Preparation method of double-layer composite film and product and application of double-layer composite film

Abstract

The invention discloses a preparation method of a double-layer composite membrane. The preparation method comprises the following steps: S1, green synthesis of SiO2 NPs; s2, MIC and MFC screening of TEO essential oil; s3, a CS-TEO / SA-SiO2 NPs double-layer composite film is prepared, and the composite film is prepared; and S4, preparing the double-layer composite film. The prepared chitosan (CS)-thyme essential oil (TEO) / sodium alginate (SA)-nano silicon dioxide (SiO2 NPs) is a novel composite double-layer film, and compared with a single CS-TEO / SA double-layer composite film, the composite double-layer film added with green synthetic SiO2 NPs has good mechanical performance and low water vapor permeability. The method has important application value and prospect in the field of fruit and vegetable preservation.

Description

Technical Field

[0001] The invention belongs to the technical field of nano material preparation and fruit preservation, and particularly relates to a preparation method of a double-layer composite film, a product thereof and an application thereof. Background Art

[0002] Fruits are rich in vitamin C, polyphenols, and dietary fiber. They are highly sought after by consumers, and market demand for fruit has steadily increased in recent years. However, postharvest fruit loss remains a technical challenge. During storage, fruit ripening is accompanied by an increase in ethylene content, which degrades macromolecules within the fruit, causing the flesh to soften, acidity to decrease, and sweetness to increase. This depletes the fruit of its edible and commercial value, leading to significant economic losses for the food industry.

[0003] Currently, common methods for preserving fruits and vegetables include refrigeration and chemical preservation methods, such as spraying pesticides. However, the high cost of refrigeration and low-temperature preservation, while the food safety and environmental pollution associated with chemical preservation methods, have significantly limited their application. Green preservation technologies using edible polymers or bio-based packaging materials are emerging as a new alternative to traditional preservation methods because they are safe, non-toxic, and biodegradable. They can act as a physical barrier, reducing the fruit's respiration rate, enzyme activity, and metabolic rate, thereby extending its shelf life.

[0004] Chitosan (CS), a polysaccharide extracted from crustacean shells, possesses natural antimicrobial properties, excellent film-forming properties, and biodegradability. However, the antimicrobial properties of CS films still cannot meet the requirements for fruit preservation. Existing results show that when plant essential oils are added to preservative materials, the essential oils are slowly released onto the food surface, thereby increasing the antimicrobial properties of the film. Sodium alginate (SA), a natural polysaccharide biopolymer, has been increasingly used as a packaging material for food preservation due to its excellent film-forming properties, low toxicity, biodegradability, and outstanding moisture resistance. Furthermore, SA films have poor mechanical strength and antimicrobial properties. Single packaging materials are easily affected by their own structure and have certain deficiencies in gas barrier properties, water resistance, and strength. Therefore, developing a composite film matrix with improved mechanical properties to improve fruit preservation technology is of great social value. Summary of the Invention

[0005] In order to solve the above problems, the present invention provides a preparation method of a double-layer composite membrane, its products and applications. The double-layer composite membrane is a chitosan-thyme essential oil (TEO) / sodium alginate-nanosilica (SiO2NPs) composite double-layer membrane, which has the advantages of good film-forming properties, low toxicity, biodegradability, etc., while also having gas barrier properties, water resistance and moderate strength.

