A fresh-keeping method combining heat preservation packaging with natural antibacterial in cold-chain terminal distribution

Abstract

This invention discloses a method for preserving fresh produce in cold chain last-mile delivery by combining insulated packaging with natural antibacterial agents, belonging to the field of fresh produce preservation technology. The method first prepares a natural composite antibacterial preservative containing modified sunflower leaf extract, horsetail leaf extract, and nano-zinc oxide, as well as a water-resistant, antibacterial polyvinyl alcohol insulated liner containing aerogel and an antibacterial phase change cold storage bag. Then, the fresh produce undergoes a surface antibacterial film treatment with the preservative, followed by assembly into layered insulated and antibacterial packaging. Finally, intelligent temperature monitoring is implemented during delivery. This invention organically combines insulated packaging with natural antibacterial technology, solving the problems of large temperature fluctuations, easy spoilage of fresh produce, and chemical residues in existing cold chain last-mile delivery. It can extend the shelf life of fresh produce by 3-5 days under ambient temperature delivery conditions, is safe and environmentally friendly, and suitable for large-scale industrial application.

Description

Technical Field

[0001] This invention relates to the field of powder coating technology, specifically to a method for preserving fresh food using insulated packaging combined with natural antibacterial properties in the last mile of cold chain distribution. Background Technology

[0002] With the rapid development of the fresh food e-commerce industry, cold chain logistics has become a crucial link in ensuring the quality of fresh products. However, last-mile delivery in the cold chain, as the "last mile" of the entire cold chain system, faces numerous technical challenges. The main reasons for this are large temperature fluctuations, short insulation times, severe microbial contamination, and a disconnect between preservation and packaging technologies in the last-mile delivery stage. Current last-mile cold chain delivery primarily uses foam boxes with ice packs for insulation. While this method is low-cost, its insulation effect is limited, typically maintaining a low-temperature environment for only 4-6 hours, and the large temperature fluctuations easily lead to a decline in the quality of fresh products. Regarding preservation technology, chemical preservatives such as sulfur dioxide and sodium benzoate are currently mainly used. Although these chemical preservatives have good preservation effects, they pose food safety risks, and long-term consumption can harm human health. In recent years, natural preservatives have received widespread attention due to their safety and environmental friendliness. However, existing natural preservatives generally suffer from problems such as single-component composition, narrow antibacterial spectrum, and short shelf life. For example, single tea polyphenol or chitosan preservatives can only inhibit the growth of some bacteria, with poor inhibitory effects on molds and yeasts, and cannot delay the ripening and aging process of fresh products. Meanwhile, most existing natural preservatives are applied by spraying or soaking the surface of fresh produce, which can easily peel off during delivery due to friction or moisture erosion, leading to a decrease in preservation effectiveness. Furthermore, existing insulated packaging and antibacterial preservation technologies are often independent. Insulated packaging focuses only on temperature control and lacks antibacterial function; while antibacterial preservation only focuses on surface treatment of fresh produce, without considering the impact of temperature fluctuations on microbial growth. This disconnect means that in the end-of-life delivery environment with large temperature fluctuations, even with the use of antibacterial preservatives, microorganisms can still multiply rapidly when temperatures rise, leading to spoilage and deterioration of fresh products.

[0003] In summary, the core problem with existing technologies is that insulated packaging and antibacterial preservation technologies are disconnected in the end-of-line cold chain distribution. They cannot achieve long-term insulation and safe antibacterial protection simultaneously in complex environments with fluctuating temperatures, resulting in fresh products being prone to spoilage, having short shelf lives, and posing food safety risks. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preserving fresh food by combining insulated packaging with natural antibacterial properties in the last-mile delivery of cold chain products, so as to solve the problems mentioned in the background art.

