results are as follows: (1) The results of the apparent changes are as follows: On day 9 of storage, the mangoes in Comparative Example 1 (blank group) showed scattered black spots on their peels, and the peel color darkened significantly; Comparative Example 2 (single γ-PGA) showed a small number of black spots; Comparative Example 3 (chitosan / nano zinc oxide) showed a small number of black spots, but the degree was less than that in Comparative Example 2; the mangoes in Example 1 had no obvious black spots on their peels and their color was bright. On day 13 of storage, some fruits in Comparative Example 1 began to rot; Comparative Examples 2 and 3 showed more black spots; the fruits in Example 1 showed only scattered black spots and remained intact. On day 15 of storage, the fruits in Comparative Example 1 showed a large number of black spots and were severely rotten; the fruits in Comparative Examples 2 and 3 showed obvious rot; the fruits in Example 1 did not show a large number of black spots on their peels and did not rot.

[0051] (2) The results of the disease index (%) are as follows: Table 1 Disease Index As can be seen, the disease index of Example 1 was significantly lower than that of each pair of proportions at all time points (p<0.05).

[0052] (3) The results of the decay rate (%) are as follows: Table 2. Rot Rate Example 1 showed a decay rate of only 16.7% after 15 days of storage, which was much lower than Comparative Example 1 (66.7%), as well as Comparative Example 2 (41.7%) and Comparative Example 3 (25.0%).

[0053] (4) The results of the quality loss rate (%) are as follows: After 15 days of storage, the mass loss rate was approximately 11.2% for Comparative Example 1, approximately 10.5% for Comparative Example 2, approximately 10.1% for Comparative Example 3, and approximately 9.8% for Example 1. Example 1 had the lowest mass loss rate.

[0054] (5) The results of fruit peel color difference (after 15 days of storage) are as follows: Table 3. Fruit Peel Color Difference As can be seen, Example 1 has the highest brightness, the lowest redness and yellowness, and the best preservation of peel color.

[0055] (6) Hardness (after 15 days of storage) results are as follows: Table 4 Hardness Data The peel and flesh firmness of Example 1 were significantly higher than those of the comparative examples (p<0.05), indicating that softening was effectively delayed.

[0056] (7) The titratable acid content (g / kg, after 15 days of storage) results are as follows: Comparative Example 1: 0.20; Comparative Example 2: 0.31; Comparative Example 3: 0.44; Example 1: 0.53. Example 1 had the highest titratable acid content, which was 2.65 times that of Comparative Example 1.

[0057] (8) The soluble solids content (%, after 15 days of storage) results are as follows: Comparative Example 1: 9.0; Comparative Example 2: 10.2; Comparative Example 3: 11.1; Example 1: 12.1. The soluble solids content of Example 1 was significantly higher than that of the comparative examples.

[0058] (9) The results of malondialdehyde (MDA) content (μmol / g, after 15 days of storage) are as follows: Comparative Example 1: 7.75; Comparative Example 2: 6.82; Comparative Example 3: 6.05; Example 1: 5.35.

[0059] Example 1 had the lowest MDA content, indicating the least degree of membrane lipid peroxidation.

[0060] (10) The total polyphenol content (mg / g, after 15 days of storage) results are as follows: Comparative Example 1: 0.234; Comparative Example 2: 0.278; Comparative Example 3: 0.305; Example 1: 0.335. Example 1 had the highest total polyphenol content and the best retention of antioxidant capacity.

[0061] Effect Analysis Compared with Comparative Example 1 (untreated), the γ-polyglutamic acid / nano zinc oxide composite coating treatment in Example 1 significantly delayed the appearance of black spots and the rotting process, reducing the rotting rate by approximately 75% after 15 days of storage; significantly inhibited the increase of disease index; effectively reduced the quality loss rate and decreased moisture evaporation; delayed the decline in peel brightness and inhibited reddening and yellowing; significantly inhibited the decrease in peel and pulp firmness, maintaining a firm texture; significantly delayed the decrease in titratable acid, soluble solids, and total polyphenol content; and significantly inhibited malondialdehyde accumulation and reduced the degree of membrane lipid peroxidation. Compared with Comparative Example 2 (single γ-PGA) and Comparative Example 3 (chitosan / nano zinc oxide), Example 1 showed superior preservation effects in all indicators, confirming the synergistic effect of γ-polyglutamic acid and nano zinc oxide.

[0062] Example 2 A method for storing and preserving mangoes according to the present invention includes: (1) The preparation of raw materials is the same as in Example 1.

[0063] (2) Pretreatment is the same as in Example 1.

[0064] (3) The compound preservative solution is prepared as follows: ① Pre-dispersion of nano zinc oxide: Weigh nano zinc oxide (particle size 1~100 nm, purity ≥99%) and add it to pure water accounting for 20% of the total volume of the composite preservation liquid (for example, to prepare 1 L of preservation liquid, take 200 mL of pure water). Disperse it ultrasonically for 15 min at a frequency of 80 kHz and a power of 200 W to obtain a primary dispersion of nano zinc oxide.

[0065] ② Dissolution of γ-polyglutamic acid: Dissolve γ-polyglutamic acid (molecular weight 80,000) in the remaining pure water (800 mL), and stir at 450 rpm for 40 min in a 30℃ water bath until completely dissolved to obtain a γ-polyglutamic acid solution.

[0066] ③ Gradient dropwise mixing: The primary dispersion of nano zinc oxide prepared in step ① is slowly added to the γ-polyglutamic acid solution prepared in step ② at a stirring speed of 500 rpm and a dropwise rate of 20 mL / min. After the addition is complete, stirring is continued for 20 min.

