Preparation method of konjac glucomannan coating film

Abstract

The invention discloses a preparation method of a konjac glucomannan-based coating film. The preparation method comprises the following steps: (1) preparing materials: 4 parts of konjac glucomannan, 3-4 parts of chitosan, 2-3 parts of high-ester pectin, 0.5-0.6 part of calcium carbonate and 0.8-1.1 parts of glycerol; (2) chitosan is dissolved in a 1% organic acid solution, then konjac glucomannan and high-ester pectin are added and stirred evenly to form a mixed solution, and the mass fraction of solute in the mixed solution is 2-3%; and (3) carrying out modification treatment on the mixed solution, then adding glycerol and calcium carbonate, and uniformly stirring to obtain the film coating solution. The coating film is not easy to dissolve, has excellent mechanical properties, and can keep fruits fresh by inhibiting ethylene synthesis, maintaining mango quality, delaying fruit ripening and maintaining fragrance release.

Description

Technical Field

[0001] The invention relates to the technical field of konjac glucomannan coating, in particular to a preparation method of konjac glucomannan coating. Background Art

[0002] Mango (Mangifera indica L.), as a popular tropical fruit, has a unique flavor and rich nutritional value, which is vital to human health and import and export markets. However, the burst of ethylene production and the increase of respiration rate in post-harvest mangoes cause the oxidation and decomposition of organic matter in the fruit into carbon dioxide and water, releasing a large amount of energy, resulting in the loss of nutrients during storage and transportation, rapid ripening and aging of the fruit, and shortened shelf life. Ethylene is recognized as the most important endogenous regulatory factor in the ripening process of climacteric fruits. The production of ethylene can be inhibited by physical and chemical methods, thereby reducing the adverse effects of its rapid ripening and aging. In addition, aroma components, as an important component of mango quality, determine the unique taste of the fruit. Therefore, it is meaningful to study the effects of mango preservation treatment technology and ethylene evolution on the quality and volatile components of mangoes during post-harvest storage and transportation.

[0003] In recent years, renewable polymers based on natural bio-based ingredients have become potential coating materials for preserving fruits and vegetables. The film material can form an extremely thin polymer film on the surface of fruits and vegetables, inhibiting the gas exchange and metabolic process of fruits and vegetables by regulating ethylene evolution, reducing respiration intensity, reducing water loss, and extending shelf life. Konjac glucomannan (KGM) is a natural polysaccharide polymer with a stable and abundant source, and is also a film-forming agent. It not only has high swelling and viscosity-increasing capabilities, but is also edible and can be used as a plastic wrap for mangoes.

[0004] However, due to the presence of a large number of hydroxyl groups on mannose residues and glucose, it is highly hydrophilic, and the resulting membrane barrier, mechanical properties, and antibacterial properties are weak, limiting its application in preservation. Chen et al. ( Front. Nutr. 9:911542) Orthophosphate is used as a phosphating agent, and it is reacted with glucomannan under microwave radiation to achieve esterification and obtain highly hydrophilic phosphate, which effectively improves the solubility, rheology and film-forming properties of natural konjac. However, there are still certain problems in the use of glucomannan coating: during the storage of fruits and vegetables, breathing and external moisture will cause the coating to dissolve or break; in addition, the coating strength is not enough, and bumping during transportation will cause mechanical damage to the coating, affecting the preservation effect. Summary of the invention

[0005] In view of this, the purpose of the present invention is to provide a method for preparing a konjac glucomannan coating in view of the deficiencies in the prior art. The coating is not easily soluble, has excellent mechanical properties, and can preserve the fruit by inhibiting ethylene synthesis, maintaining mango quality, delaying fruit ripening and maintaining aroma release.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions: A method for preparing a konjac glucomannan-based coating comprises the following steps: (1) Ingredients: 4 parts of konjac glucomannan, 3-4 parts of chitosan, 2-3 parts of high-ester pectin, 0.5-0.6 parts of calcium carbonate, and 0.8-1.1 parts of glycerol; (2) Dissolve chitosan in 1% organic acid solution, then add konjac glucomannan and high-ester pectin and stir evenly to form a mixed solution, wherein the mass fraction of solute in the mixed solution is 2-3%; (3) After the above mixed liquid is modified, glycerol and calcium carbonate are added and stirred evenly to obtain a coating liquid; the fruit is soaked in the coating liquid and dried to form a coating, which can be used to preserve the fruit.