[0006] To achieve the above object, the present invention provides the following technical solutions: In a first aspect, the present invention provides a method for preparing a double-layer composite membrane, comprising the following steps: S1 Green Synthesis of SiO2NPs S1-1: Mixing roxburghii powder with deionized water, heating and stirring, cooling to room temperature, centrifuging, filtering the obtained liquid to obtain roxburghii extract, and refrigerating; S1-2: Mixing the roxburghii extract with a sodium metasilicate solution, adjusting the pH, heating and stirring, and centrifuging the resulting precipitate, washing, drying, and calcining to obtain SiO2NPs; S2 TEO essential oil MIC and MFC screening After the TEO essential oil is screened, it is ready for use; S3 Preparation of CS-TEO / SA-SiO2NPs double-layer composite membrane S3-1 Preparation of CS-TEO membrane CS and glycerol were mixed with acetic acid aqueous solution and stirred to obtain chitosan solution CS, and then TEO with the selected minimum bactericidal concentration was added to the chitosan solution and named CS-TEO; S3-2 Preparation of SA-SiO2NPs film SA was placed in ionized water, glycerol was added, and then SiO2NPs were added and stirred to obtain SA-SiO2NPs solution; Preparation of S4 double-layer composite membrane The CS-TEO emulsion was poured into a Petri dish and dried to form a CS-TEO film. The SA-SiO2NPs solution was then cast on top of the CS-TEO film. After drying, the formed film was peeled off from the Petri dish to obtain a double-layer composite film.

[0007] Furthermore, in step S1-1, the material-liquid ratio of the roxburghii powder to the deionized water is 1:15-30 (g / mL), the heating temperature is 55-60°C, and the refrigeration temperature is 3-5°C.

[0008] Furthermore, in step S1-2, the volume ratio of the roxburghii extract to the sodium metasilicate solution is 1:2-3, and the concentration of the sodium metasilicate solution is 0.1 mol / L.

[0009] Furthermore, in step S1-2, washing is performed by alternating deionized water and ethanol solution for 3 to 6 times, the drying temperature is 70 to 90° C., and the calcination condition is calcination in a tube furnace at 450 to 550° C. for 3 to 5 h.

[0010] Furthermore, in step S2, the MIC and MFC screening of TEO essential oil is specifically carried out by preparing different concentrations of TEO using a two-fold serial broth dilution method, adding the above TEO and 1×106 indivual A. alternata Spores∙mL -1 The suspension was then cultured in a constant temperature shaker. The MIC value was determined by visual observation using the lowest concentration at which no fungal growth was observed in potato dextrose broth. After the MIC experiment, the culture liquid was taken and inoculated onto PDA medium. The culture was observed for colony formation and the concentration at which no colony growth was observed was used as the MFC.

[0011] Furthermore, in step S3-1, the feed ratio of CS and glycerol to the acetic acid aqueous solution is 1.5-2.5 g:1.5-2 ml:100 ml, the concentration of the acetic acid aqueous solution is 2% (v / v), and the amount of TEO added at the minimum bactericidal concentration is 0.3-0.4 μL / mL based on the chitosan solution.

[0012] Furthermore, in step S3-2, the material-liquid ratio of SA to deionized water is 1-3:100 (g / ml), the amount of glycerol added is 2-3% (v / v), and the mass ratio of SA to SiO2NPs is 75:1-40:1.

[0013] Furthermore, in step S4, the mass ratio of CS-TEO to SA-SiO2NPs is 1-2:1-2.

[0014] In a second aspect, the present invention provides a double-layer composite membrane prepared by the above method.

[0015] In a third aspect, the present invention provides an application of the above-mentioned double-layer composite film for preserving fruits, especially Provence tomatoes.

[0016] The beneficial effects obtained by the present invention are: The invention has reasonable design, simple preparation method and the following advantages.

