Composite coating film preservative liquid and preparation method and application thereof

Abstract

The present application relates to the field of food preservation technology, in particular to a kind of compound coating film preservative and its preparation method and application.Preservative includes the following mass fraction of raw materials: carboxymethylated nanocellulose 0.1-0.5 parts, chitosan quaternary ammonium salt 0.5-2.0 parts, sodium alginate 0.3-1.0 parts, extract of cypress leaf obtained by eutectic solvent extraction 0.05-0.3 parts, plasticizer 0.1-0.5 parts, acidity regulator 0.1-0.3 parts, water 80-120 parts.The present application constructs a compound coating film preservative with multiple network structure by electrostatic self-assembly of carboxymethylated nanocellulose and chitosan quaternary ammonium salt, post-crosslinking of sodium alginate and calcium ions, and combining the antioxidant and antibacterial activity of cypress leaf extract.The coating film preservative can effectively inhibit postharvest water loss of fragrant pear, delay fruit softening, reduce browning and rot, and significantly prolong cold storage shelf life.

Description

Technical Field

[0001] This invention relates to the field of food preservation technology, and in particular to a composite coating preservation liquid, its preparation method, and its application. Background Technology

[0002] Fragrant pears are beloved by consumers for their thin skin, delicate flesh, abundant juice, sweet taste, and rich aroma. However, as a climacteric fruit, fragrant pears experience vigorous post-harvest metabolism. Combined with their extremely thin skin and high transpiration rate, they begin to shrivel and lose water within 3-5 days of harvest at room temperature, and start to brown and soften extensively within 7-10 days, resulting in a shelf life of less than 10 days at room temperature. Even under refrigerated conditions (0-4℃), the spoilage rate remains high after 30 days of storage, severely hindering the industrialization and market circulation of fragrant pears. Therefore, developing efficient preservation technologies suitable for fragrant pears is of great significance.

[0003] Edible coating technology has become a research hotspot in the field of fruit and vegetable preservation in recent years. By forming a semi-permeable film on the surface of the fruit, the gas composition around the fruit can be regulated, water evaporation can be inhibited, and microbial infection can be reduced, thereby delaying the ripening and senescence of the fruit. Currently, commonly used edible coating matrices mainly include polysaccharides, proteins, and lipids. Among them, polysaccharides have received widespread attention due to their wide availability, good film-forming properties, and high biocompatibility. However, existing edible coating solutions still have the following technical shortcomings in practical applications: First, the moisture-barrier properties of the coating are insufficient. For thin-skinned, juicy fruits like fragrant pears, post-harvest moisture evaporation is the primary factor leading to quality deterioration. The applicant's preliminary experiments found that fragrant pears treated with a single chitosan coating still experienced a high weight loss rate under refrigeration, indicating that polysaccharide film-forming matrices alone have limited ability to block water vapor and are insufficient to adequately inhibit rapid post-harvest water loss. Second, the mechanical strength of the coating is limited. Coatings formed from single polysaccharide film-forming materials are often brittle or lack strength, easily developing microcracks or even breakage during fruit handling and storage, leading to a loss of the coating's barrier function. Third, the compatibility between the film-forming matrix and functional additives is poor. To impart antibacterial or antioxidant functions to the coating, it is often necessary to add active ingredients such as plant extracts or essential oils. The applicant's preliminary experiments observed that some active ingredients, when mixed with hydrophilic polysaccharide matrices, are prone to phase separation or aggregation, resulting in uneven coatings and affecting the sustained-release effect of the active ingredients. Fourth, the preparation or application conditions of some coating solutions are quite demanding. For example, some film-forming materials can only dissolve under acidic conditions, which may irritate the fruit peel; the extraction of some active ingredients requires the use of organic solvents, which poses a safety hazard due to solvent residue.