[0007] Furthermore, the modification step is as follows: Disodium hydrogen phosphate, sodium dihydrogen phosphate and urea are dissolved in water and stirred evenly, wherein the molar ratio of the disodium hydrogen phosphate to the sodium dihydrogen phosphate is 1:2, and the urea is 4-5% of the total amount of the disodium hydrogen phosphate and the sodium dihydrogen phosphate. The pH is adjusted to 5.0 to obtain a modifier, and the modifier is added to the mixed solution and stirred for reaction for 30-40 minutes. The molar ratio of phosphate to konjac glucomannan is 1-1.2:1.

[0008] Furthermore, the reaction was stirred for 30-40 min and then microwaved for 7-10 min.

[0009] Furthermore, the calcium carbonate is surface treated, and the specific steps are as follows: (1) adding water to calcium chloride to form a 2-3 wt% calcium chloride solution; uniformly mixing polysorbate 80 and paraffin oil to form an oil phase; emulsifying the calcium chloride solution and the oil phase for 5-10 minutes to form an emulsion, wherein the weight ratio of the paraffin oil to water is 0.6-0.7:1; (2) dripping sodium carbonate solution into the emulsion drop by drop, stirring and reacting for 5 minutes after dripping, centrifuging and washing the precipitate, drying it, and grinding it to obtain the product; the molar ratio of calcium chloride, sodium carbonate and polysorbate 80 is 1:1:0.4-0.5.

[0010] Furthermore, the organic acid in step (2) is malic acid or citric acid.

[0011] An application of the coating film in fruit preservation is to soak the fruit in the coating liquid for 3-5 minutes, and form a film after drying, which can be used for fruit preservation.

[0012] The beneficial effects of the present invention are: 1. The present invention discloses a konjac glucomannan-based coating film, which adopts three macromolecular substances, namely, konjac glucomannan, chitosan and high-ester pectin, to prepare a ternary blend film. The prepared coating film has excellent mechanical properties, good water resistance and excellent preservation performance.

[0013] 2. Adding high-ester pectin to konjac glucomannan has good film-forming properties and gas barrier properties, and has good compatibility with chitosan and konjac glucomannan. There are a large number of hydroxyl and carboxyl groups on the main chain of high-ester pectin, which can interact with konjac glucomannan and chitosan through electrostatic interactions and hydrogen bonds to form a more stable network structure, thereby increasing the mechanical properties and stability of the coating.

[0014] 3. The present application uses phosphate to carry out esterification cross-linking reaction of konjac glucomannan, chitosan and high ester pectin. During the modification process, the cross-linked network between the three macromolecular chains is more dense, which increases the density and mechanical properties of the coating. In addition, the phosphate groups are grafted onto the macromolecular chains, and there is mutual repulsion between the negatively charged phosphate groups, and the effective volume of the groups increases, the macromolecular chains expand, and the stability of the coating liquid is stronger.