[0067] ④ pH adjustment: Add food-grade sodium citrate (or sodium bicarbonate) to the mixture obtained in step ③, adjust the pH value to 8.0, and continue stirring for 10 min.

[0068] ⑤ High-pressure homogenization: The mixture obtained in step ④ is homogenized twice by a high-pressure homogenizer at a pressure of 35 MPa, and cooled to 25°C after each homogenization.

[0069] ⑥ Degassing by standing: The homogenized liquid obtained in step ⑤ is allowed to stand at room temperature for 45 minutes to degas, thus obtaining the composite preservative liquid.

[0070] (4) Soaking treatment Same as Example 1: Soaking time 10 min.

[0071] (5) Air drying to form a film Same as Example 1: air dry at 25°C for 1.5 hours.

[0072] (6) Storage Same as Example 1: Store at room temperature (25°C) for 15 days.

[0073] Effect Experiment I. Stability Test of Coating Solution Experimental Method: The composite preservative solutions prepared in Example 2 and Example 1 were placed in 100 mL transparent glass graduated cylinders and allowed to stand at room temperature (25℃) for 48 hours. The stratification and precipitation were observed. Samples were taken from 5 cm below the liquid surface at 0 h, 24 h, and 48 h, respectively, and the following measurements were taken: Sedimentation rate: Take 10 mL of coating solution into a centrifuge tube, centrifuge at 3000 rpm for 10 min, and measure the percentage of the precipitate volume to the total volume.

[0074] Particle size distribution: The average particle size (D50) and polydispersity index (PDI) of nano zinc oxide were determined using a dynamic light scattering particle size analyzer.

[0075] Zeta potential: The zeta potential of the coating solution is measured using an electrophoretic light scattering instrument (reflecting dispersion stability).

[0076] The results are as follows: Table 5 Stability of Coating Solution The sedimentation rate of the coating solution in Example 2 after standing for 48 hours was only 2.1%, far lower than that of the comparative examples; it had the smallest average particle size (95 nm) and the lowest PDI (0.18), indicating that the nano-zinc oxide was uniformly dispersed; the highest absolute value of the Zeta potential (-38.5 mV) indicated strong electrostatic repulsion and a stable dispersion system. This demonstrates that the six-step sequential operation of the present invention (ultrasonic pre-dispersion, gradient dropwise addition, pH adjustment, high-pressure homogenization, cooling, and degassing) synergistically solved the sedimentation and stratification problem of nano-zinc oxide.

[0077] II. Comparison of mango preservation effects after coating Experimental Methods: Mangoes were coated with the composite preservative solution prepared in Example 2 (prepared fresh and used immediately, without standing) (soaked for 10 minutes, then air-dried to form a film). Uncoated mangoes served as a blank control. Thirty mangoes were stored at 25℃ for 15 days in each group. Rot rate, disease index, peel color difference, firmness, titratable acid, soluble solids, MDA, and total polyphenols were measured (methods were the same as in Example 1). The focus was on investigating the effect of different coating solution preparation methods on the preservation effect.

[0078] The results are as follows: Table 6. Comparison of preservation effects of different treatment groups of mangoes after 15 days of storage. In Example 2, the mangoes treated for 15 days of storage had a rot rate of only 10%, a disease index of 45.8%, a peel brightness L*=55.6, a flesh firmness of 62.8 g, a titratable acid of 0.58 g / kg, a soluble solids content of 13.2%, an MDA content of 4.85 μmol / g, and a total polyphenol content of 0.362 mg / g. All indicators were significantly improved.

[0079] III. Batch Consistency Test of Coating Solution Experimental methods: Three batches of coating solution were prepared independently according to the methods of Example 2 and Example 1. The sedimentation rate (48h), average particle size and Zeta potential of each batch were measured, and the inter-batch coefficient of variation (CV%) was calculated.

[0080] Table 7. Results of batch-to-batch consistency of coating solutions As can be seen, the batch-to-batch coefficient of variation in Example 2 is much smaller than that in Example 1, indicating that the preparation method of the present invention has excellent reproducibility and process stability, and is suitable for industrial production.

[0081] Example 3 A method for storing and preserving mangoes according to the present invention includes: (1) Raw material preparation Same as Example 1. Keitt mango variety, green-ripe stage.

[0082] (2) Pretreatment Same as Example 1: Rinse with clean water, absorb with absorbent paper, and air dry naturally.

[0083] (3) Surface wetting modification ① Preparation of surface wetting modification solution: Weigh cocamidopropyl betaine, add it to an ethanol aqueous solution with a volume ratio of 10%, stir until completely dissolved, and prepare a wetting modification solution with a mass-volume concentration of 0.5%.

[0084] ② Spraying: Spray the modified liquid evenly onto the surface of the mango using a spray method. The spraying amount is 10 mL per kilogram of mango, and let it stand for 2 minutes.

[0085] ③ Pre-drying: Place the sprayed mangoes in an environment of 25℃ and 50% relative humidity for 4 minutes to pre-dry, forming a polar gradient transition interface.

[0086] (4) Preparation of compound preservative solution Same as step (3) in Example 2: ultrasonic pre-dispersion (nano zinc oxide in water 20%, 80 kHz, 200 W, 15 min) → γ-PGA dissolution (30℃, 450 rpm, 40 min) → gradient addition (500 rpm, 20 mL / min) → pH adjustment to 8.0 (sodium citrate) → high pressure homogenization (35 MPa, 2 times, cooling to 25℃) → standing for degassing for 45 min.