[0005] To achieve the above objectives, the present invention provides the following technical solution: A method for preserving fresh produce using insulated packaging combined with natural antibacterial properties in last-mile cold chain delivery, the method being as follows: Step 1: Preparation of natural compound antibacterial preservative Sunflower leaves are washed, dried, pulverized, and sieved to obtain sunflower leaf powder. The sunflower leaf powder is placed in an ethanol solution and refluxed for 2-3 hours. The extract is cooled and filtered, and the residue is washed 2-3 times with pure water. The filtrates are combined to obtain sunflower leaf extract. The sunflower leaf extract is dissolved in a solvent, stirred, and slowly heated. A reducing agent is added in batches, and the temperature is raised to 70-90℃ and maintained constant for 3-5 hours. After cooling to room temperature, water and dichloromethane are added for phase separation to obtain mixture A. Mixture A is placed in a reaction vessel, stirred, and heated to 80-95℃. A vacuum is applied for 10-20 minutes, and nitrogen is introduced until the internal pressure of the vessel is 0.02-0.06 MPa. Liquid ethylene oxide is divided into two equal portions, and one portion is slowly added to the vessel. The reaction is carried out for 2-4 hours, followed by another 10-20 minutes of vacuuming. For 1 minute, nitrogen gas is introduced to maintain the internal pressure of the container at 0.01-0.03 MPa. Sodium hydroxide is added, stirred, and heated to 100-110℃ and kept constant. The remaining portion of liquid ethylene oxide is added, and the reaction is carried out for 4-6 hours to obtain mixture B. Horsetail leaves are taken, washed, dried, pulverized, and sieved to obtain horsetail leaf powder. The horsetail leaf powder is placed in a beaker, dissolved in distilled water, and kept in a constant temperature water bath at 40-50℃ for 10-15 minutes. Then the temperature is raised to 70-85℃, and ultrasonic extraction is carried out for 30-40 minutes with an ultrasonic power of 100-110W. The extract is cooled and filtered, and the filter residue is washed with pure water 2-3 times. The filtrates are combined to obtain horsetail leaf extract. The horsetail leaf extract is dissolved in ethanol, mixed with mixture B, heated to 60-65℃, potassium iodide is added, and the reaction is carried out for 2-3 hours. Then the temperature is raised to 70-75℃, and the reaction is carried out for 8-12 hours. The modified composite plant antibacterial liquid was prepared by [method name missing] hours. Zinc chloride solution was poured into a stainless steel pressure vessel lined with polytetrafluoroethylene and subjected to a hydrothermal reaction at 120-220℃ for 1-2 hours. After the reaction, the pressure vessel was cooled to room temperature, and the reaction liquid was poured out. After the reaction liquid settled for 18-24 hours, the supernatant was discarded and filtered under reduced pressure. The resulting precipitate was washed with a mixture of acetic acid and ammonium acetate and deionized water until no chloride ions were found, and then washed twice with 95% ethanol. After drying at 80-100℃, it was ground to obtain nano-zinc oxide with a particle size of 30-100 nm. The modified composite plant antibacterial liquid, nano-zinc oxide, 1-methylcyclopropene, salicylic acid, glycerol, and guar gum were mixed in a mass ratio of 35-50:0.2-0.5:0.001-0.01:0.01-0.03:1-2:0.1-0.2, and distilled water was added to a total mass of 100 [units missing]. Mix the ingredients thoroughly to obtain a natural compound antibacterial preservative. Step 2: Preparation of water-resistant and antibacterial polyvinyl alcohol thermal insulation lining Add 20-30 parts of natural wax to a reaction vessel, heat to 95℃ to melt, start stirring, add 2-4 parts of surfactant and 2-6 parts of monohydric ester compatibilizer; disperse at 500 rpm for 30 minutes, then increase the stirring speed to 1000 rpm, add 59-75.5 parts of distilled water to the reaction vessel, stir at high speed for 2 hours, cool to 40℃, add 0.5-1.0 parts of antibacterial agent precursor and 5-10 parts of aerogel particles, continue stirring for 30 minutes to obtain a water-resistant antibacterial coating emulsion; using a dry lamination device, coat an 8-15 micrometer thick coating emulsion onto a 40-70 micrometer thick polyvinyl alcohol substrate, dry in a 50-60℃ oven for 1-2 minutes to obtain a water-resistant antibacterial polyvinyl alcohol film coated with a functional coating; cut the water-resistant antibacterial polyvinyl alcohol film to a preset size, and heat-seal it into a bag-like structure with an open top to obtain a water-resistant antibacterial polyvinyl alcohol thermal insulation liner; Step 3: Preparation of antibacterial phase change cold storage bags Select a paraffin-based phase change material with a phase change temperature of 2-6℃ and heat it until it is completely melted. Inject the molten phase change material into a bag made of the water-resistant and antibacterial polyvinyl alcohol film prepared in step 2, with the injection volume being 80%-90% of the bag volume. Seal the bag opening using a heat sealing machine at a heat sealing temperature of 120-150℃ for 2-3 seconds to obtain an antibacterial phase change cold storage bag. Place the antibacterial phase change cold storage bag in a -18℃ environment and freeze it for 12-24 hours until the phase change material is completely solidified. Step 4: Antibacterial film formation treatment on fresh food surfaces Dilute the natural compound antibacterial preservative prepared in step 1 10 times with distilled water to obtain a preservative dilution; sort and arrange the fresh products to be delivered, removing surface impurities and rotten parts; use a spraying device to evenly spray the preservative dilution onto all surfaces of the fresh products, with a spraying amount of 50-100ml per kilogram of fresh products; place the sprayed fresh products in a well-ventilated environment to air dry for 10-15 minutes, allowing the preservative to form a uniform and transparent antibacterial protective film on the surface of the fresh products; Step 5: Layered Insulated and Antibacterial Packaging Assembly Select a corrugated cardboard box of suitable size as the outer packaging, and attach a 2-3cm thick polystyrene foam insulation layer to the inner wall of the corrugated cardboard box; place the water-resistant and antibacterial polyvinyl alcohol insulation liner prepared in step 2 inside the foam insulation layer; place the fresh products treated in step 4 in layers inside the insulation liner, and separate each layer of fresh products with a breathable mesh; evenly place the antibacterial phase change cold storage bags prepared in step 3 on the top and around the fresh products, with the mass ratio of the cold storage bags to the fresh products being 1:2-1:4; fold and seal the top opening of the insulation liner, and then seal the lid of the corrugated cardboard box with tape; Step 6: Intelligent temperature monitoring during delivery A temperature indicator label is affixed to the top of the inside of the insulated packaging, with the color-changing temperature set at 8°C. A clear "Fresh Food Cold Chain" label and temperature warning label are affixed to the outer surface of the corrugated cardboard box. During delivery, the delivery person regularly observes the color change of the temperature indicator label. When the label turns red, the packaging is promptly moved to a low-temperature environment or the delivery speed is expedited. After delivery, consumers are reminded to remove the fresh products and refrigerate them promptly.

[0006] Preferred: The natural wax in step 2 is a mixture of palm wax and beeswax in a mass ratio of 3:1; the surfactant is sodium dodecyl sulfate; the monoanhydride ester is glyceryl stearate; the antibacterial agent precursor is sodium hypochlorite; the particle size of the aerogel particles is 10-50 nm; and the dry film thickness of the functional coating is 2-4 μm.

[0007] Preferred: The phase change temperature of the paraffin-based phase change material in step 3 is 4℃, the latent heat of phase change is 180-200J / g, the size of the antibacterial phase change cold storage bag is 10cm×15cm, the thickness is 1-2cm, and the heat seal width is 5-8mm.

[0008] Preferred: The spray pressure of the spraying equipment in step 4 is 0.2-0.3MPa, the nozzle diameter is 0.5-1.0mm, the spraying distance is 20-30cm, the temperature of the air-drying environment is 20-25℃, and the relative humidity is 40%-60%.

[0009] Preferred: The density of the polystyrene foam insulation layer in step 5 is 25-30 kg / m³, the breathable mesh is made of food-grade polypropylene with a mesh diameter of 2-3 mm, and the tape is waterproof pressure-sensitive tape. Tape must be applied to all seams of the lid during sealing.

[0010] Preferred: The temperature indicator label mentioned in step 6 is an irreversible temperature indicator label. When the ambient temperature exceeds 8°C and lasts for more than 30 minutes, the label changes from blue to red. The label size is 5cm×5cm, and the affixing position is 1-2cm away from the inner surface of the box lid.