[0004] In summary, developing a coating preservative solution with good moisture barrier properties, suitable mechanical strength, good component compatibility, and mild preparation and use conditions is an urgent problem to be solved in this field. Summary of the Invention

[0005] The purpose of this invention is to address the problems existing in the prior art by providing a composite coating preservative liquid, its preparation method, and its application. A coating with a multi-network structure is constructed through the electrostatic self-assembly of carboxymethylated nanocellulose and chitosan quaternary ammonium salt, post-crosslinking of sodium alginate and calcium ions, and functional compounding with Platycladus orientalis leaf extract, thereby improving preservation performance.

[0006] To achieve the above objectives, the present invention provides a composite coating preservative liquid, comprising the following raw materials in parts by weight: Carboxymethylated nanocellulose 0.1-0.5 parts, chitosan quaternary ammonium salt 0.5-2.0 parts, sodium alginate 0.3-1.0 parts, arborvitae leaf extract obtained by eutectic solvent extraction 0.05-0.3 parts, plasticizer 0.1-0.5 parts, acidity regulator 0.1-0.3 parts, water 80-120 parts.

[0007] In an optional embodiment, the method for preparing the carboxymethylated cellulose nanoparticles includes: (1) Mix cellulose nanofibers, a first solvent and an alkaline solution, and alkalize to obtain an intermediate product; (2) The intermediate product, etherifying agent and second solvent are mixed and reacted to obtain carboxymethylated cellulose nanoparticles.

[0008] In an optional embodiment, in step (1), the cellulose nanofibers have a length of 200-500 nm and a diameter of 10-30 nm; the first solvent is selected from isopropanol; the alkaline solution is selected from sodium hydroxide solution, and the mass fraction of the sodium hydroxide solution is 15-25%; the mass-volume ratio of the cellulose nanofibers, the first solvent and the alkaline solution is (3-7) g : (150-250) mL : (15-25) g; the alkalization temperature is 20-30℃ and the time is 20-40 min.

[0009] In an optional embodiment, in step (2), the etherifying agent is selected from chloroacetic acid; the second solvent is selected from isopropanol; the mass-volume ratio of the etherifying agent and the second solvent is (5-10) g : (15-25) mL; the mass ratio of the etherifying agent and the cellulose nanofibers in step (1) is (5-10) : (3-7); the reaction temperature is 50-70℃ and the time is 2-4 h.

[0010] In an optional embodiment, in step (2), after the reaction is completed, the pH is adjusted to neutral with acetic acid, filtered, and the obtained solid is washed more than twice with an ethanol solution of 75-85% by mass. After washing, it is dispersed in water, dialyzed for 40-55 hours, and finally freeze-dried to obtain carboxymethylated nanocellulose.

[0011] In an optional embodiment, the chitosan quaternary ammonium salt is selected from hydroxypropyltrimethylammonium chloride chitosan.

[0012] In an optional embodiment, the eutectic solvent comprises choline chloride and lactic acid; the molar ratio of choline chloride to lactic acid is 1:(1-2).

[0013] In an optional embodiment, the method for preparing the Platycladus orientalis leaf extract includes: Choline chloride and lactic acid were mixed at a molar ratio of 1:(1-2) and stirred evenly at 50-70℃ to obtain a eutectic solvent. Dried Platycladus orientalis leaves were pulverized and passed through a 40-80 mesh sieve. The eutectic solvent (mass-volume ratio of Platycladus orientalis leaves to eutectic solvent was 1 g:(10-20) mL) was added, and ultrasonic extraction was performed at 60-80℃ and 30-50 kHz for 1-2 h. The supernatant was collected by centrifugation (6000-10000 r / min, 10-20 min), and ethanol (2-4 times the volume of the supernatant) was added. The mixture was allowed to stand at 0-4℃ for precipitation for 2-12 h. The precipitate was collected by centrifugation, washed 1-2 times with ethanol, and freeze-dried to obtain the Platycladus orientalis leaf extract.