[0087] (5) Add plasticizer and stabilizer Add plasticizers and stabilizers to the prepared compound preservation solution: add 2 g of glycerin, 1 g of sorbitol and 0.3 g of cocamidopropyl betaine per liter of compound preservation solution, and stir until completely dissolved.

[0088] (6) Immersion coating The mangoes treated in step (3) are completely immersed in the composite preservative solution obtained in step (5) for 10 minutes, so that the surface of the mangoes is evenly covered with the preservative solution.

[0089] (7) Gradient humidity control drying Remove the soaked mangoes and place them in a gradient humidity-controlled drying chamber for drying in the following stages: First stage: Temperature 30℃, relative humidity 35%, drying for 8 minutes; Second stage: Temperature 25℃, relative humidity 55%, drying for 20 minutes; Third stage: Temperature 22℃, relative humidity 70%, drying for 45 minutes.

[0090] (8) Storage The dried mangoes were placed in a cardboard box and stored at 25°C and approximately 60% relative humidity for 15 days.

[0091] To verify the effect, the following comparison ratio was set: Comparative Example 4 (without surface wetting modification): Except for omitting steps (1)-(3) (i.e., not spraying and pre-drying the surface wetting modification liquid), the other operations are the same as in Example 3 (still using plasticizer stabilizer, atomized spraying, and gradient drying).

[0092] Comparative Example 5 (without plasticizer stabilizer): Except for omitting step (4) (i.e., not adding glycerol, sorbitol, or cocamidopropyl betaine), the rest of the operation is the same as in Example 3 (surface wetting modification, atomized spraying, and gradient drying are still performed).

[0093] Comparative Example 6 (single plasticizer, using only glycerin): except that step (4) is changed to "only 2 g of glycerin is added to each liter of compound preservative liquid (without sorbitol and cocamidopropyl betaine)", the rest of the operation is the same as in Example 3.

[0094] Comparative Example 7 (natural air drying, no gradient humidity control): Except for step (6) which was changed to "the sprayed mango was placed under 25°C ventilation and air dried naturally for 2 hours (same as the air drying method of Example 1)," the rest of the operation was the same as Example 3 (surface wetting modification and the addition of plasticizer stabilizer were still carried out).

[0095] Comparative Example 8 (blank control, no coating): Same as Comparative Example 1: Kettering was washed and stored directly without any coating treatment.

[0096] Effect Experiment I. Coating coverage, crack rate and adhesion test Experimental Methods: Thirty mangoes were taken from each group. After the coating was completely dry (Example 3 and comparative examples), the following measurements were performed: Coating Coverage: The mangoes were stained with 0.1% methylene blue solution, washed, and photographed. The percentage of stained area was calculated (unstained areas of the coating, and stained areas showing white areas). Crack Rate: The surface of the mango peel was observed under a 100x magnifying glass, and the percentage of fields of view showing cracks in the coating was recorded (5 fields of view from the equatorial region were taken for each mango). Adhesion (Friction Peeling Rate): A cotton swab dipped in pure water was used to rub the coating surface back and forth 10 times with a constant force (approximately 2 N). The difference in mango mass before and after rubbing (mass of detached coating) was measured, and the peeling rate was calculated.

[0097] Table 8. Coating coverage, crack rate and adhesion results (after 0 days of storage and coating drying). As can be seen, Example 3 had the highest coverage (97.8%), the lowest crack rate (3.5%), and the lowest peeling rate (4.2%). Comparative Example 4 had a coverage of less than 75% due to the lack of wetting modification; Comparative Example 5 had a crack rate as high as 42% due to the lack of plasticizer; Comparative Example 6 (glycerol only) had a certain anti-cracking effect, but it was still far inferior to the ternary composite plasticizer system of Example 3; Comparative Example 7 (naturally air-dried) had a crack rate as high as 35%, therefore, gradient humidity control drying was essential.

[0098] II. Comparison of coating integrity degradation during storage Experimental method: Mangoes coated with the film were stored at 25℃. Samples were taken on days 3, 7, 11 and 15. The coating coverage was measured using the method described above (methylene blue staining method) to evaluate the peeling and damage of the coating during storage.

[0099] Table 9. Coating integrity results (coverage, %) The coating of Example 3 maintained a coverage of over 80% after 15 days of storage, while the coverage of the comparative examples decreased sharply. Comparative Example 4 (without wetting modification) had only 18.6% coverage after 15 days due to the initial discontinuity of the coating and weak adhesion; Comparative Example 7 (natural air drying) had large areas of coating peel off due to crack propagation; the gradient drying and composite plasticizing system of Example 3 enabled the coating to maintain excellent integrity and adhesion durability.

[0100] III. Comparison of overall preservation effects after 15 days of storage Table 10 Comparison of overall preservation effects (15 days of storage) Compared to Comparative Example 4 (without wetting modification), the spoilage rate of Example 3 decreased from 15.0% to 8.5%, a reduction of 43%; compared to Comparative Example 7 (natural air drying), the spoilage rate decreased from 10.0% to 8.5%, a reduction of 15%; and compared to Comparative Example 5 (without plasticizer), the crack rate decreased significantly. The overall preservation effect of Example 3 is significantly better than that of each comparative example, and shows a clear improvement compared to Example 2 (10.0%).

[0101] Example 4 A method for storing and preserving mangoes according to the present invention includes: Pick green and ripe Keitt mangoes, wash off surface impurities with clean water, absorb surface moisture with absorbent paper, and air dry naturally in a ventilated place.

[0102] Surface wetting modification: Prepare a 10% ethanol aqueous solution of cocamidopropyl betaine with a mass volume concentration of 0.5%, and spray it evenly on the surface of mango by spraying. The spraying amount is 10 mL per kilogram of mango. After standing for 2 min, place it in an environment of 25℃ and 50% relative humidity for pre-drying for 4 min.