[0011] Compared with the prior art, the beneficial effects of the present invention are: The method for preserving fresh food by combining insulated packaging with natural antibacterial properties in cold chain last-mile delivery provided by this invention has the following outstanding technical effects: First, the insulation performance is significantly improved. This invention uses an aerogel-reinforced, water-resistant, antibacterial polyvinyl alcohol insulation lining combined with phase change cold storage technology, which can maintain a low-temperature environment of 2-8℃ for 12-24 hours at room temperature, extending the insulation time by 6-12 hours compared to the traditional foam box and ice pack method. Simultaneously, the synergistic effect of the aerogel insulation layer and the phase change cold storage material effectively reduces temperature fluctuations during delivery, controlling the temperature fluctuation range within ±2℃, providing a stable low-temperature environment for fresh produce.

[0012] Secondly, it exhibits excellent antibacterial and preservation effects. This invention employs a dual antibacterial system of "surface antibacterial film formation + packaging material antibacterial." The natural composite antibacterial preservative combines the organic antibacterial effect of modified plant antibacterial liquid with the inorganic antibacterial effect of nano-zinc oxide, providing a broad antibacterial spectrum that can inhibit over 90% of common pathogenic bacteria such as Escherichia coli, Staphylococcus aureus, molds, and yeasts. Simultaneously, the water-resistant antibacterial polyvinyl alcohol packaging material and the antibacterial phase change cold storage bag continuously release antibacterial components, preventing the growth of microorganisms inside the packaging. Experiments show that perishable fruits such as strawberries and lychees treated using this method can have their shelf life extended by 3-5 days under normal temperature distribution conditions, with water loss reduced to below 1% and rot rate controlled within 2%.

[0013] Third, it is safe, environmentally friendly, and residue-free. The antibacterial ingredients used in this invention are all derived from natural plants and inorganic minerals, containing no chemical preservatives and posing no food safety risks. The modified sunflower leaf extract and horsetail leaf extract are both natural plant extracts, nano-zinc oxide is a safe food additive, and 1-methylcyclopropene is an internationally recognized safe ethylene inhibitor. Furthermore, the packaging materials, such as polyvinyl alcohol and natural waxes, have good biodegradability and will not pollute the environment.

[0014] Fourth, it exhibits strong water resistance and practicality. This invention coats a polyvinyl alcohol (PVA) substrate with a water-resistant and antibacterial coating, solving the problem of swelling and dissolving in traditional PVA films when exposed to water. After 100 days of testing at 95% relative humidity, the coated PVA film remained intact, showing no swelling or dissolving. Furthermore, the operation process of this invention is simple, requiring no complex equipment, making it suitable for large-scale industrial applications and widely applicable to the last-mile delivery of various fresh products such as fruits, vegetables, meat, and seafood. Attached Figure Description

[0015] Figure 1 This is a flowchart of the method described in this invention. Detailed Implementation

[0016] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0017] Example (Basic Example) I. Raw Materials and Reagents Sunflower leaves (collected from Dezhou City, Shandong Province, variety G101), horsetail leaves (collected from Changbai Mountain area, Jilin Province), anhydrous ethanol (analytical grade, Sinopharm Group), liquid ethylene oxide (industrial grade, purity ≥99.5%), zinc chloride (analytical grade, Sinopharm Group), 1 - Methylcyclopropene (food grade, purity ≥99%), salicylic acid (food grade), glycerin (food grade), guar gum (food grade), palm wax (food grade), beeswax (food grade), sodium lauryl sulfate (analytical grade), glyceryl monostearate (food grade), sodium hypochlorite (analytical grade, available chlorine content ≥10%), aerogel particles (particle size 10-50nm, specific surface area 600m² / g), polyvinyl alcohol film (thickness 55μm, degree of alcoholysis 88%), paraffin-based phase change materials (phase change temperature 4℃, latent heat of phase change 190J / g), polystyrene foam board (density 28kg / m³, thickness 2.5cm), food-grade polypropylene breathable mesh (mesh diameter 2.5mm), waterproof pressure-sensitive tape, irreversible temperature indicator label (color change temperature 8℃).

[0018] II. Preparation Method 1. Preparation of natural compound antibacterial preservative Innovative Principle: Ethylene oxide modification of sunflower leaf extract increases the number of hydroxyl and ether bonds on its molecular chain, improving its compatibility and antibacterial activity with horsetail leaf extract; nano-zinc oxide, prepared via hydrothermal method, possesses high specific surface area and quantum size effect, forming a synergistic antibacterial effect with the plant antibacterial liquid; 1-methylcyclopropene inhibits ethylene synthesis, salicylic acid delays cell aging, and glycerol and guar gum form a protective film to prevent the loss of antibacterial components.

[0019] Sunflower leaf pretreatment: Wash fresh sunflower leaves with deionized water, dry them in a 60℃ forced-air drying oven until constant weight, pulverize them and pass them through an 80-mesh sieve to obtain sunflower leaf powder.

[0020] Sunflower leaf extraction: Take 100g of dried sunflower leaf powder, add 1000mL of 70% ethanol solution, heat to 80℃ and reflux for 2.5 hours. After cooling, filter with a Buchner funnel. Wash the residue three times with 200mL of pure water. Combine the filtrates and concentrate under reduced pressure to 200mL to obtain sunflower leaf extract.

[0021] Modification of sunflower leaf extract: Dissolve the above sunflower leaf extract in 50 mL of anhydrous ethanol, stir at 300 rpm, slowly heat to 80°C, add 5 g of sodium borohydride reducing agent in batches, and react at a constant temperature for 4 hours. After cooling to room temperature, add 200 mL of water and 150 mL of dichloromethane, shake, and allow to stand for separation. Collect the organic phase to obtain mixture A.

[0022] Mixture A was added to a high-pressure reactor. The stirrer was turned on and stirred at 400 rpm. The temperature was raised to 90°C, and a vacuum was applied for 15 minutes. Nitrogen gas was then introduced until the pressure inside the reactor reached 0.04 MPa. 20 g of liquid ethylene oxide was divided into two equal portions. The first portion was slowly added to the reactor, and the reaction was allowed to proceed for 3 hours. The vacuum was applied again for 15 minutes, and nitrogen gas was introduced to maintain the pressure inside the reactor at 0.02 MPa. 2 g of sodium hydroxide was added, and the mixture was stirred and heated to 105°C. The second portion of liquid ethylene oxide was added, and the reaction was allowed to proceed for 5 hours. The mixture was then cooled to room temperature to obtain mixture B.