[0015] 4. Calcium carbonate is added to the film, which can reduce the permeability of the film, reduce the oxygen in the film, and inhibit the respiration of the fruit. On the other hand, the calcium carbonate of the present application is prepared by a two-phase precipitation method. After the calcium chloride solution is emulsified with the oil phase and polysorbate-80 to form an oil-in-water emulsion, sodium carbonate is dropped to form a precipitation at the interface of the oil-in-water emulsion, and then after drying, washing and grinding, calcium carbonate particles with one side lipophilic and the other side hydrophilic are formed. After being dispersed in a polymer solution to form a coating, the hydrophilic end migrates inward and the lipophilic end migrates outward to the surface of the film, which plays a role in structural reinforcement, can absorb external impact force, and increase the toughness of the film; in addition, the hydrophilicity of the film surface can be reduced and the stability can be increased. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 The curves of Example 4 and Comparative Example 1 on the quality of Jinhuang mango at the post-harvest maturity stage are shown; wherein 1-1 is weight loss; 1-2 is total soluble solids; 1-3 is titratable acid; 1-4 is hardness; 1-5 is color difference (ΔE); and 1-6 is ethylene.

[0017] Figure 2 The effect curve of Example 4 and Comparative Example 1 on the quality of Kate mango during post-harvest maturity; 2-1 weight loss; 2-2 total soluble solids; 2-3 titratable acid; 2-4 hardness; 2-5 color difference (ΔE); 2-6 ethylene. DETAILED DESCRIPTION

[0018] The present invention will be further described below in conjunction with the accompanying drawings and embodiments. Example 1

[0019] A method for preparing a konjac glucomannan-based coating comprises the following steps: (1) Ingredients: 4 parts of konjac glucomannan, 3 parts of chitosan, 3 parts of high-ester pectin, 0.5 parts of calcium carbonate, and 0.8 parts of glycerol; high-ester pectin refers to pectin with a carboxyl esterification degree greater than 50%; (2) Dissolve chitosan in 1 wt% malic acid solution, then add konjac glucomannan and high ester pectin and stir evenly to form a mixed solution, wherein the mass fraction of solute in the mixed solution is 2%; (3) After the above mixed liquid is modified, glycerol and calcium carbonate are added and stirred evenly to obtain a coating liquid; the fruit is soaked in the coating liquid and dried to form a coating, which can be used to preserve the fruit.

[0020] The steps of modification treatment are: Disodium hydrogen phosphate, sodium dihydrogen phosphate and urea are dissolved in water and stirred evenly, wherein the molar ratio of the disodium hydrogen phosphate to the sodium dihydrogen phosphate is 1:2, and the urea is 4% of the total amount of the disodium hydrogen phosphate and the sodium dihydrogen phosphate. The pH is adjusted to 5.0 to obtain a modifier, and the modifier is added to the mixed solution and stirred for reaction for 30 minutes. The mixed solution is subjected to microwave treatment for 7 minutes, the microwave power is 300 w, and the molar ratio of phosphate to konjac glucomannan is 1:1.

[0021] Calcium carbonate is surface treated in the following steps: (1) adding calcium chloride to water to form a 2.5 wt% calcium chloride solution; uniformly mixing polysorbate 80 and paraffin oil to form an oil phase; emulsifying the calcium chloride solution and the oil phase for 5 minutes to form an emulsion, wherein the weight ratio of paraffin oil to water is 0.6:1; (2) dripping sodium carbonate solution into the emulsion drop by drop, stirring and reacting for 5 minutes after dripping, centrifuging and washing the precipitate, drying it, and grinding it to obtain the product; the molar ratio of calcium chloride, sodium carbonate and polysorbate 80 is 1:1:0.4. Example 2

[0022] A method for preparing a konjac glucomannan-based coating comprises the following steps: (1) Ingredients: 4 parts of konjac glucomannan, 3.5 parts of chitosan, 2.5 parts of high-ester pectin, 0.55 parts of calcium carbonate, and 0.9 parts of glycerol; (2) Dissolve chitosan in 1% malic acid solution, then add konjac glucomannan and high-ester pectin and stir evenly to form a mixed solution, wherein the mass fraction of solute in the mixed solution is 2.5%; (3) After the above mixed solution is modified, glycerol and calcium carbonate are added and stirred evenly to obtain a coating solution.