[0014] In one optional embodiment, the plasticizer is selected from at least one of glycerol and sorbitol; the acidity regulator is selected from at least one of acetic acid and lactic acid.

[0015] The present invention also provides a method for preparing the aforementioned composite coating preservative liquid, comprising the following steps: The raw materials are mixed to obtain the composite coating preservation liquid.

[0016] In an optional implementation, mixing the raw materials includes: S1. Mix carboxymethylated nanocellulose and water from the first part, and sonicate at 150-250W for 10-20 min to obtain dispersion A; mix chitosan quaternary ammonium salt, acidity regulator, and water from the second part, and stir until completely dissolved to obtain solution B; mix sodium alginate, plasticizer, and water from the third part, and stir to dissolve in a water bath at 50-60℃ to obtain solution C; mix the arborvitae leaf extract obtained from eutectic solvent extraction with water from the fourth part, and stir until completely dissolved to obtain solution D; S2. Under stirring conditions, mix solution B with dispersion A and react at 20-40℃ for 20-40 min; then add solution C and continue stirring for 10-20 min; then add solution D and continue stirring for 5-10 min; add water to the formula amount to obtain composite coating preservation liquid.

[0017] In one optional implementation, the mass ratio of the first part of water, the second part of water, the third part of water, and the fourth part of water is (2-4):(3-5):(1-3):(0.3-0.7).

[0018] The present invention also provides the application of the aforementioned composite coating preservative liquid in fruit preservation.

[0019] In an alternative implementation, the fruit is selected from fragrant pears.

[0020] The present invention also provides a method for preserving fruit using the aforementioned composite coating preservative liquid, comprising the following steps: The fruit is immersed in the composite coating preservation liquid, then removed and air-dried, and then immersed in a solution containing calcium ions, and removed and air-dried again.

[0021] Specifically: Select fragrant pears that are 8-9 ripe, without mechanical damage, and uniform in size. Wash off surface dust with clean water and allow to air dry naturally. Immerse the fruit in a composite coating preservative solution at room temperature for 2-5 minutes, remove and drain excess liquid, then place in a ventilated area to air dry naturally at room temperature (30-60 minutes). Next, immerse the fruit in a 0.5-2% calcium chloride solution at room temperature for 30-60 seconds, remove and drain excess liquid, then place in a ventilated area to air dry naturally at room temperature (30-60 minutes). Finally, place in a plastic turnover basket, cover with a plastic bag (unsealed), and store in a cold storage at 0-4℃ and 85-95% relative humidity.

[0022] Compared with the prior art, the present invention has the following beneficial effects: (1) In this invention, carboxymethylated nanocellulose and chitosan quaternary ammonium salt form a polyelectrolyte composite network through electrostatic interaction in aqueous solution. At the same time, after coating, sodium alginate forms an ion gel network on the fruit surface through calcium ion crosslinking. The dual network structure makes the coating dense and continuous, which can effectively inhibit the water loss caused by transpiration in thin-skinned and juicy fruits such as fragrant pears after harvest.

[0023] (2) Carboxymethylated nanocellulose has a high aspect ratio and high crystallinity. When uniformly dispersed in the coating network, it can play a nano-reinforcing role, significantly improving the tensile strength and puncture resistance of the coating, and reducing the risk of cracks or damage to the coating during fruit handling and storage.

[0024] (3) Carboxymethylated nanocellulose carries a negative charge, and chitosan quaternary ammonium salt carries a positive charge. The two can spontaneously and uniformly combine in a neutral aqueous solution, avoiding the phase separation or agglomeration phenomenon that occurs between functional additives and film-forming matrix due to polarity differences in traditional coatings.

[0025] (4) Chitosan quaternary ammonium salt has broad-spectrum antibacterial activity and can inhibit a variety of bacteria and fungi that cause fruit spoilage; Platycladus orientalis leaf extract is rich in flavonoids and has good free radical scavenging ability, which can delay the oxidative browning of fruit. The synergistic effect of the two helps to reduce the occurrence of postharvest diseases and quality deterioration.