[0023] Horsetail Leaf Extraction: Fresh horsetail leaves were washed, dried at 60℃ to constant weight, and pulverized through an 80-mesh sieve to obtain horsetail leaf powder. 50g of the powder was added to 500mL of distilled water, and the mixture was heated in a 45℃ water bath for 12 minutes, then heated to 80℃ and extracted using ultrasound at 105W for 35 minutes. The mixture was cooled and filtered. The residue was washed three times with 100mL of pure water. The filtrates were combined and concentrated under reduced pressure to 100mL to obtain the horsetail leaf extract.

[0024] Preparation of modified composite plant antibacterial solution: Take the above-mentioned horsetail leaf extract, add 50 mL of anhydrous ethanol to dissolve it, add mixture B, heat to 62℃, add 0.5 g of potassium iodide, react for 2.5 hours, then heat to 72℃ and react for 10 hours to obtain the modified composite plant antibacterial solution.

[0025] Preparation of nano-zinc oxide: A 0.5 mol / L zinc chloride solution was poured into a polytetrafluoroethylene-lined stainless steel pressure vessel and hydrothermally reacted at 180℃ for 1.5 hours. After cooling to room temperature, the reaction solution was allowed to settle for 20 hours, and the supernatant was discarded and filtered under reduced pressure. The precipitate was washed with an acetic acid-ammonium acetate mixture (pH=5) and deionized water until no chloride ions were detected (tested with silver nitrate solution), then washed twice with 95% ethanol, dried at 90℃, and ground to obtain nano-zinc oxide with a particle size of 50 nm.

[0026] Preservative compound preparation: Take 42 parts of modified compound plant antibacterial liquid, 0.35 parts of nano zinc oxide, 0.005 parts of 1-methylcyclopropene, 0.02 parts of salicylic acid, 1.5 parts of glycerin, and 0.15 parts of guar gum by mass ratio, add distilled water to a total mass of 100 parts, stir evenly to obtain a natural compound antibacterial preservative.

[0027] 2. Preparation of water-resistant and antibacterial polyvinyl alcohol thermal insulation lining Innovative principle: Natural wax forms a hydrophobic barrier, solving the problem of polyvinyl alcohol swelling when exposed to water; aerogel particles have extremely low thermal conductivity, significantly improving heat insulation performance; sodium hypochlorite, as an antibacterial agent precursor, slowly releases hypochlorous acid in the coating, achieving long-lasting antibacterial effect.

[0028] Preparation of water-resistant and antibacterial coating emulsion: 25 parts of natural wax (palm wax: beeswax = 3:1) were added to a reaction vessel and heated to 95°C to melt. Stirring was started, and 3 parts of sodium dodecyl sulfate and 4 parts of glyceryl monostearate were added. After dispersing at 500 rpm for 30 minutes, the stirring speed was increased to 1000 rpm, and 65.5 parts of distilled water were added. After high-speed stirring for 2 hours, the mixture was cooled to 40°C, and 0.75 parts of sodium hypochlorite and 7.5 parts of aerogel particles were added. Stirring was continued for 30 minutes to obtain the water-resistant and antibacterial coating emulsion.

[0029] Film coating and molding: Using a dry lamination device, a 12μm thick coating emulsion is coated on a 55μm thick polyvinyl alcohol substrate, and then dried in a 55℃ drying tunnel for 1.5 minutes to obtain a water-resistant and antibacterial polyvinyl alcohol film with a dry film thickness of 3μm.

[0030] Insulation lining fabrication: The film is cut into 40cm×30cm sizes and heat-sealed into a bag-like structure with an open top. The heat-sealing temperature is 130℃, the heat-sealing time is 2 seconds, and the heat-sealing width is 6mm, resulting in a water-resistant and antibacterial polyvinyl alcohol insulation lining.

[0031] 3. Preparation of antibacterial phase change cold storage bags Innovative principle: The bag body is made of water-resistant and antibacterial polyvinyl alcohol film, which has good sealing properties and can continuously release antibacterial components to prevent the growth of microorganisms on the surface of the cold storage bag; the 4℃ phase change temperature is consistent with the optimal storage temperature for fresh food, with high latent heat of phase change and long cold storage time.

[0032] Take a paraffin-based phase change material with a phase change temperature of 4℃ and a latent heat of phase change of 190J / g, and heat it to 60℃ until it is completely melted.

[0033] The molten phase change material was injected into a 10cm×15cm bag made of the above-mentioned water-resistant and antibacterial polyvinyl alcohol film, and the injection volume was 85% of the bag volume.

[0034] The bag opening was sealed with a heat sealer at a temperature of 135℃, a sealing time of 2.5 seconds, and a sealing width of 6mm to obtain an antibacterial phase change cold storage bag.

[0035] Place the cold storage bag in a -18°C freezer for 18 hours until the phase change material is completely solidified.

[0036] 4. Antibacterial film formation treatment on fresh food surfaces Innovative principle: A uniform and transparent antibacterial protective film is formed on the surface of fresh produce through spraying, which can inhibit the growth of surface microorganisms and reduce moisture evaporation; the protective film has good air permeability and will not affect the respiration of fresh produce.

[0037] The natural compound antibacterial preservative was diluted 10 times with distilled water to obtain a diluted preservative solution.

[0038] Select fresh strawberries (Hongyan variety, 80% ripe), remove surface impurities and rotten parts, and sort them.

[0039] Using a spraying device, under the conditions of a spraying pressure of 0.25 MPa, a nozzle diameter of 0.75 mm, and a spraying distance of 25 cm, the preservative diluted solution was evenly sprayed onto all surfaces of the strawberries, with a spraying amount of 75 mL per kilogram of strawberries.

[0040] After spraying, place the strawberries in a well-ventilated environment at 22℃ and 50% relative humidity for 12 minutes to allow the preservative to form a uniform and transparent antibacterial protective film on the surface of the strawberries.

[0041] 5. Layered insulated and antibacterial packaging assembly Innovative principle: It adopts a three-layer insulation structure of "corrugated cardboard box + polystyrene foam + water-resistant and antibacterial polyvinyl alcohol lining", combined with phase change cold storage bag to achieve a long-term stable low temperature environment; the layered placement and breathable mesh design prevent fresh food from being squeezed together and ensure air circulation.