[0103] Preparation of the composite preservative solution: Add nano zinc oxide (particle size 1~100 nm) to 20% of the final volume of pure water and ultrasonically disperse for 15 min at 80 kHz and 200 W; dissolve γ-polyglutamic acid (molecular weight 50,000~100,000) in the remaining pure water and stir at 450 rpm for 40 min in a 30℃ water bath; while stirring at 500 rpm, slowly add the nano zinc oxide dispersion to the γ-polyglutamic acid solution at a rate of 20 mL / min, and continue stirring for 20 min after the addition is complete; adjust the pH to 8.0 with sodium citrate; homogenize twice under high pressure at 35 MPa, and cool to 25℃ after each homogenization; allow to stand for 45 min to remove bubbles.

[0104] Add plasticizer and stabilizer: Add plasticizer and stabilizer to the above compound preservation liquid, add 2 g of glycerin, 1 g of sorbitol and 0.3 g of cocamidopropyl betaine per liter, and stir to dissolve.

[0105] Immersion coating: Immerse the surface-wetting modified mangoes completely in the above-mentioned composite preservative solution for 10 minutes to ensure that the mango surface is evenly covered with the preservative solution.

[0106] Gradient humidity control drying: Take out the soaked mangoes and place them in a gradient humidity control drying chamber: First stage: 30℃, 35% relative humidity, drying for 8 min; Second stage: 25℃, 55% relative humidity, drying for 20 min; Third stage: 22℃, 70% relative humidity, drying for 45 min.

[0107] Post-ion crosslinking treatment: Prepare a 20% ethanol aqueous solution of calcium lactate with a mass-volume concentration of 0.5%, and spray it evenly on the dried mango coating surface by atomization spraying at a spraying rate of 20 mL / kg and a pressure of 0.2 MPa; then dry it at 30℃ and 40% relative humidity for 20 min; finally, equilibrate it in an environment of 25℃ and 60% relative humidity for 36 hours.

[0108] Storage: Place the processed mangoes in a cardboard box and store them at 25°C and approximately 60% relative humidity for 15 days.

[0109] Comparative Example 9: No ionic crosslinking post-treatment (Example 3).

[0110] Comparative Example 10: Calcium lactate (final concentration 0.5%) was added directly when preparing the composite preservative solution, without any surface spraying post-treatment.

[0111] Comparative Example 11: The crosslinking agent treatment solution used pure water instead of 20% ethanol aqueous solution, and the rest was the same as in Example 4.

[0112] Comparative Example 12: The equilibration step was omitted (the product was stored directly after drying), and the rest was the same as in Example 4.

[0113] Effect Experiment 1. Coating performance The coating was peeled off from the surface of mangoes stored for 0 days, and the swelling rate (weight gain after 24 hours of immersion in water) and the retention rate of nano-zinc oxide (EDTA elution-ICP-OES) were measured. The results are as follows: Table 11 Coating Performance Example 4 exhibited the lowest swelling rate (120%) and the highest retention rate of nano-zinc oxide (88%). This is due to the Ca coating on the surface. 2+ It forms an ionic cross-linking network with the carboxyl groups on the γ-PGA molecular chain, while Ca 2+ Bridging anchors the nano-zinc oxide within the network, inhibiting its dissolution. Comparative Example 10 (pre-crosslinked) shows that Ca... 2+ The crosslinking agent combines with γ-PGA during the preparation stage of the coating solution, resulting in excessive viscosity of the coating solution and uneven film formation during immersion; in Comparative Example 11, the pure water solvent causes the crosslinking agent droplets to spread too quickly and penetrate too deeply on the coating surface.

[0114] 2. Antibacterial activity Wipe the coating surface (1 cm) with a sterile cotton swab during storage. 2 After elution, the sample is spread onto PDA medium, and colony forming units (CFU / cm²) are counted. 2 (logarithmic value) Table 12 Antibacterial activity Example 4 showed the lowest colony count throughout the entire process, at only 4.2 log CFU / cm³ at 15 days. 2 The control ratio was 7.8, while the control ratio for Example 9 was as high as 7.8. This is because in Example 4, the nano-zinc oxide was coated with Ca... 2+ Bridging and anchoring, slow loss, and long-lasting antibacterial activity. Comparative Example 10 showed uneven coating due to pre-crosslinking, with insufficient exposure of nano-zinc oxide in some areas; Comparative Examples 11 and M had poor crosslinking effects, and nano-zinc oxide was easily lost.

[0115] 3. Resistant to moisture absorption and softening in high humidity environments The coated mango was placed in an environment of 25℃ and 85% relative humidity, and the coating weight gain rate and peel strength (180° peel force) were measured. Table 13 Resistance to hygroscopic softening under high humidity conditions Example 4 showed the least weight gain (only 9.8% after 7 days) and the highest peel strength (28.5 N / m). The ionic crosslinking network restricted the molecular migration of plasticizers (glycerol, sorbitol), reducing their accumulation and moisture absorption on the coating surface, while enhancing the interfacial adhesion between the coating and the fruit peel.