[0023] The steps of modification are: Disodium hydrogen phosphate, sodium dihydrogen phosphate and urea are dissolved in water and stirred evenly, wherein the molar ratio of the disodium hydrogen phosphate to the sodium dihydrogen phosphate is 1:2, and the urea is 4-5% of the total amount of the disodium hydrogen phosphate and the sodium dihydrogen phosphate. The pH is adjusted to 5.0 to obtain a modifier, and the modifier is added to the mixed solution and stirred for reaction for 35 minutes. The mixed solution is subjected to microwave treatment for 10 minutes, the microwave power is 300 w, and the molar ratio of phosphate to konjac glucomannan is 1.1:1.

[0024] Calcium carbonate is surface treated in the following steps: (1) adding calcium chloride to water to form a 2.8 wt% calcium chloride solution; uniformly mixing polysorbate 80 and paraffin oil to form an oil phase; emulsifying the calcium chloride solution and the oil phase for 8 minutes to form an emulsion, wherein the weight ratio of paraffin oil to water is 0.65:1; (2) dripping sodium carbonate solution into the emulsion drop by drop, stirring and reacting for 5 minutes after dripping, centrifuging and washing the precipitate, drying it, and grinding it to obtain the product; the molar ratio of calcium chloride, sodium carbonate and polysorbate 80 is 1:1:0.45. Example 3

[0025] A method for preparing a konjac glucomannan-based coating comprises the following steps: (1) Ingredients: 4 parts of konjac glucomannan, 4 parts of chitosan, 2 parts of high-ester pectin, 0.56 parts of calcium carbonate, and 0.9 parts of glycerol; (2) Dissolve chitosan in 1% citric acid solution, then add konjac glucomannan and high-ester pectin and stir evenly to form a mixed solution, wherein the mass fraction of solute in the mixed solution is 2.5%; (3) After the above mixed solution is modified, glycerol and calcium carbonate are added and stirred evenly to obtain a coating solution.

[0026] The steps of modification are: Disodium hydrogen phosphate, sodium dihydrogen phosphate and urea are dissolved in water and stirred evenly, wherein the molar ratio of the disodium hydrogen phosphate to the sodium dihydrogen phosphate is 1:2, and the urea is 4.5% of the total amount of the disodium hydrogen phosphate and the sodium dihydrogen phosphate. The pH is adjusted to 5.0 to obtain a modifier, and the modifier is added to the mixed solution and stirred for reaction for 35 minutes. The mixed solution is subjected to microwave treatment for 9 minutes, and the microwave power is 300 w. The molar ratio of phosphate to konjac glucomannan is 1-1.2:1.

[0027] Calcium carbonate is surface treated in the following steps: (1) adding calcium chloride to water to form a 2.2 wt% calcium chloride solution; uniformly mixing polysorbate 80 and paraffin oil to form an oil phase; emulsifying the calcium chloride solution and the oil phase for 5-10 minutes to form an emulsion, wherein the weight ratio of paraffin oil to water is 0.65:1; (2) dripping sodium carbonate solution into the emulsion drop by drop, stirring and reacting for 5 minutes after dripping, centrifuging and washing the precipitate, drying it, and grinding it to obtain the product; the molar ratio of calcium chloride, sodium carbonate and polysorbate 80 is 1:1:0.45. Example 4

[0028] A method for preparing a konjac glucomannan-based coating comprises the following steps: (1) Ingredients: 4 parts of konjac glucomannan, 4 parts of chitosan, 2 parts of high-ester pectin, 0.6 parts of calcium carbonate, and 1.1 parts of glycerol; (2) Dissolve chitosan in 1% malic acid solution, then add konjac glucomannan and high-ester pectin and stir evenly to form a mixed solution, wherein the mass fraction of solute in the mixed solution is 3%; (3) After the above mixed solution is modified, glycerol and calcium carbonate are added and stirred evenly to obtain a coating solution.