[0026] (5) All raw materials used in this invention are food-grade or of natural origin, and the main components are biodegradable; the extract of arborvitae leaves is extracted using choline chloride-lactic acid, which is a green and environmentally friendly process; the preparation and use of the coating liquid are mild and have little irritation to the fruit peel.

[0027] (6) Through the design of multiple network structures, the coating film plays a synergistic role in moisture barrier, enhancement, antibacterial and antioxidant properties. It can effectively inhibit post-harvest water loss and wrinkling of fragrant pears, delay the softening of the flesh, reduce browning and rot, significantly extend its cold storage shelf life and enhance its commercial value. Attached Figure Description

[0028] Figure 1 This is a comparison chart of the preservation effects of different treatment groups in this invention. Detailed Implementation

[0029] The following embodiments are provided to better understand the present invention and are not limited to the described embodiments. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0030] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0031] In the following embodiments and comparative examples of the present invention, "water" is "deionized water".

[0032] Example 1 This embodiment provides a composite coating preservative liquid, comprising the following raw materials in parts by weight: Carboxymethylated nanocellulose 0.3 parts, chitosan quaternary ammonium salt (hydroxypropyltrimethylammonium chloride chitosan) 1.2 parts, sodium alginate 0.6 parts, arborvitae leaf extract obtained by eutectic solvent extraction 0.1 parts, plasticizer (glycerin) 0.3 parts, acidity regulator (acetic acid) 0.2 parts, water 100 parts.

[0033] The preparation methods of carboxymethylated cellulose nanoparticles include: Take 500 mL of cellulose nanofiber aqueous dispersion (containing 5 g of cellulose nanofibers, with a length of 200-500 nm and a diameter of 10-30 nm), centrifuge and concentrate, then disperse in 200 mL of isopropanol, add 20 g of 20% sodium hydroxide solution, and stir at 25 °C for 30 min to alkalize and obtain the intermediate product.

[0034] 7.5 g of chloroacetic acid was dissolved in 20 mL of isopropanol and slowly added dropwise to the above intermediate product. The mixture was stirred in a water bath at 60 °C for 3 h. After the reaction was complete, the pH was adjusted to 7 with glacial acetic acid, and the mixture was filtered. The resulting solid was washed three times with 80% ethanol solution. The washed solid was dispersed in water and transferred to a dialysis bag (molecular weight cutoff of 3500 Da) for dialysis for 48 h. Finally, it was freeze-dried to obtain carboxymethylated cellulose nanoparticles.

[0035] The preparation method of Platycladus orientalis leaf extract obtained by eutectic solvent extraction includes: Choline chloride and lactic acid were mixed at a molar ratio of 1:1.5 and stirred evenly at 60℃ to obtain a eutectic solvent. Arborvitae leaves were collected, washed, dried, and pulverized through a 60-mesh sieve. 50g of Arborvitae leaf powder was added to 750mL of the eutectic solvent, and extracted with ultrasound-assisted extraction at 70℃ and 40kHz for 1.5h. The extract was centrifuged (8000r / min, 15min) to collect the supernatant. Ethanol, three times the volume of the supernatant, was added, and the mixture was allowed to stand at 4℃ for 12h to precipitate. The precipitate was collected by centrifugation, washed twice with ethanol, and freeze-dried to obtain the Arborvitae leaf extract.

[0036] This embodiment also provides a method for preparing the above-mentioned composite coating preservative liquid, including the following steps: Carboxymethylated nanocellulose was mixed with 28 parts of water and sonicated at 200W for 15 min to obtain dispersion A; chitosan quaternary ammonium salt, acidity regulator and 38 parts of water were mixed and stirred at room temperature until completely dissolved to obtain solution B; sodium alginate, plasticizer and 18 parts of water were mixed and stirred in a water bath at 55℃ to obtain solution C; arborvitae leaf extract obtained by eutectic solvent extraction was mixed with 4 parts of water and stirred at room temperature until completely dissolved to obtain solution D.