[0116] 4. Overall preservation effect Table 14 Overall Preservation Effect After 15 Days of Storage Table 14 Comprehensive Preservation Effect The decay rate in Example 4 (5.8%) was 32% lower than that in Example 3 (8.5%), while the coating coverage increased from 92.0% to 93.2%. This is attributed to the enhanced mechanical integrity and adhesion durability of the coating due to the ionic cross-linking network, while the high retention rate of nano-zinc oxide provided long-lasting antibacterial protection. Due to the improved coating integrity, the loss of fruit moisture and nutrients was reduced, and the retention of firmness, titratable acid, soluble solids, and total polyphenols was significantly improved. The malondialdehyde content decreased by 19.2%, indicating a reduction in membrane lipid peroxidation, which is related to the intact, crack-free coating, moderate regulation of oxygen and ethylene, and the synergistic antioxidant effect of nano-zinc oxide.

[0117] Example 5 A method for storing and preserving mangoes according to the present invention includes: Pick green and ripe Keitt mangoes, wash off surface impurities with clean water, absorb surface moisture with absorbent paper, and air dry naturally in a ventilated place.

[0118] Preparation of the composite preservative solution: All γ-polyglutamic acid used has a weight-average molecular weight (Mw). Based on the average molecular weight, high molecular weight γ-polyglutamic acid (molecular weight 100,000) and low molecular weight γ-polyglutamic acid (molecular weight 50,000) are mixed at a mass ratio of 1:1, resulting in a total mass-volume concentration of 2.5% (i.e., 2.5 g per liter, of which 1.25 g is high molecular weight and 1.25 g is low molecular weight). Ammonium carbonate (food grade) with a mass-volume concentration of 0.2% is added as a pore-forming agent. Nano-zinc oxide (particle size 1~100 nm) with a mass-volume concentration of 0.05% is added. Using deionized water as the solvent, the mixture is stirred at 300 rpm for 20 minutes with a magnetic stirrer until completely dispersed and homogeneous.

[0119] The mangoes were treated with the above-mentioned compound preservative solution by soaking (same as in Example 1): soaking time was 10 minutes, ensuring that the preservative solution completely covered the fruit. After removal, the mangoes were naturally air-dried at 25°C under ventilation for 1.5 hours to form a coating. The treated mangoes were then placed in a cardboard box and stored at 25°C and approximately 60% relative humidity for 15 days.

[0120] Comparative Example 13 (Example 1): Using a single molecular weight γ-PGA 80000 at a concentration of 2.5%, nano zinc oxide 0.05%, and no pore-forming agent.

[0121] Comparative Example 14: Only high molecular weight γ-PGA (100,000, concentration 2.5%) was used, with no low molecular weight components and no pore-forming agents.

[0122] Comparative Example 15: Only low molecular weight γ-PGA (50000, concentration 2.5%) was used, without pore-forming agent.

[0123] Comparative Example 16: A compound γ-PGA (100000:50000=1:1) was used, but without the addition of ammonium carbonate (no pore-forming agent).

[0124] Comparative Example 17: High molecular weight γ-PGA (100,000) was used alone, with 0.2% ammonium carbonate (with pore-forming agent).

[0125] Effect Experiment 1. Place each group of mangoes in a sealed desiccator (25℃) and determine the acetaldehyde + ethanol content in the pulp after 15 days of storage: Table 18 Acetaldehyde + Ethanol Content in Fruit Pulp Comparative Example 14 (pure 100,000) had the highest molecular weight, the densest coating, and severe anaerobic respiration, with acetaldehyde + ethanol reaching 245 mg / kg. Comparative Example 15 (pure 50,000) had the lowest molecular weight, a loose coating, weaker anaerobic respiration, but rapid moisture loss. Comparative Example 13 (80,000) was between the two, with a slight fermented odor. Comparative Example 16 (compound without pore-forming agent) had an anaerobic respiration product of 105 mg / kg. Comparative Example 17 (pure 100,000 + pore-forming agent) still had an acetaldehyde + ethanol content of 80 mg / kg. Example 5, by compounding 100,000 and 50,000 and adding ammonium carbonate (which decomposes to release CO2 and form micropores), reduced the anaerobic respiration product to 48 mg / kg, with no off-odor.

[0126] 2. The moisture loss was tested, and the results are as follows: Table 19 Comparison of Quality Loss Rates The mass loss rate of Example 5 (8.5%) was lower than that of Comparative Example 13 (9.8%) and Comparative Example 15 (13.8%), and comparable to that of Comparative Example 17, indicating that the pore-forming agent did not significantly increase water loss, and the molecular weight blending ensured that the coating film density remained good.

[0127] 3. Overall preservation effect after 15 days of storage Table 20 Comparison of Overall Preservation Effects The rot rate in Example 5 (8.0%) was 52% lower than that in Example 1 (16.7%). This is because the molecular weight blend (100,000 + 50,000) formed a multi-level structure where "high molecular weight provides a dense framework, and low molecular weight forms defects at the ends of the molecular chains." Combined with in-situ pore formation by ammonium carbonate, this resulted in suitable air permeability of the coating. The fruit maintained normal aerobic respiration, with no accumulation of toxic metabolites, normal release of endogenous ethylene, a significantly delayed aging process, and an improvement of approximately 10-20% in various quality indicators, while malondialdehyde (MDA) decreased by 27%.

[0128] 4. Optimization experiments of different compounding ratios and pore-forming agent concentrations Based on Example 5, the compounding ratio (100000:50000) and ammonium carbonate concentration were changed, and the results are as follows: Table 21 Optimization experimental results for different compounding ratios and pore-forming agent concentrations The best overall effect (rot rate 8.0%) was achieved when the compound ratio was 1:1 and ammonium carbonate was 0.2%. If the ratio was too low (1:0.5) or the pore-forming agent was too low, the air permeability was still insufficient; if the ratio was too high (1:2) or the pore-forming agent was too high, the air permeability was too strong, and the rot rate increased, because the water loss was accelerated or microorganisms were more likely to invade.