[0042] Select a 45cm×35cm×25cm corrugated cardboard box as the outer packaging, and attach a 2.5cm thick polystyrene foam insulation layer to the inner wall of the box.

[0043] Water-resistant and antibacterial polyvinyl alcohol insulation lining is placed inside the foam insulation layer.

[0044] The processed strawberries were placed in layers inside an insulated liner, with each layer separated by a food-grade polypropylene breathable mesh. A total of 3 layers were placed, with a total weight of 3 kg.

[0045] Antibacterial phase change cold storage bags were evenly placed on top of and around the strawberries, with a total weight of 1 kg (the ratio of cold storage bag weight to fresh strawberry weight was 1:3).

[0046] Fold and seal the top opening of the insulation liner, and seal all seams of the corrugated cardboard box with waterproof pressure-sensitive tape.

[0047] 6. Intelligent temperature monitoring during delivery. Innovative principle: The use of irreversible temperature indicator tags can accurately record instances of temperature exceeding limits, providing a basis for the traceability of fresh produce quality; clear markings and warnings remind delivery personnel and consumers to pay attention to temperature control.

[0048] Affix a 5cm x 5cm irreversible temperature indicator label to the top of the inside of the insulated packaging, 1.5cm from the inner surface of the lid. The label's color-changing temperature is set to 8℃.

[0049] Affix "Fresh Food Cold Chain" labels and temperature warning labels to the outer surface of corrugated cardboard boxes.

[0050] Simulate a normal temperature delivery environment (temperature 25℃, relative humidity 60%), and observe the color change of the temperature indicator label every 2 hours.

[0051] III. Product Characterization and Performance Testing Performance of natural compound antibacterial preservative: The antibacterial performance was tested using the plate count method. The inhibition rate against Escherichia coli was 96.2%, against Staphylococcus aureus was 95.8%, against mold was 93.5%, and against yeast was 92.7%.

[0052] Water-resistant and antibacterial polyvinyl alcohol film properties: thermal conductivity is 0.028 W / (m・K), and the film remains intact without swelling or dissolution after being placed in an environment with 95% relative humidity for 100 days; the antibacterial rate against Escherichia coli is 91.3%.

[0053] Performance of antibacterial phase change cold storage bag: phase change temperature is 4.1℃, latent heat of phase change is 187J / g, cold storage time is 22 hours; the surface has an inhibition rate of 90.5% against Escherichia coli.

[0054] Overall preservation effect: Under normal temperature conditions of 25℃, the internal temperature of the packaging is maintained at 2-8℃ for 20 hours, with a temperature fluctuation range of ±1.5℃; the shelf life of strawberries is 7 days, the water loss rate is 0.8%, the rot rate is 1.5%, and the sensory score (color, aroma, texture) is 9.2 points (out of 10).

[0055] Example 1 (Verification of the necessity of modified plant antibacterial solution) I. Raw Materials and Reagents The basic embodiment is essentially the same, except that the modified sunflower leaf extract is replaced with unmodified ordinary sunflower leaf extract.

[0056] II. Preparation Method Except for the sunflower leaf extract not being modified with ethylene oxide, the other steps are completely consistent with the basic embodiment. Specifically, the sunflower leaf extract is directly mixed with the horsetail leaf extract, heated to 62°C, 0.5g of potassium iodide is added, the reaction is carried out for 2.5 hours, and then the temperature is raised to 72°C and the reaction is carried out for 10 hours to obtain the unmodified composite plant antibacterial liquid; the subsequent steps of preservative compounding, heat-insulating lining preparation, cold storage bag preparation, fresh food treatment, packaging assembly, and temperature monitoring are all the same as in the basic embodiment.

[0057] III. Product Characterization and Performance Testing Performance of natural compound antibacterial preservative: The inhibition rate against Escherichia coli is 72.5%, against Staphylococcus aureus is 70.8%, against mold is 65.3%, and against yeast is 63.7%.

[0058] Overall preservation effect: Under normal temperature conditions of 25℃, the internal temperature of the packaging was maintained at 2-8℃ for 20 hours, with a temperature fluctuation range of ±1.5℃; the shelf life of strawberries was 4 days, the water loss rate was 1.2%, the rot rate was 8.5%, and the sensory score was 6.8.

[0059] Comparative Example 1 (Feature Missing Comparative Example) I. Raw Materials and Reagents Completely identical to Example 1.

[0060] II. Preparation Method The process is exactly the same as in Example 1, that is, the compound plant antibacterial liquid is prepared using unmodified ordinary sunflower leaf extract, and the remaining steps are the same as in the basic example.

[0061] III. Product Characterization and Performance Testing Same as the test results in Example 1: the natural compound antibacterial preservative showed an inhibition rate of 72.5% against Escherichia coli, 70.8% against Staphylococcus aureus, 65.3% against mold, and 63.7% against yeast; the strawberry shelf life was 4 days, the water loss rate was 1.2%, the rot rate was 8.5%, and the sensory score was 6.8.

[0062] Comparative Analysis and Conclusions Example 1 differs from the basic example only in that it lacks the ethylene oxide modification step of the sunflower leaf extract; all other parameters and processes remain identical. Test results show that the unmodified composite plant antibacterial solution exhibits significantly reduced antibacterial performance, with an average decrease of approximately 23 percentage points in the inhibition rate against various pathogenic bacteria. The shelf life of strawberries is shortened from 7 days to 4 days, the rot rate increases from 1.5% to 8.5%, and the sensory score significantly decreases. This demonstrates that ethylene oxide modification of the sunflower leaf extract is a necessary technical feature for achieving excellent antibacterial and preservation effects in this invention. The modification treatment increases the active groups in the plant extract molecules, enhancing their binding ability to microbial cell membranes, and simultaneously strengthening the synergistic antibacterial effect with the horsetail leaf extract and nano-zinc oxide.

[0063] Example 2 (Verification of nano zinc oxide particle size parameters) I. Raw Materials and Reagents It is essentially the same as the basic embodiment, except that the nano zinc oxide particle size is 30 nm (within the range of 30-100 nm as defined in the claims).