[0037] Under stirring conditions, solution B is slowly poured into dispersion A and reacted at 25°C for 30 minutes; then solution C is added and stirring is continued for 15 minutes; then solution D is added and stirring is continued for 5 minutes; water is added to the formula amount (100 parts) to obtain the composite coating preservation liquid.

[0038] Example 2 This embodiment provides a composite coating preservative liquid, comprising the following raw materials in parts by weight: Carboxymethylated nanocellulose 0.5 parts, chitosan quaternary ammonium salt (hydroxypropyltrimethylammonium chloride chitosan) 1.5 parts, sodium alginate 0.8 parts, arborvitae leaf extract obtained by eutectic solvent extraction 0.2 parts, plasticizer (sorbitol) 0.4 parts, acidity regulator (lactic acid) 0.25 parts, water 110 parts.

[0039] The preparation methods for carboxymethylated nanocellulose and the preparation methods for Platycladus orientalis leaf extract obtained by eutectic solvent extraction are the same as in Example 1.

[0040] This embodiment also provides a method for preparing the above-mentioned composite coating preservative liquid, including the following steps: Carboxymethylated nanocellulose was mixed with 30 parts of water and sonicated at 200W for 15 min to obtain dispersion A; chitosan quaternary ammonium salt, acidity regulator and 45 parts of water were mixed and stirred at room temperature until completely dissolved to obtain solution B; sodium alginate, plasticizer and 20 parts of water were mixed and stirred in a water bath at 55℃ to obtain solution C; arborvitae leaf extract obtained by eutectic solvent extraction was mixed with 5 parts of water and stirred at room temperature until completely dissolved to obtain solution D.

[0041] Under stirring conditions, solution B is slowly poured into dispersion A and reacted at 25°C for 30 minutes; then solution C is added and stirring is continued for 15 minutes; then solution D is added and stirring is continued for 5 minutes; water is added to the formula amount (110 parts) to obtain the composite coating preservation liquid.

[0042] Example 3 This embodiment provides a composite coating preservative liquid, comprising the following raw materials in parts by weight: Carboxymethylated nanocellulose 0.15 parts, chitosan quaternary ammonium salt (hydroxypropyltrimethylammonium chloride chitosan) 0.8 parts, sodium alginate 0.4 parts, arborvitae leaf extract obtained by eutectic solvent extraction 0.08 parts, plasticizer (glycerin) 0.2 parts, acidity regulator (acetic acid) 0.15 parts, water 90 parts.

[0043] The preparation methods for carboxymethylated nanocellulose and the preparation methods for Platycladus orientalis leaf extract obtained by eutectic solvent extraction are the same as in Example 1.

[0044] This embodiment also provides a method for preparing the above-mentioned composite coating preservative liquid, including the following steps: Carboxymethylated nanocellulose was mixed with 25 parts water and sonicated at 200W for 15 min to obtain dispersion A; chitosan quaternary ammonium salt, acidity regulator and 30 parts water were mixed and stirred at room temperature until completely dissolved to obtain solution B; sodium alginate, plasticizer and 15 parts water were mixed and stirred in a water bath at 55℃ to obtain solution C; arborvitae leaf extract obtained by eutectic solvent extraction was mixed with 3 parts water and stirred at room temperature until completely dissolved to obtain solution D.

[0045] Under stirring conditions, solution B is slowly poured into dispersion A and reacted at 25°C for 30 minutes; then solution C is added and stirring is continued for 15 minutes; then solution D is added and stirring is continued for 5 minutes; water is added to the formula amount (90 parts) to obtain the composite coating preservation liquid.