[0129] 5. Molecular weight selection is based on experiments. The results of comparing different combinations of γ-PGA molecular weights (fixed blending ratio 1:1, ammonium carbonate 0.2%) are as follows: Table 22 Results of different molecular weight combinations As can be seen, the combination of γ-PGA molecular weights of 100,000 and 50,000 yields the best results. The moderate molecular weight difference (2 times) ensures a sufficient number of defects at the molecular chain ends while maintaining the overall continuity of the coating. An excessively large difference (100,000 vs. 30,000) results in a softer coating with insufficient barrier properties; a difference that is too small (120,000 vs. 40,000) or an overall high difference (150,000 vs. 50,000) provides limited improvement in air permeability.

[0130] In summary, Example 5, by combining γ-PGA with molecular weights of 100,000 and 50,000 (1:1) and adding 0.2% ammonium carbonate as a pore-forming agent, reduced the mango spoilage rate after 15 days of storage from 16.7% in Example 1 to 8.0% (a reduction of 52%), and the anaerobic respiration product from 97.9 mg / kg to 48 mg / kg, without changing the soaking and air-drying processes, resulting in a comprehensive improvement in quality indicators. This method is simple to operate, low in cost, and effectively solves the problem of balancing coating density and breathability.

[0131] Example 6 Select Wogan tangerines that are uniformly ripe (80% ripe) and free from pests, diseases, and mechanical damage. Wash off any surface impurities with clean water, absorb the surface moisture with absorbent paper, and air dry naturally in a ventilated place.

[0132] The pretreated Wogan tangerines were placed in a 55℃ hot air circulating drying oven for 1 minute, and then removed and allowed to cool naturally to room temperature. Hot air treatment can deactivate pathogens such as Penicillium and Pseudomonas spp. lurking on the surface of the peel, while inducing the fruit to produce heat shock proteins and activating its own disease resistance defense system.

[0133] Preparation of the composite preservative solution: Add nano-zinc oxide (particle size 1-100 nm) to 20% of the final volume of pure water and ultrasonically disperse for 15 min at 80 kHz and 200 W. Dissolve γ-polyglutamic acid (weight average molecular weight 80,000) in the remaining pure water and stir at 450 rpm for 40 min in a 30℃ water bath. While stirring at 500 rpm, slowly add the nano-zinc oxide dispersion to the γ-polyglutamic acid solution at a rate of 20 mL / min, and continue stirring for 20 min after the addition is complete. Adjust the pH to 8.0 with sodium citrate. Homogenize twice under high pressure at 35 MPa, cooling to 25℃ after each homogenization. Allow to stand for 45 min to remove bubbles. Add β-cyclodextrin (0.5% by volume) and tea polyphenols (0.1% by volume) to the above composite preservative solution and stir until completely dissolved. β-Cyclodextrin is used to encapsulate essential oil components that may be released from the oil cells of Wogan mandarin oranges, preventing the essential oils from damaging the coating structure and reducing odor; tea polyphenols, as natural antioxidants and antibacterial agents, synergistically enhance the effects with nano zinc oxide.

[0134] After heat treatment and cooling, completely immerse the Wogan mandarins in the aforementioned compound preservative solution for 10 minutes, gently turning them during this time to ensure the fruit surface is evenly covered with the preservative solution. Remove the Wogan mandarins and air-dry them naturally at 25°C under ventilation for 1.5 hours, until a transparent, non-sticky compound coating forms on the surface. Place the coated Wogan mandarins in cardboard boxes and store them at 25°C and approximately 60% relative humidity for 30 days.

[0135] Scale settings Comparative Example 18 (blank control): Wogan oranges were not coated with any film and were simply washed and stored directly.

[0136] Comparative Example 19 (Hot air treatment only): Wogan oranges were treated with hot air at 55°C for 1 minute, but no coating was applied.

[0137] Comparative Example 20 (coated but without β-cyclodextrin and tea polyphenols): The composite preservative solution of Example 1 (γ-polyglutamic acid + nano zinc oxide, without β-cyclodextrin and tea polyphenols) was used, without hot air treatment, and otherwise the same as in Example 6.

[0138] Comparative Example 21 (Hot air treatment + coating but without β-cyclodextrin and tea polyphenols): Hot air treatment was performed first, and then the composite preservative liquid of Example 1 was used for coating. The rest was the same as in Example 6.

[0139] Comparative Example 22 (Hot air treatment + coating containing β-cyclodextrin but without tea polyphenols): Hot air treatment was performed first, and only β-cyclodextrin (0.5%) was added to the coating solution, without adding tea polyphenols. The rest was the same as in Example 6.

[0140] Comparative Example 23 (Hot air treatment + coating containing tea polyphenols but without β-cyclodextrin): Hot air treatment was performed first, and only tea polyphenols (0.1%) were added to the coating solution, without adding β-cyclodextrin. The rest was the same as in Example 6.

[0141] Effect Experiment The results of index measurements for each treatment group after 30 days of storage are as follows.

[0142] Table 23. Overall Preservation Effect of Wogan Tangerines after 30 Days of Storage As shown in Table 23, the rot rate of Example 6 was only 3.3%, significantly lower than that of the comparative examples; it had the lowest mass loss rate (6.8%), the highest peel brightness (L*=65.5), the best pulp firmness retention (28.5 N), and the highest content of titratable acid and soluble solids. Example 6 had an odor intensity of 0, indicating no odor, while Comparative Example 20 (coating without heat treatment) had an odor intensity of 4 (irritating odor from citrus essential oil volatilization), and Comparative Example 21 (heat treatment + coating without additives) had an odor intensity of 3. This indicates that heat treatment and β-cyclodextrin have a synergistic effect on odor control.