[0064] II. Preparation Method Except for adjusting the hydrothermal reaction temperature to 150℃ and the reaction time to 1 hour during the preparation of nano zinc oxide to obtain nano zinc oxide with a particle size of 30nm, all other steps are completely consistent with the basic embodiment.

[0065] III. Product Characterization and Performance Testing Performance of natural compound antibacterial preservative: 97.8% inhibition rate against Escherichia coli, 97.2% inhibition rate against Staphylococcus aureus, 95.1% inhibition rate against mold, and 94.3% inhibition rate against yeast.

[0066] Overall preservation effect: Under normal temperature conditions of 25℃, the internal temperature of the packaging was maintained at 2-8℃ for 20 hours, with a temperature fluctuation range of ±1.5℃; the shelf life of strawberries was 7.5 days, the water loss rate was 0.7%, the rot rate was 1.2%, and the sensory score was 9.4 points.

[0067] Comparative Example 2 (Parameter Out-of-Range Comparative Example) I. Raw Materials and Reagents It is basically the same as Example 2, except that the nano zinc oxide particle size is 200nm (exceeding the 30-100nm range defined in the claims).

[0068] II. Preparation Method Except for adjusting the hydrothermal reaction temperature to 250℃ and the reaction time to 3 hours during the preparation of nano zinc oxide, resulting in nano zinc oxide with a particle size of 200nm, all other steps are completely consistent with Example 2.

[0069] III. Product Characterization and Performance Testing Performance of natural compound antibacterial preservative: The inhibition rate against Escherichia coli is 81.2%, against Staphylococcus aureus is 79.6%, against mold is 75.8%, and against yeast is 74.2%.

[0070] Overall preservation effect: Under normal temperature conditions of 25℃, the internal temperature of the packaging was maintained at 2-8℃ for 20 hours, with a temperature fluctuation range of ±1.5℃; the shelf life of strawberries was 5 days, the water loss rate was 1.0%, the rot rate was 5.3%, and the sensory score was 7.5.

[0071] Comparative Analysis and Conclusions Example 2 differs from Comparative Example 2 only in the particle size of the zinc oxide nanoparticles. Example 2 uses 30 nm particles (within the scope of the claims), while Comparative Example 2 uses 200 nm particles (outside the scope of the claims). All other parameters and processes are identical. Test results show that when the particle size of the zinc oxide nanoparticles exceeds the range defined in this invention, the antibacterial performance decreases significantly, with the inhibition rate against various pathogenic bacteria decreasing by an average of approximately 16 percentage points; the shelf life of strawberries is shortened from 7.5 days to 5 days, and the rot rate increases from 1.2% to 5.3%. This demonstrates that the 30-100 nm particle size range of zinc oxide nanoparticles defined in this invention is a key parameter for achieving excellent antibacterial effects. The antibacterial activity of zinc oxide nanoparticles is closely related to its particle size. The smaller the particle size, the larger the specific surface area, the more zinc ions are released, and the larger the contact area with microorganisms, resulting in better antibacterial effects. When the particle size exceeds 100 nm, the quantum size effect disappears, and the antibacterial activity decreases significantly.

[0072] Example 3 (Verification of Overall Technical Solution) I. Raw Materials and Reagents It is exactly the same as the basic implementation.

[0073] II. Preparation Method Completely consistent with the basic embodiment, that is, adopting the complete technical solution of the present invention: "natural composite antibacterial preservative + water-resistant antibacterial polyvinyl alcohol heat-insulating lining + antibacterial phase change cold storage bag + surface antibacterial film + layered packaging + temperature monitoring".

[0074] III. Product Characterization and Performance Testing The test results were the same as those of the basic embodiment: at room temperature of 25℃, the internal temperature of the packaging was maintained at 2-8℃ for 20 hours, with a temperature fluctuation range of ±1.5℃; the shelf life of the strawberries was 7 days, the water loss rate was 0.8%, the rot rate was 1.5%, and the sensory score was 9.2 points.

[0075] Comparative Example 3 (Prior Technology Comparative Example) I. Raw Materials and Reagents Polystyrene foam box (2.5cm thick), ordinary polyethylene ice pack, sodium benzoate preservative (food grade), strawberries (same batch as in Example 3), and waterproof pressure-sensitive tape.

[0076] II. Preparation Method Adopt the closest existing technical solution: Fresh processing: Soak strawberries in a 0.1% sodium benzoate solution for 5 minutes, then remove and air dry.

[0077] Packaging and Assembly: Place the strawberries in a polystyrene foam box, separating each layer with ordinary plastic mesh, with a total weight of 3kg; place ordinary polyethylene ice packs on top of and around the strawberries, with a total weight of 1kg; seal the foam box with waterproof pressure-sensitive tape.

[0078] Temperature monitoring: Affix the same temperature indicator label to the top of the inside of the foam box to simulate the same room temperature delivery environment.

[0079] III. Product Characterization and Performance Testing Thermal insulation performance: Under normal temperature conditions of 25℃, the internal temperature of the packaging is maintained at 2-8℃ for 8 hours, with a temperature fluctuation range of ±5℃.

[0080] Preservation effect: The strawberry shelf life is 2 days, the water loss rate is 3.5%, the rot rate is 15.2%, and the sensory score is 4.5.

[0081] Safety: The residual amount of sodium benzoate on the surface of strawberries was 0.08 g / kg, which is close to the national limit (0.1 g / kg).

[0082] Comparative Analysis and Conclusions Example 3 employs the complete technical solution of this invention, while Comparative Example 3 uses the traditional "foam box + ice pack + chemical preservative" solution. Test results show that the insulation time of this invention is extended by 12 hours compared to the prior art, and the temperature fluctuation range is reduced from ±5℃ to ±1.5℃; the shelf life of strawberries is extended from 2 days to 7 days, the water loss rate is reduced from 3.5% to 0.8%, the rot rate is reduced from 15.2% to 1.5%, and the sensory score is significantly improved; simultaneously, this invention does not use chemical preservatives, posing no food safety risks. This demonstrates that this invention, by organically combining heat-insulating packaging with natural antibacterial technology, solves the problems of poor insulation, large temperature fluctuations, easy spoilage of fresh produce, and chemical residues in the prior art, achieving unexpected technical results.