[0046] Comparative Example 1 This comparative example provides a composite coating preservative liquid, which differs from Example 1 only in that the addition of carboxymethylated nanocellulose is omitted and replaced with an equal amount of water. Specifically, it includes the following raw materials in parts by weight: 1.2 parts of chitosan quaternary ammonium salt (hydroxypropyltrimethylammonium chloride chitosan), 0.6 parts of sodium alginate, 0.1 parts of Platycladus orientalis leaf extract obtained by eutectic solvent extraction, 0.3 parts of plasticizer (glycerin), 0.2 parts of acidity regulator (acetic acid), and 0.3 parts of water.

[0047] The preparation method of the Platycladus orientalis leaf extract obtained by eutectic solvent extraction is the same as that in Example 1.

[0048] This comparative example also provides a method for preparing the above-mentioned composite coating preservative liquid, including the following steps: Solution B is obtained by mixing chitosan quaternary ammonium salt, acidity regulator and 38 parts water, stirring at room temperature until completely dissolved; solution C is obtained by mixing sodium alginate, plasticizer and 18 parts water, stirring in a water bath at 55°C; and solution D is obtained by mixing arborvitae leaf extract obtained by eutectic solvent extraction with 4 parts water, stirring at room temperature until completely dissolved.

[0049] Under stirring conditions, add solution C to solution B and stir for 15 minutes; then add solution D and continue stirring for 5 minutes; add water to make up to the formula amount (100.3 parts) to obtain the composite coating preservation liquid.

[0050] Comparative Example 2 This comparative example provides a composite coating preservation liquid, which differs from Example 1 only in that the addition of the Platycladus orientalis leaf extract obtained by eutectic solvent extraction is omitted and replaced with an equal amount of water. Specifically, it includes the following raw materials in parts by weight: Carboxymethylated nanocellulose 0.3 parts, chitosan quaternary ammonium salt (hydroxypropyltrimethylammonium chloride chitosan) 1.2 parts, sodium alginate 0.6 parts, plasticizer (glycerol) 0.3 parts, acidity regulator (acetic acid) 0.2 parts, water 100.1 parts.

[0051] The preparation method of carboxymethylated nanocellulose is the same as that in Example 1.

[0052] This comparative example also provides a method for preparing the above-mentioned composite coating preservative liquid, including the following steps: Carboxymethylated nanocellulose and 28 parts of water were mixed and ultrasonicated at 200W for 15 min to obtain dispersion A; chitosan quaternary ammonium salt, acidity regulator and 38 parts of water were mixed and stirred at room temperature until completely dissolved to obtain solution B; sodium alginate, plasticizer and 18 parts of water were mixed and stirred in a water bath at 55℃ to dissolve to obtain solution C.

[0053] Under stirring conditions, solution B was slowly poured into dispersion A and reacted at 25°C for 30 minutes; then solution C was added and stirring was continued for 15 minutes; water was added to the formula amount (100.1 parts) to obtain the composite coating preservation liquid.

[0054] Comparative Example 3 This comparative example provides a composite coating preservative liquid, which differs from Example 1 only in that the addition of chitosan quaternary ammonium salt is omitted and replaced with an equal amount of water. Specifically, it includes the following raw materials in parts by weight: Carboxymethylated nanocellulose 0.3 parts, sodium alginate 0.6 parts, Platycladus orientalis leaf extract obtained by eutectic solvent extraction 0.1 parts, plasticizer (glycerin) 0.3 parts, acidity regulator (acetic acid) 0.2 parts, water 101.2 parts.

[0055] The preparation methods for carboxymethylated nanocellulose and the preparation methods for Platycladus orientalis leaf extract obtained by eutectic solvent extraction are the same as in Example 1.