[0029] The steps of modification treatment are: Disodium hydrogen phosphate, sodium dihydrogen phosphate and urea are dissolved in water and stirred evenly, wherein the molar ratio of the disodium hydrogen phosphate to the sodium dihydrogen phosphate is 1:2, and the urea is 5% of the total amount of the disodium hydrogen phosphate and the sodium dihydrogen phosphate. The pH is adjusted to 5.0 to obtain a modifier, and the modifier is added to the mixed solution and stirred for reaction for 40 minutes. The mixed solution is subjected to microwave treatment for 10 minutes, the microwave power is 300 w, and the molar ratio of phosphate to konjac glucomannan is 1.2:1.

[0030] Calcium carbonate is surface treated in the following steps: (1) adding calcium chloride to water to form a 2.2 wt% calcium chloride solution; uniformly mixing polysorbate 80 and paraffin oil to form an oil phase; emulsifying the calcium chloride solution and the oil phase for 10 minutes to form an emulsion, wherein the weight ratio of paraffin oil to water is 0.7:1; (2) dripping sodium carbonate solution into the emulsion drop by drop, stirring and reacting for 5 minutes after dripping, centrifuging and washing the precipitate, drying it, and grinding it to obtain the product; the molar ratio of calcium chloride, sodium carbonate and polysorbate 80 is 1:1:0.5.

[0031] Comparative Example 1 Comparative Example 1 is a comparative example of Example 4, except that: A method for preparing a konjac glucomannan-based coating comprises the following steps: (1) Ingredients: 10 parts of konjac glucomannan and 1.1 parts of glycerol; (2) Dissolve konjac glucomannan in 1% malic acid solution with a solute mass fraction of 3%; (3) Then add glycerin evenly to obtain the coating solution.

[0032] Comparative Example 2 Comparative Example 2 is a comparative example of Example 4, except that: In Comparative Example 2, the calcium carbonate powder was not surface treated.

[0033] Comparative Example 3 Comparative Example 3 is a comparative example of Example 4, except that: In Comparative Example 3, no calcium carbonate was added.

[0034] Performance Testing 90 g of the coating liquid prepared in Examples 1-4 and Comparative Examples 1-3 was poured onto a glass plate with a film-forming area of ​​16 cm×16 cm and spread evenly, then dried at 25°C for 24 h, and the water solubility, mechanical properties, water vapor permeability (WVP), antioxidant properties and antibacterial properties of the formed films were tested.

[0035] Water solubility test refers to Nair, SB, etc. ( Starch‐Stärke , 69 (1-2), 2017) method; the determination method of tensile strength and elongation at break is referenced by Liu, Y. et al. ( International Journal of Biology Macromolecules , 267, 131292.2024); the water vapor transmission rate WVP was calculated according to the national standard GB / T 1037-1988 "Test method for water vapor transmission of plastic films and sheets (cup method)", and the antioxidant activity was evaluated by the DPPH free radical scavenging activity assay (reference Liu, Y., etc.). Antibacterial test: Under sterile conditions, small circular filter paper pieces were immersed in the coating solution for 3 min, and then placed on a prefabricated double-layer culture dish containing bacteria; the culture dish was placed in an incubator set at different temperatures; after 1-2 days, the size of the inhibition zone around the disc was observed. The diameter of each inhibition zone was measured using a vernier caliper. The measurement was repeated three times and the average value was calculated. The test data is shown in Table 1.