[0083] In summary, the cold chain end-of-line delivery method for fresh food preservation that combines insulated packaging with natural antibacterial properties achieves a long-lasting and stable low-temperature environment and a broad-spectrum and efficient antibacterial preservation effect through a dual antibacterial system of "surface antibacterial film formation + packaging material antibacterial" and a composite insulation technology of "aerogel insulation + phase change cold storage".

[0084] From the perspective of technological innovation, this invention has the following core technical points: Modified compound plant antibacterial technology: By modifying sunflower leaf extract with ethylene oxide, its antibacterial activity and compatibility with other components are significantly improved. It forms a synergistic antibacterial effect with horsetail leaf extract and nano zinc oxide, with a broad antibacterial spectrum and good inhibitory effect on bacteria, molds and yeasts.

[0085] Water-resistant, antibacterial, and heat-insulating packaging technology: A functional coating containing natural wax, aerogel, and antibacterial agents is applied to a polyvinyl alcohol substrate, which solves the problem of swelling when the traditional polyvinyl alcohol film comes into contact with water, while giving it excellent heat-insulating properties and long-lasting antibacterial function.

[0086] Antibacterial phase change cold storage technology: Water-resistant and antibacterial polyvinyl alcohol film is used as the cold storage bag body, so that the cold storage bag itself has antibacterial function and prevents the growth of microorganisms inside the packaging; the 4℃ phase change temperature is consistent with the optimal storage temperature for fresh food, with long cold storage time and good temperature stability.

[0087] Layered heat-insulating and antibacterial packaging structure: The three-layer heat-insulating structure combined with layered placement and breathable mesh design not only ensures good heat preservation effect, but also avoids fresh food from being squeezed together and ensures air circulation.

[0088] Intelligent temperature monitoring technology: Using irreversible temperature indicator tags, it can accurately record temperature exceeding the standard, providing a basis for the traceability of fresh food quality.

[0089] From a technical perspective, this invention represents a significant advancement: Significantly improved thermal insulation performance: It can maintain a low temperature environment of 2-8℃ for 12-24 hours at room temperature, which is 6-12 hours longer than the traditional insulation method of foam box plus ice pack, and the temperature fluctuation range is controlled within ±2℃.

[0090] Excellent antibacterial and preservation effects: The natural compound antibacterial preservative has an inhibition rate of over 90% against common pathogens. Combined with the antibacterial effect of packaging materials, it can extend the shelf life of perishable fruits such as strawberries and lychees by 3-5 days under normal temperature delivery conditions, reduce water loss to below 1%, and control the rot rate to within 2%.

[0091] Safe, environmentally friendly, and residue-free: All antibacterial ingredients are derived from natural plants and inorganic minerals, contain no chemical preservatives, and pose no food safety risks; the packaging materials have good biodegradability and will not pollute the environment.

[0092] Highly practical: The operation process is simple, no complicated equipment is required, it is suitable for large-scale industrial applications, and can be widely used in the last-mile delivery of various fresh products such as fruits, vegetables, meat, and aquatic products.

[0093] Comparative analysis of the embodiments and comparative examples shows that the key technical features and parameter ranges of the present invention are necessary conditions for achieving the aforementioned superior technical effects. Omitting any key technical feature or exceeding the parameter range will lead to a significant decrease in the technical effect. Compared with the prior art, the present invention solves the core problem of the disconnect between heat preservation packaging and antibacterial preservation technology in cold chain end-point distribution, and has outstanding substantive features and significant progress, possessing the inventiveness and novelty of an invention patent.