[0056] This comparative example also provides a method for preparing the above-mentioned composite coating preservative liquid, including the following steps: Carboxymethylated nanocellulose was mixed with 28 parts of water and sonicated at 200W for 15 min to obtain dispersion A; acidity regulator was mixed with 38 parts of water and stirred at room temperature until completely dissolved to obtain solution B; sodium alginate, plasticizer and 18 parts of water were mixed and stirred in a water bath at 55℃ to obtain solution C; arborvitae leaf extract obtained by eutectic solvent extraction was mixed with 4 parts of water and stirred at room temperature until completely dissolved to obtain solution D.

[0057] Under stirring conditions, solution B is slowly poured into dispersion A and reacted at 25°C for 30 minutes; then solution C is added and stirring is continued for 15 minutes; then solution D is added and stirring is continued for 5 minutes; water is added to the formula amount (100 parts) to obtain the composite coating preservation liquid.

[0058] Experimental Example 1 Select fragrant pears grown in Chengde that are 8-9 ripe, free from mechanical damage, and uniform in size. Wash off the surface dust with clean water and let them air dry naturally. The average weight of a single fruit is about 95g.

[0059] The fruits were randomly divided into 7 groups, with 30 fruits in each group. Groups 1, 2, 3, Comparative Example 1, 2, and 3 were treated with their respective prepared composite coating preservation solutions, while the blank control group was treated with deionized water. Treatment method: The fruits were immersed in the corresponding treatment solution for 3 minutes, drained of excess liquid, and air-dried at room temperature in a ventilated area for 40 minutes. Then, they were immersed in a 1% calcium chloride solution for 45 seconds, drained of excess liquid, and air-dried at room temperature in a ventilated area for 40 minutes. Finally, they were placed in plastic turnover baskets, covered with PE preservation bags (unsealed), and stored in a cold storage at 2°C and 90% relative humidity for 45 days. After storage, the following performance tests were conducted: (1) Weight loss rate: Weigh the initial weight (m0) of each fruit before storage and weigh the weight (m1) of each fruit after 45 days of storage. Weight loss rate (%) = (m0-m1) / m0×100%. Take the average weight loss rate of 30 fruits in each group.

[0060] (2) Hardness: Measured using a TA.XT Plus texture analyzer. Probe model P / 5 (cylindrical, 5mm diameter). Test parameters: Pre-test speed 1.0mm / s, test speed 1.0mm / s, post-test speed 5.0mm / s, puncture depth 8mm, trigger force 5g. After removing the peel from each fruit, three points were evenly selected at the equator for puncture, and the average value was taken as the hardness value of the fruit. The average hardness of 30 fruits in each group was taken, in Newtons (N).

[0061] (3) Rot rate: The criterion for judging rot is the presence of visible mold spots, water-soaked soft rot, or oozing juice on the fruit surface. The number of rotten fruits in each group (n) is counted, and the rot rate (%) is calculated as n / 30 × 100%.

[0062] (4) Browning Index: Visual assessment was used. The degree of browning of the fruit was divided into 5 levels: Level 0: No browning; Level 1: Browning area < 1 / 4 of the total fruit surface area; Level 2: Browning area ≥ 1 / 4 and < 1 / 2; Level 3: Browning area ≥ 1 / 2 and < 3 / 4; Level 4: Browning area ≥ 3 / 4. Browning Index = Σ (browning level × number of fruits at that level) / (total number of fruits × 4).

[0063] The test results are recorded in Table 1. Additionally, to more clearly and intuitively demonstrate the differences in preservation effects among the groups, a comparison chart of the preservation effects of each treatment group was created, as shown below. Figure 1 As shown.

[0064] Table 1 Test Results

[0065] From the above results, we can conclude that: The blank control group had the highest weight loss rate (15.6%), the lowest hardness (3.1N), the highest rot rate (52.3%), and the most severe browning (3.9), indicating that the fragrant pears without coating preservation had basically lost their commercial value after 45 days of refrigeration.