[0017] The present invention provides a method for preparing a double-layer composite membrane comprising the following raw materials: CS, SA, TEO, and greenly synthesized SiO2NPs. Compared to a CS-TEO / SA double-layer composite membrane without SiO2NPs, the composite membrane provided by the present invention can slow the rate of weight loss, decay, titratable acid content, and vitamin C content of fruit, effectively retaining nutrients in the fruit and extending its storage life. This method has significant application value and prospects in the field of fruit and vegetable preservation. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a scanning electron microscope (SEM) image of SiO2 NPs prepared in an embodiment of the present invention; Figure 2This is a screening diagram of the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MFC) of TEO essential oil in the embodiment of the present invention; Figure 3 This is a graph showing the effect of double-layer composite films of SiO2NPs with different addition amounts on the appearance of tomatoes in an embodiment of the present invention; Figure 4 This is a graph showing the effect of double-layer composite films of SiO2NPs with different addition amounts on the tomato rot index in an embodiment of the present invention; Figure 5 This is a graph showing the effect of double-layer composite films of SiO2NPs with different addition amounts on the weight loss rate of tomatoes in an embodiment of the present invention; Figure 6 This is a graph showing the effect of double-layer composite films of SiO2NPs with different addition amounts on the titratable acidity of tomatoes in an embodiment of the present invention; Figure 7 This is a graph showing the effect of double-layer composite films of SiO2NPs with different addition amounts on the vitamin C content of tomatoes in an embodiment of the present invention; DETAILED DESCRIPTION Before discussing the exemplary embodiments in more detail, it should be mentioned that some exemplary embodiments are described as processes or methods depicted as flow charts. Although the flow charts describe the various operations as sequential processes, many of the operations therein can be implemented in parallel, concurrently, or simultaneously. In addition, the order of the various operations can be rearranged. The process can be terminated when its operation is completed, but can also have additional steps not included in the accompanying drawings. The process can correspond to a method, function, procedure, subroutine, subprogram, etc.

[0019] It should be understood that although the terms "first," "second," and the like may be used herein to describe various elements, these elements should not be limited by these terms. These terms are used solely to distinguish one element from another. For example, a first element may be referred to as a second element, and similarly, a second element may be referred to as a first element, without departing from the scope of the exemplary embodiments. The term "and / or" as used herein includes any and all combinations of one or more of the listed associated items.

[0020] The present invention provides a method for preparing a double-layer composite membrane, comprising the following steps: S1 Green Synthesis of SiO2NPs S1-1: Mixing roxburghii powder with deionized water, heating and stirring, cooling to room temperature, centrifuging, filtering the obtained liquid to obtain roxburghii extract, and refrigerating; S1-2: Mixing the roxburghii extract with a sodium metasilicate solution, adjusting the pH, heating and stirring, and centrifuging the resulting precipitate, washing, drying, and calcining to obtain SiO2NPs; S2 TEO essential oil MIC and MFC screening After the TEO essential oil is screened, it is ready for use; S3 Preparation of CS-TEO / SA-SiO2NPs double-layer composite membrane S3-1 Preparation of CS-TEO membrane CS and glycerol were mixed with acetic acid aqueous solution and stirred to obtain chitosan solution CS, and then TEO with the selected minimum bactericidal concentration was added to the chitosan solution and named CS-TEO; S3-2 Preparation of SA-SiO2NPs film SA was placed in ionized water, glycerol was added, and then SiO2NPs were added and stirred to obtain SA-SiO2NPs solution; Preparation of S4 double-layer composite membrane The CS-TEO emulsion was poured into a Petri dish and dried to form a CS-TEO film. The SA-SiO2NPs solution was then cast on top of the CS-TEO film. After drying, the formed film was peeled off from the Petri dish to obtain a double-layer composite film.

[0021] Materials used in the present invention Rosa roxburghii fruit was purchased from Guizhou Qihong Business Co., Ltd., Na₂SiO₃·9H₂O, sodium alginate (SA), and CS were purchased from Sinopharm Chemical Reagent Co., Ltd., thyme essential oil (TEO) was purchased from Jiangxi Yuanshangcao Spice Co., Ltd., and Provence tomatoes were purchased from Haiyang Jinsheng E-Commerce Co., Ltd. Alternaria alternata was obtained from the Biological Laboratory of Changzhou University, and the pesticide DuPont Isoquinone was purchased from Shouguang Zhishang Agricultural Materials Co., Ltd.

[0022] Other materials are of regular models and specifications purchased from the reagent company.

[0023] Example 1 S1 Green Synthesis of SiO2NPs S1-1 Rosa roxburghii powder and deionized water were mixed at a solid-liquid ratio of 1:20 (g / mL), stirred in a water bath at 60°C for 60 min, cooled to room temperature, centrifuged at 12,000 rpm, and the resulting liquid was filtered to obtain a brown filtrate as the Rosa roxburghii extract, which was stored at 4°C in the dark.