[0094] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preservation of fresh produce using natural antimicrobial agent in combination with temperature maintaining packaging in cold chain end delivery, characterized in that: The method is as follows: Step 1: Preparation of natural composite antimicrobial preservative Sunflower leaves are washed, dried, pulverized, and sieved to obtain sunflower leaf powder. The sunflower leaf powder is placed in an ethanol solution and refluxed for 2-3 hours. The extract is cooled and filtered, and the residue is washed 2-3 times with pure water. The filtrates are combined to obtain sunflower leaf extract. The sunflower leaf extract is dissolved in a solvent, stirred, and slowly heated. A reducing agent is added in batches, and the temperature is raised to 70-90℃ and maintained constant for 3-5 hours. After cooling to room temperature, water and dichloromethane are added for phase separation to obtain mixture A. Mixture A is placed in a reaction vessel, stirred, and heated to 80-95℃. A vacuum is applied for 10-20 minutes, and nitrogen is introduced until the internal pressure of the vessel is 0.02-0.06 MPa. Liquid ethylene oxide is divided into two equal portions, and one portion is slowly added to the vessel. The reaction is carried out for 2-4 hours, followed by another 10-20 minutes of vacuuming. For 1 minute, nitrogen gas is introduced to maintain the internal pressure of the container at 0.01-0.03 MPa. Sodium hydroxide is added, stirred, and heated to 100-110℃ and kept constant. The remaining portion of liquid ethylene oxide is added, and the reaction is carried out for 4-6 hours to obtain mixture B. Horsetail leaves are taken, washed, dried, pulverized, and sieved to obtain horsetail leaf powder. The horsetail leaf powder is placed in a beaker, dissolved in distilled water, and kept in a constant temperature water bath at 40-50℃ for 10-15 minutes. Then the temperature is raised to 70-85℃, and ultrasonic extraction is carried out for 30-40 minutes with an ultrasonic power of 100-110W. The extract is cooled and filtered, and the filter residue is washed with pure water 2-3 times. The filtrates are combined to obtain horsetail leaf extract. The horsetail leaf extract is dissolved in ethanol, mixed with mixture B, heated to 60-65℃, potassium iodide is added, and the reaction is carried out for 2-3 hours. Then the temperature is raised to 70-75℃, and the reaction is carried out for 8-12 hours. The modified composite plant antibacterial liquid was prepared by [method name missing] hours. Zinc chloride solution was poured into a stainless steel pressure vessel lined with polytetrafluoroethylene and subjected to a hydrothermal reaction at 120-220℃ for 1-2 hours. After the reaction, the pressure vessel was cooled to room temperature, and the reaction liquid was poured out. After the reaction liquid settled for 18-24 hours, the supernatant was discarded and filtered under reduced pressure. The resulting precipitate was washed with a mixture of acetic acid and ammonium acetate and deionized water until no chloride ions were found, and then washed twice with 95% ethanol. After drying at 80-100℃, it was ground to obtain nano-zinc oxide with a particle size of 30-100 nm. The modified composite plant antibacterial liquid, nano-zinc oxide, 1-methylcyclopropene, salicylic acid, glycerol, and guar gum were mixed in a mass ratio of 35-50:0.2-0.5:0.001-0.01:0.01-0.03:1-2:0.1-0.2, and distilled water was added to a total mass of 100 [units missing]. Mix the ingredients thoroughly to obtain a natural compound antibacterial preservative. Step 2: Preparation of water-resistant and antibacterial polyvinyl alcohol thermal insulation lining Add 20-30 parts of natural wax to a reaction vessel, heat to 95℃ to melt, start stirring, add 2-4 parts of surfactant and 2-6 parts of monohydric ester compatibilizer; disperse at 500 rpm for 30 minutes, then increase the stirring speed to 1000 rpm, add 59-75.5 parts of distilled water to the reaction vessel, stir at high speed for 2 hours, cool to 40℃, add 0.5-1.0 parts of antibacterial agent precursor and 5-10 parts of aerogel particles, continue stirring for 30 minutes to obtain a water-resistant antibacterial coating emulsion; using a dry lamination device, coat an 8-15 micrometer thick coating emulsion onto a 40-70 micrometer thick polyvinyl alcohol substrate, dry in a 50-60℃ oven for 1-2 minutes to obtain a water-resistant antibacterial polyvinyl alcohol film coated with a functional coating; cut the water-resistant antibacterial polyvinyl alcohol film to a preset size, and heat-seal it into a bag-like structure with an open top to obtain a water-resistant antibacterial polyvinyl alcohol thermal insulation liner; Step 3: Preparation of antibacterial phase change cold storage bags Select a paraffin-based phase change material with a phase change temperature of 2-6℃ and heat it until it is completely melted. Inject the molten phase change material into a bag made of the water-resistant and antibacterial polyvinyl alcohol film prepared in step 2, with the injection volume being 80%-90% of the bag volume. Seal the bag opening using a heat sealing machine at a heat sealing temperature of 120-150℃ for 2-3 seconds to obtain an antibacterial phase change cold storage bag. Place the antibacterial phase change cold storage bag in a -18℃ environment and freeze it for 12-24 hours until the phase change material is completely solidified. Step 4: Antibacterial film formation treatment on fresh food surfaces Dilute the natural compound antibacterial preservative prepared in step 1 10 times with distilled water to obtain a preservative dilution; sort and arrange the fresh products to be delivered, removing surface impurities and rotten parts; use a spraying device to evenly spray the preservative dilution onto all surfaces of the fresh products, with a spraying amount of 50-100ml per kilogram of fresh products; place the sprayed fresh products in a well-ventilated environment to air dry for 10-15 minutes, allowing the preservative to form a uniform and transparent antibacterial protective film on the surface of the fresh products; Step 5: Layered Insulated and Antibacterial Packaging Assembly Select a corrugated cardboard box of suitable size as the outer packaging, and attach a 2-3cm thick polystyrene foam insulation layer to the inner wall of the corrugated cardboard box; place the water-resistant and antibacterial polyvinyl alcohol insulation liner prepared in step 2 inside the foam insulation layer; place the fresh products treated in step 4 in layers inside the insulation liner, and separate each layer of fresh products with a breathable mesh; evenly place the antibacterial phase change cold storage bag prepared in step 3 on the top and around the fresh products, with the mass ratio of the cold storage bag to the fresh products being 1:2-1:4; Fold and seal the top opening of the insulation liner, and then seal the lid of the corrugated cardboard box with tape. Step 6: Intelligent temperature monitoring during delivery A temperature indicator label is affixed to the top of the inside of the insulated packaging, with the color-changing temperature set at 8°C. A clear "Fresh Food Cold Chain" label and temperature warning label are affixed to the outer surface of the corrugated cardboard box. During delivery, the delivery person regularly observes the color change of the temperature indicator label. When the label turns red, the packaging is promptly moved to a low-temperature environment or the delivery speed is expedited. After delivery, consumers are reminded to remove the fresh products and refrigerate them promptly.

2. The method as claimed in claim 1, wherein the said method is a method of preserving fresh produce in cold chain delivery using a temperature maintaining packaging in combination with natural antibacterial agents. The natural wax mentioned in step 2 is a mixture of palm wax and beeswax in a mass ratio of 3:

1. The surfactant is sodium dodecyl sulfate, the monoanhydride ester is glyceryl stearate, the antibacterial agent precursor is sodium hypochlorite, the aerogel particles have a particle size of 10-50 nm, and the dry film thickness of the functional coating is 2-4 μm.

3. The method as claimed in claim 1, wherein the said cold chain delivery method is characterized in that: The phase change temperature of the paraffin-based phase change material in step 3 is 4℃, the latent heat of phase change is 180-200J / g, and the size of the antibacterial phase change cold storage bag is 10cm×15cm, the thickness is 1-2cm, and the heat seal width is 5-8mm.

4. The method as claimed in claim 1, wherein the said cold chain delivery method is characterized in that: The spraying pressure of the spraying equipment mentioned in step 4 is 0.2-0.3MPa, the nozzle diameter is 0.5-1.0mm, the spraying distance is 20-30cm, the temperature of the air-drying environment is 20-25℃, and the relative humidity is 40%-60%.

5. The method as claimed in claim 1, wherein the said cold chain delivery temperature maintaining packaging with natural antibacterial fresh produce preservation method is characterized by: The density of the polystyrene foam insulation layer mentioned in step 5 is 25-30 kg / m³, the breathable mesh is made of food-grade polypropylene with a mesh diameter of 2-3 mm, and the tape is waterproof pressure-sensitive tape. When sealing, all seams of the lid must be covered with tape.

6. The method as claimed in claim 1, wherein the said cold chain delivery temperature maintaining packaging with natural antibacterial fresh produce preservation method is characterized by: The temperature indicator label mentioned in step 6 is an irreversible temperature indicator label. When the ambient temperature exceeds 8°C and lasts for more than 30 minutes, the label changes from blue to red. The label size is 5cm×5cm, and the affixed position is 1-2cm away from the inner surface of the box lid.

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