[0066] The performance indicators of Comparative Examples 1 and 3 were significantly worse than those of the groups in the Examples, indicating that the composite network structure formed by the electrostatic self-assembly of carboxymethylated nanocellulose and chitosan quaternary ammonium salt is the key to imparting excellent moisture-proof properties and mechanical strength to the coating film. Comparative Example 2 showed better weight loss and hardness than Comparative Examples 1 and 3, but its rot rate and browning index were still significantly higher than those of the groups in the Examples, suggesting that the Platycladus orientalis leaf extract provides important synergistic antioxidant and antibacterial effects.

[0067] The indicators of Examples 1-3 are significantly better than those of all comparative examples, indicating that the composite coating preservation liquid provided by the present invention can significantly inhibit post-harvest moisture evaporation of fragrant pears, delay fruit softening, reduce spoilage and browning, and has excellent preservation performance.

[0068] In summary, this invention constructs a composite coating preservation liquid with a multi-network structure through the electrostatic self-assembly of carboxymethylated nanocellulose and chitosan quaternary ammonium salt, post-crosslinking of sodium alginate and calcium ions, and the combination of the antioxidant and antibacterial activities of Platycladus orientalis leaf extract. This coating preservation liquid can effectively inhibit postharvest water loss in fragrant pears, delay fruit softening, reduce browning and decay, significantly extend the refrigerated shelf life, and utilizes green and safe raw materials and a mild preparation process, demonstrating excellent preservation performance and promising application prospects.

[0069] Finally, it should be noted that the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A composite coating preservative liquid, characterized in that, The raw materials include the following parts by weight: Carboxymethylated nanocellulose 0.1-0.5 parts, chitosan quaternary ammonium salt 0.5-2.0 parts, sodium alginate 0.3-1.0 parts, arborvitae leaf extract obtained by eutectic solvent extraction 0.05-0.3 parts, plasticizer 0.1-0.5 parts, acidity regulator 0.1-0.3 parts, water 80-120 parts.

2. The composite coating preservative liquid according to claim 1, characterized in that, The method for preparing the carboxymethylated cellulose nanoparticles includes: (1) Mix cellulose nanofibers, a first solvent and an alkaline solution, and alkalize to obtain an intermediate product; (2) The intermediate product, etherifying agent and second solvent are mixed and reacted to obtain carboxymethylated cellulose nanoparticles.

3. The composite coating preservative liquid according to claim 2, characterized in that, In step (1), the first solvent is selected from isopropanol; the alkaline solution is selected from sodium hydroxide solution, and the mass fraction of the sodium hydroxide solution is 15-25%; the alkalization temperature is 20-30℃ and the time is 20-40min.

4. The composite coating preservative liquid according to claim 2, characterized in that, In step (2), the etherifying agent is selected from chloroacetic acid; the second solvent is selected from isopropanol; the reaction temperature is 50-70℃ and the time is 2-4h.

5. The composite coating preservative liquid according to claim 1, characterized in that, The chitosan quaternary ammonium salt is selected from hydroxypropyltrimethylammonium chloride chitosan.

6. The composite coating preservative liquid according to claim 1, characterized in that, The eutectic solvent includes choline chloride and lactic acid; the molar ratio of choline chloride to lactic acid is 1:(1-2).

7. The composite coating preservative liquid according to claim 1, characterized in that, The plasticizer is selected from at least one of glycerol and sorbitol; the acidity regulator is selected from at least one of acetic acid and lactic acid.

8. The method for preparing the composite coating preservative liquid according to any one of claims 1-7, characterized in that, Includes the following steps: The raw materials are mixed to obtain the composite coating preservation liquid.

9. The application of the composite coating preservative liquid according to any one of claims 1-7 in fruit preservation.

10. A method for preserving fruit using the composite coating preservative liquid according to any one of claims 1-7, characterized in that, Includes the following steps: The fruit is immersed in the composite coating preservation liquid, then removed and air-dried, and then immersed in a solution containing calcium ions, and removed and air-dried again.

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