[0036] Table 1 Performance test data

[0037] The coating liquid prepared by the present application has strong stability and can be stored at room temperature for 6 days without stratification. The prepared film is colorless, transparent, and has strong flexibility. Its solubility in water is 12.15-14.25%. The ternary composite film and phosphorylation modification increase the hydrophobicity of the film and reduce the solubility of the material in water. The low-solubility film can effectively prevent external moisture from entering the fruit and reduce the evaporation of internal moisture, thereby maintaining the freshness and texture of the fruit. In addition, the low-solubility film can better isolate air and microorganisms and reduce the risk of oxidation and corruption. The tensile strength of the film is 67-70.15MPa, the elongation at break is 19-21%, and the mechanical properties are excellent. If the water vapor permeability is too high, moisture can easily penetrate, which is not conducive to fruit preservation. If it is too low, it will also cause anaerobic respiration of fruits and vegetables. The water vapor permeability of the present application is 0.071-0.086g·mol / (d·m 2 ·Pa), suitable for preserving fruits and vegetables. The free radical scavenging rate of the film of the present application may be attributed to the enhanced hydrogen supply capacity after phosphorylation, resulting in the formation of a stable DPPH-H complex. In addition, it has a strong inhibitory effect on Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus), which may be attributed to the antibacterial effect of calcium carbonate. In addition, the positive and negative charged groups on the polysaccharide molecular chain interact with the bacterial cell wall to cause bacterial death.

[0038] Comparative Examples 1-3 are the comparative examples of Example 4, wherein Comparative Example 1 has only adopted Konjac Glucomannan and glycerol, and Konjac Glucomannan film water solubility is higher, easily absorbs water and dissolves, and water vapor permeability is high, mechanical property is poor, and antibacterial property and free radical scavenging rate are all lower. In conjunction with Comparative Examples 2-3 and Example 4, it can be seen that adding calcium carbonate can increase the mechanical property and antibacterial property of film; and after calcium carbonate is surface treated, its tensile strength and elongation at break can be greatly improved again, and water vapor permeability and solubility are also significantly reduced, illustrating that the surface treatment process of the application is very large for the mechanical property, hydrophilicity and water vapor permeability of film, and can greatly increase the mechanical strength and fresh-keeping property of film.

[0039] Application test The coating solution of Example 4 and Comparative Example 1 of the present application was used for mango preservation test. The mango varieties (Jinhuang and Kate) used were taken from the Guangzhou Fruit World Mango Orchard, where the average annual rainfall is 1500-2000 mm. Three healthy plants with similar growth conditions were selected for each variety, totaling 9 plants. The treatment groups (pre-harvest and post-harvest) were soaked in the film solution. A random complete block design was adopted, and the orchard was divided into two blocks, "Jinhuang" and "Kate". The soil type and moisture conditions in each block were similar, representing the same mango variety. Three healthy and as consistent as possible plants were randomly selected in each block, and all mangoes on each tree were randomly numbered on average, and were set as the control group CK: soaked in the coating solution of Comparative Example 1; pre-harvest KGM: pre-harvest soaked in the coating solution of Example 4; post-harvest KGM: post-harvest mango soaked in the coating solution of Example 4, and each treatment was repeated three times.

[0040] The specific method is: Pre-harvest group: Two months before harvest (May 2022, 30-45 days after fruit setting), mangoes in the pre-harvest treatment group were soaked in the coating liquid for 5 minutes every 20 days, repeated three times to ensure uniform coverage of the fruit surface. Control group: The same operation as pre-harvest, mangoes were soaked in the coating liquid for 5 minutes every 20 days, repeated three times to ensure uniform coverage. On June 10, 2022, 5 mangoes were harvested from the east, west, south and north directions of the pre-harvest treatment group and the control group. The selected mangoes had uniform growth, size and color, no disease and insect spots, and a maturity of 70%. A 2 cm long fruit stalk was retained during harvesting, and each mango tree was repeated three times. A total of 180 mangoes were collected from each group. Within 2 hours after harvesting, pre-cooling was carried out in a cold storage at 11-12℃ for 12 hours. After air drying, the fruit was refrigerated at 24±2℃ and 70%-80% relative humidity for 16 days.