[0024] S1-2: 100 mL of the roxburghii extract was mixed with 200 mL of a 0.1 mol / L sodium metasilicate solution. The pH was adjusted to 7, and the mixture was heated and stirred at 90°C in a water bath for 4 hours. The color change was observed to confirm the formation of nanoparticles. The resulting precipitate was centrifuged at 8000 rpm and washed three times with deionized water and ethanol alternately. The precipitate was dried in an 80°C oven and calcined in a tube furnace at 500°C for 3 hours to obtain white powdered SiO2 NPs.

[0025] Scanning electron microscopy (SEM, FEI Quanta FEG 250) was used to observe the morphology and size of SiO2NPs particles.

[0026] S2 TEO essential oil minimum inhibitory concentration (MIC) and minimum fungicidal concentration (MFC) screening TEO was prepared at a concentration range of 0.02, 0.04, 0.08, 0.16, 0.32, 0.64, 1.28, and 2.56 μL / mL using two-fold serial broth dilution.

[0027] To each flask, add a concentration of TEO and 1 mL of 1 × 10 6 indivual A. alternata Spores∙mL -1 The suspension was then incubated at 25°C in a constant temperature shaker (150 rpm) for 3 days. The MIC value was determined by visual observation as the lowest concentration at which no fungal growth was observed in potato dextrose broth (PDB) medium. After the MIC experiment, 100 μL of the 24-hour incubation liquid was inoculated onto PDA medium and incubated at 25°C for 24 hours. Colony formation was observed and the concentration at which no colony growth was observed was designated as the minimum concentration (MFC) for future use.

[0028] S3 Preparation of CS-TEO / SA-SiO2NPs double-layer composite membrane S3-1 Preparation of CS-TEO membrane 2 g of CS and 1.5 mL of glycerol were mixed with 100 mL of 2% (v / v) acetic acid in water. The mixture was stirred at 800 rpm on a magnetic stirrer for 1 hour to obtain a chitosan solution (CS). TEO at a minimum bactericidal concentration (MFC) of 0.32 μL / mL was then added to the chitosan solution, which was designated CS-TEO.

[0029] S3-2 Preparation of SA-SiO2NPs film 2 g of SA was placed in 100 mL of deionized water (80°C), 2% (v / v) glycerol was added, and then 0, 20, 40, and 80 mg of SiO2NPs were added, respectively. The mixture was stirred at 800 rpm for 1 h. Finally, SA-SiO2NPs solutions were obtained and named SA-SiO20, SA-SiO220, SA-SiO240, and SA-SiO280, respectively.

[0030] Preparation of S4 double-layer composite membrane 12 g of CS-TEO emulsion was poured into a 9 mm Petri dish and dried at room temperature to form a CS-TEO film (to ensure the film was not completely dry and still maintained a certain viscosity). Then, 12 g each of four solutions (SA-SiO20, SA-SiO220, SA-SiO240, and SA-SiO280) were cast on top of the CS-TEO film. All samples were then dried for 12 hours. Finally, the formed film was peeled off the Petri dish to obtain a bilayer composite membrane, which was designated CTSS-0, CTSS-20, CTSS-40, and CTSS-80 membranes, respectively.

[0031] The membrane performance of the double-layer composite membranes CTSS-0, CTSS-20, CTSS-40 and CTSS-80 prepared in the examples was tested. 1. Experimental Design The double-layer composite film is tested for performance, including water vapor transmission rate (WVP), oxygen permeability (OP), carbon dioxide permeability (CDP) and tensile fracture performance. The specific steps are as follows: WVP: Film samples were sealed on top of a test container (2.8 cm diameter, 11.5 cm height) containing 5 g of anhydrous calcium chloride and weighed. They were then placed in a chamber maintained at a constant temperature (20°C) and 75% relative humidity for 48 hours before being weighed. WVP was calculated as follows:

[0032] Where: ΔM is the added weight of the experimental container (g), d is the film thickness (m), t is the time (s), A The effective area of the film (m 2 ), ΔP is the water vapor pressure between the two sides of the film at 20 °C (2339 Pa).