[0143] Table 24. Retention rate of essential oil in fruit peel and inhibition effect against Penicillium wilt (after 30 days of storage) In Table 24, Example 6 showed the highest essential oil retention rate in the peel (72.8%), with infection rates of Penicillium and Green mold both at only 1.7%. Hot air treatment (55℃×1min) could inactivate latent pathogens on the peel surface and induce defensive responses such as heat shock proteins and cell wall thickening in the fruit, thereby reducing the infection rates of Penicillium and Green mold. The essential oil retention rate of Comparative Example 22 (containing β-cyclodextrin but not tea polyphenols) (68.5%) was lower than that of Example 6 (72.8%), indicating that tea polyphenols further protected the peel aroma by inhibiting the oxidative rancidity of essential oils; the essential oil retention rate of Comparative Example 23 (containing tea polyphenols but not β-cyclodextrin) (52.1%) was much lower than that of Example 6, and the off-odor intensity was still 2.5, indicating that the encapsulation effect of β-cyclodextrin on the essential oil was the key to eliminating off-odors.

[0144] This embodiment addresses three key technical challenges in the storage of Wogan oranges and employs corresponding solutions: One issue is the easy proliferation of Penicillium and Green mold on the surface of Wogan tangerines. Hot air treatment (55℃×1min) can passivate the latent Penicillium and Green mold spores on the peel surface, while inducing the fruit to produce heat shock proteins, activating its own disease resistance defense system. Studies have shown that heat treatment can significantly increase the cell wall thickness and lignin content of citrus fruits, providing a physical barrier against pathogen infection. The synergistic effect of heat treatment and γ-polyglutamic acid / nano zinc oxide coating reduced the Penicillium infection rate from 35.0% to 1.7% and the Green mold infection rate from 28.3% to 1.7%.

[0145] Secondly, there is the problem of essential oils released from the ruptured oil cells of Wogan tangerines, damaging the coating structure and producing off-odors. Wogan tangerine peels are rich in oil cells, which are prone to rupture during post-harvest processing and storage. The released terpene essential oils, such as limonene and γ-terpinene, can erode the coating, causing peeling and flaking, while also producing an irritating odor. In this embodiment, β-cyclodextrin (0.5%) was added to the coating solution. Its hydrophobic cavities can encapsulate essential oil molecules, forming stable inclusion complexes, preventing direct contact between the essential oils and the coating and thus preventing damage to the coating structure, while also inhibiting the volatilization of essential oils and the generation of off-odors. Experiments showed that Comparative Example 20 (coating without hot air treatment and without β-cyclodextrin) had an odor intensity as high as level 4, while Example 6 (containing β-cyclodextrin) had an odor intensity of level 0, and the essential oil retention rate of the peel increased from 55.6% to 72.8%.

[0146] Thirdly, there is the problem of insufficient antioxidant capacity of the coating. In this embodiment, tea polyphenols (0.1%) were added to the coating solution. As a natural antioxidant, tea polyphenols can scavenge free radicals and inhibit membrane lipid peroxidation, while also synergistically inhibiting bacteria with nano-zinc oxide. In Table 23, the malondialdehyde content of Example 6 was significantly lower than that of Comparative Example 22 (containing β-cyclodextrin but not tea polyphenols), indicating that the addition of tea polyphenols further reduced the degree of membrane lipid peroxidation and delayed fruit senescence.

[0147] In summary, Example 6, through the synergistic effect of hot air treatment (55℃×1min), γ-polyglutamic acid / nano zinc oxide composite coating, β-cyclodextrin, and tea polyphenols, reduced the spoilage rate of Wogan oranges stored for 30 days to 3.3%, increased the peel essential oil retention rate to 72.8%, and reduced the infection rates of Penicillium and Green mold to 1.7%, with no off-odors produced. Compared with the closest existing technology (chitosan coating or simple heat treatment), this solution is the first to apply hot air treatment, γ-polyglutamic acid / nano zinc oxide coating, β-cyclodextrin essential oil encapsulation, and tea polyphenol antioxidant synergistically to the preservation of Wogan oranges. It solves the technical problems of easy water loss, easy mold growth, and essential oil damage to the coating and off-odors during the storage of Wogan oranges, significantly extending the shelf life. Moreover, all materials used are food-grade, safe, and residue-free.

[0148] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. It can be applied to various fields suitable for the present invention. Further modifications can be readily implemented by those skilled in the art.

Claims

1. A method for storing and preserving fruit, characterized in that, Includes the following steps: Step 1: Prepare a compound preservative solution, wherein the compound preservative solution contains γ-polyglutamic acid and nano zinc oxide; wherein the molecular weight of the γ-polyglutamic acid is 50,000~100,000, and the particle size of the nano zinc oxide is 1~100 nm. Step 2: Soak or spray the harvested green-ripe fruit in the compound preservative solution prepared in Step 1 for 5-15 minutes to cover the fruit surface with the preservative solution. Step 3: Remove the soaked fruit and dry it at 20~30℃ to form a γ-polyglutamic acid / nano zinc oxide composite coating on the fruit surface; Step 4: Place the coated fruit in a storage environment for preservation.

2. The method according to claim 1, characterized in that, In the composite preservative solution described in step 1, the mass-volume concentration of γ-polyglutamic acid is 0.1%~5%, and the mass-volume concentration of nano zinc oxide is 0.01%~1%.

3. The method according to claim 1 or 2, characterized in that, In step 3, the drying process is natural air drying, which takes 1 to 2 hours, until a transparent, non-sticky γ-polyglutamic acid / nano zinc oxide composite coating is formed on the surface of the fruit.