[0041] Post-harvest group: Mangoes in the post-harvest treatment group were harvested and pre-cooled using the same methods and standards as the pre-harvest treatment group. After pre-cooling, undamaged fruits were selected, rinsed three times with clean water, and air-dried to remove surface moisture. Then they were soaked in the coating solution for 5 minutes. After the surface was air-dried, the mangoes were refrigerated at 24±2℃ and 70%-80% relative humidity for 16 days.

[0042] Experimental method: Samples were analyzed at 0, 4, 8, 12 and 16 days to determine their weight loss rate, soluble solids, titratable acidity, firmness index, color difference (∆E) and ethylene production. Twenty mangoes were used in each treatment group to determine the weight loss rate, and 5 mangoes were taken to determine the remaining physiological parameters. The measurements were repeated three times. After peeling, the pulp was cut into small pieces of 1 cm³ to obtain a representative sample of the whole fruit. The method for determining the amount of ethylene production: Three fruits were randomly selected from each group, numbered, and placed in 6L sealed containers respectively. They were placed at 24℃ for 1 hour. A 1 mL gas sample was collected from each container using a gastight syringe and the gas was analyzed using a gas chromatograph. Results can be found at Figure 1 (Golden Brilliance) and Figure 2 (Kate).

[0043] Figure 1-1 and 2-1 show that the weight loss rate of the two mango varieties continues to increase with the extension of storage time. The weight loss rate of the control group is higher than that of KGM; and the KGM pretreatment is more effective in reducing the weight loss rate than the KGM post-treatment. The weight loss rate of Jinhuang mango is lower than that of Kate. The weight loss of fruits and vegetables is mainly attributed to two factors: water transpiration and post-harvest respiratory consumption. The transpiration of water through the numerous stomata and lenticels on the surface of the fruit is the main factor causing weight loss. Post-harvest weight loss is a common phenomenon in harvested fruits. Dehydration and shrinkage of the fruit surface will lead to a decline in overall quality and make the fruit more susceptible to pathogen invasion and cold damage. It can be seen that the coating liquid of the present application effectively reduces the weight loss rate, and the coating formed acts as a semi-permeable membrane barrier, reducing the transpiration of water through the stomata and openings on the surface of the fruit, thereby reducing the weight loss rate of stored fruits.

[0044] During storage, the total soluble solids (TSS) content of mangoes increased rapidly, but the titratable acidity (TA) decreased ( Figure 1-2 / 1-3 and Figure 2-2 / 2-3). The increase in TSS and the decrease in TA were associated with the ripening process of mango fruit. KGM-treated mangoes had higher TSS and TA values ​​than the control group. Specifically, KGM pretreatment maintained relatively high TSS levels in mangoes, while its TA value remained low. The higher acidity observed in KGM-treated fruits may be due to the delayed ripening and senescence processes induced by the KGM coating.

[0045] Figure 1 -4 and Figure 2 -4 showed that mangoes softened significantly during postharvest ripening, which was mainly due to physiological changes in cell wall composition, such as degradation, deesterification and depolymerization of pectin polysaccharides, cellulose and hemicellulose. In addition, the rich starch reserves in the pulp undergo enzymatic hydrolysis during the ripening process, reducing tissue hardness and loosening cell structure. In the two mango varieties, KGM-treated fruits showed higher hardness levels than the control group throughout the postharvest storage period. Among them, the softening rate of the pre-harvest treated samples was the slowest. The coating of the present application effectively inhibited the water loss of mangoes and created a low-oxygen and high-carbon dioxide microenvironment on the surface of the fruit. This can inhibit respiration and ethylene biosynthesis, thereby reducing the metabolism and enzyme activity associated with fruit softening, and ultimately delaying the softening of mangoes.

[0046] Color is an important indicator of mango appearance quality. The color change of mango peel during postharvest storage can be expressed by the ΔE value ( Figure 1-5 and 2-5). A larger ΔE value indicates a greater change in peel color during postharvest storage, and the ΔE value gradually increases with storage time. The ΔE value increase rate of the control group (CK) was significantly higher than that of the KGM pretreatment group and posttreatment group. KGM treatment effectively inhibited the expansion of color differences to a certain extent. This indicates that KGM peel coating helps maintain the brightness of mangoes.

[0047] During storage, the ethylene production trends of the two mango varieties first increased and then decreased ( Figure 1 -6 and 2-6). Compared with the control group, KGM treatment delayed the postharvest ripening of mangoes. Specifically, the ethylene release rate of the KGM pretreatment group was always significantly lower than that of the other two groups. This shows that the coating treatment of the present application can inhibit the release of ethylene in mangoes, possibly delay the ethylene peak, and thus delay fruit senescence.

[0048] Through the above analysis, it can be seen that the coating liquid of the present application acts on the preservation of mangoes, and the preservation effect of pre-harvest treatment is better than that of post-harvest treatment. The coating liquid significantly reduces the weight loss rate of mangoes, delays the softening of mangoes, and inhibits the release of ethylene. It can preserve the fruit by inhibiting ethylene synthesis, maintaining mango quality, delaying fruit ripening and maintaining aroma release.

[0049] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Other modifications or equivalent substitutions made to the technical solution of the present invention by ordinary technicians in the field should be included in the scope of the claims of the present invention as long as they do not depart from the spirit and scope of the technical solution of the present invention.

Claims

1. A method for preparing a konjac glucomannan-based coating, characterized in that: The following steps are involved: (1) Ingredients: 4 parts of konjac glucomannan, 3-4 parts of chitosan, 2-3 parts of high-ester pectin, 0.5-0.6 parts of calcium carbonate, and 0.8-1.1 parts of glycerol; (2) Dissolve chitosan in 1% organic acid solution, then add konjac glucomannan and high-ester pectin and stir evenly to form a mixed solution, wherein the mass fraction of solute in the mixed solution is 2-3%; (3) After the above mixed solution is modified, glycerol and calcium carbonate are added and stirred evenly to obtain a coating solution.

2. The method for preparing a konjac glucomannan-based coating according to claim 1, characterized in that: The steps of the modification treatment are: Disodium hydrogen phosphate, sodium dihydrogen phosphate and urea are dissolved in water and stirred evenly, wherein the molar ratio of the disodium hydrogen phosphate to the sodium dihydrogen phosphate is 1:2, and the urea is 4-5% of the total amount of the disodium hydrogen phosphate and the sodium dihydrogen phosphate. The pH is adjusted to 5.0 to obtain a modifier, and the modifier is added to the mixed solution and stirred for reaction for 30-40 minutes. The molar ratio of phosphate to konjac glucomannan is 1-1.2:

1.

3. The method for preparing a konjac glucomannan-based coating according to claim 2, wherein: The reaction was stirred for 30-40 min and then treated with microwave for 7-10 min.

4. The method for preparing a konjac glucomannan-based coating according to claim 1, characterized in that: The calcium carbonate is surface treated, and the specific steps are as follows: (1) adding water to calcium chloride to form a 2-3 wt% calcium chloride solution; uniformly mixing polysorbate 80 and paraffin oil to form an oil phase; emulsifying the calcium chloride solution and the oil phase for 5-10 minutes to form an emulsion, wherein the weight ratio of the paraffin oil to water is 0.6-0.7:1; (2) dripping sodium carbonate solution into the emulsion drop by drop, stirring and reacting for 5 minutes after dripping, centrifuging and washing the precipitate, drying it, and grinding it to obtain the product; the molar ratio of calcium chloride, sodium carbonate and polysorbate 80 is 1:1:0.4-0.

5.

5. The method for preparing a konjac glucomannan-based coating according to claim 1, characterized in that: The organic acid in step (2) is malic acid or citric acid.

6. An application of the coating film prepared according to claim 1 in fruit preservation, characterized in that: Soak the fruit in the coating solution for 3-5 minutes and form a film after drying.

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