4. The method according to claim 1, characterized in that, The composite preservative solution mentioned in step 1 is prepared by mixing γ-polyglutamic acid and nano zinc oxide in water.

5. The method according to claim 1, characterized in that, Before step 2, there is also a step of pre-treating the harvested fruit: washing the surface of the fruit with clean water to remove impurities, absorbing the surface moisture with absorbent paper, and then air-drying it naturally in a ventilated place.

6. The method according to claim 4, characterized in that, The preparation method of the compound preservative solution in step 1 includes the following steps in sequence: Nano zinc oxide was added to pure water accounting for 10% to 30% of the total volume of the composite preservative solution, and ultrasonically dispersed for 5 to 30 minutes at a frequency of 40 to 100 kHz and a power of 100 to 300 W to obtain a primary dispersion of nano zinc oxide; γ-polyglutamic acid was dissolved in the remaining pure water and stirred at 300 to 60 rpm in a water bath at 20 to 40°C for 20 to 60 minutes until completely dissolved to obtain a γ-polyglutamic acid solution. The prepared nano-zinc oxide primary dispersion was slowly added to the prepared γ-polyglutamic acid solution at a stirring speed of 300-800 rpm and a dropping rate of 10-30 mL / min. After the addition was complete, stirring was continued for 10-30 min. Food-grade pH adjuster was added to the resulting mixture to adjust the pH value to 7.5-8.5, and stirring was continued for 5-15 min. The mixture was homogenized 1-3 times under a pressure of 20-50 MPa using a high-pressure homogenizer, and cooled to 20-30℃ after each homogenization. The resulting homogenized solution was allowed to stand at room temperature for 30-60 min to degas, thus obtaining the composite preservative solution.

7. The method according to claim 1, characterized in that, Before step 2, there is a pretreatment step for the fruit surface, and a plasticizer and stabilizer are added to the compound preservative solution mentioned in step 1. The drying in step 3 is a gradient humidity-controlled drying. Specifically, the operations are performed in the following order: Step 201: Prepare a surface wetting modification solution, wherein the surface wetting modification solution contains 0.1% to 1% cocamidopropyl betaine by mass volume, and the solvent is an aqueous ethanol solution with a volume ratio of 5% to 15%. Step 202: Spray the prepared surface wetting and modification liquid onto the fruit surface by spraying, with a spraying amount of 5-20 mL per kilogram of fruit, and let it stand for 1-3 minutes. Step 203: Pre-dry the treated fruit in an environment of 20-30℃ and 40%-60% relative humidity for 3-5 minutes; Step 204: Prepare a compound preservation solution and add a plasticizer and stabilizer to the compound preservation solution. The amount of plasticizer and stabilizer added is: 1-3 g of glycerin, 0.5-1.5 g of sorbitol, and 0.1-0.5 g of cocamidopropyl betaine per liter of compound preservation solution. Step 205: Immerse the fruit treated in step 203 in the compound preservative solution obtained in step 204 for 5-15 minutes to cover the surface of the fruit with the preservative solution. Step 206: Remove the fruit soaked in step 205 and place it in a gradient humidity-controlled drying chamber for staged drying: Stage 1: Temperature 25~35℃, relative humidity 30%~40%, drying for 5~10 min; Stage 2: Temperature 20~30℃, relative humidity 50%~60%, drying for 15~25 min; Stage 3: Temperature 18~25℃, relative humidity 65%~75%, drying for 30~60 min, forming a composite coating on the surface of the fruit.

8. The method according to claim 7, characterized in that, Following step 206, a crosslinking agent treatment of the coating is also included, specifically comprising the following operations performed in sequence: Step 207: Prepare a crosslinking agent treatment solution. The crosslinking agent treatment solution contains a calcium salt with a mass-volume concentration of 0.1% to 1%, wherein the calcium salt is selected from at least one of calcium lactate and calcium gluconate, and the solvent is an aqueous ethanol solution with a volume ratio of 10% to 30%. Step 208: After the three-stage drying in step 206 is completed, the crosslinking agent treatment solution prepared in step 207 is evenly sprayed onto the coating surface by atomization spraying. The spraying amount is 10~30 mL per kilogram of fruit, the spraying pressure is 0.1~0.3 MPa, and the nozzle orifice diameter is 0.5~1.0 mm. Step 209: Place the fruit treated in step 208 in an environment with a temperature of 25~35℃ and a relative humidity of 35%~45% and dry for 15~30 minutes. Step 210: Place the fruit treated in step 209 in an environment with room temperature and relative humidity of 50%~65% for 24~48 hours to equilibrate.

9. The method according to any one of claims 1 to 8, characterized in that, The γ-polyglutamic acid mentioned in step 1 is composed of a high molecular weight component and a low molecular weight component in a mass ratio of 1:0.5~2, wherein the molecular weight of the high molecular weight component is 100,000 and the molecular weight of the low molecular weight component is 50,000; and the composite preservative liquid in step 1 also contains a biodegradable pore-forming agent ammonium carbonate with a mass-volume concentration of 0.05%~0.5%.

10. The method according to any one of claims 1 to 8, characterized in that, When the fruit is Wogan orange, the compound preservative liquid in step 1 also contains β-cyclodextrin with a mass-volume concentration of 0.2%~1.0% and tea polyphenols with a mass-volume concentration of 0.05%~0.2%; and before step 2, the Wogan orange is placed in hot air at 55~60℃ for 0.5~1.5 minutes, cooled, and then soaked or sprayed with a coating in step 2.

Images