[0033] Tensile fracture: The tensile strength (TS) and elongation at break (EB) of film samples (10 mm × 50 mm) were measured using a universal testing machine (CMT6103, China).

[0034] OP is measured using the deoxidant absorption method. The film sample is sealed on top of a test container containing 5 g of deoxidant. The test container containing the film sample is first weighed and then placed in a chamber at a constant temperature (25°C) and 75% relative humidity for 48 hours before being weighed. OP:

[0035] in ΔM is the added weight of the experimental container (g), d is the film thickness (m), t is the time (s), A The effective area of the film (m 2 ), ΔP is the water vapor pressure between the two sides of the film at 20°C (2339 Pa).

[0036] 2. Experimental results The microstructure of the synthesized nanoparticles was analyzed using scanning electron microscopy. Figure 1 As shown, the synthesized SiO2NPs present a spherical structure, and the average particle size of the green synthesized SiO2NPs is 13 nm, which is smaller than that of the other extracts.

[0037] By screening the concentration of TEO essential oil, such as Figure 2 As shown in (a), the MIC of TEO essential oil was 0.16 μL / mL. Figure 2 (b) The MFC shown is 0.32 μL / mL. The MFC concentration of the essential oil was ultimately selected as the amount of essential oil added to the composite membrane, which is lower than the amount of essential oil added in other studies.

[0038] Mechanical properties are key parameters for packaging films, impacting their range of use and durability. Table 1 shows that an appropriate amount of SiO2NPs can improve both the film's TS and EB. The improved TS of the film is attributed to the fact that SiO2NPs act as interstitial fillers, uniformly dispersed throughout the polymer and forming strong interfacial interactions with the film matrix through their high surface energy and large specific surface area. Furthermore, the addition of SiO2NPs promotes interfacial slip between polymer chains under tension, thereby improving the film's EB.

[0039] Table 1 TS and EB of different double-layer composite membranes

[0040] Water vapor transmission rate (WVP) is a key property of coatings, significantly influencing fruit transpiration and respiration. Table 2 shows the water vapor transmission rates of different two-layer composite films. Compared with the CTSS-0 two-layer film, the addition of SiO2NPs effectively improves the film's water vapor barrier properties. The CTSS-40 composite film exhibits the lowest WVP, a 44.7% decrease compared to CTSS-0. This is because the incorporation of SiO2NPs into the polymer matrix increases the tortuous path for water vapor diffusion. Furthermore, as the SiO2NP concentration increases, the hydrophilicity of the nanocomposite film gradually increases, enhancing the attraction of water molecules to the film and thus accelerating its passage through the membrane.

[0041] The oxygen barrier properties of the film are essential parameters for maintaining food quality. The OP data of the film are shown in Table 2. The OP of the double-layer film decreased slightly after the addition of SiO2NPs. This is because the addition of SiO2NPs increased the density of the film, thereby delaying the diffusion rate of oxygen through the polymer. However, when the content of SiO2NPs increased to 0.8 g / L, the OP of the double-layer film increased to 1.5480×10 -7 g·m -1 ·s -1， This is consistent with the trend of WVP results, because high-concentration SiO2NPs are prone to agglomeration and uneven dispersion, thus affecting the gas barrier performance.

[0042] Table 2 WVP and OP of different double-layer composite membranes

[0043] Water vapor transmission rate (WVP) is a key property of coatings, significantly influencing fruit transpiration and respiration. Table 1 shows the water vapor transmission rates of different two-layer composite films. Compared with the CTSS-0 two-layer film, the addition of SiO2NPs effectively improves the water vapor barrier properties of the films. The CTSS-40 composite film exhibits the lowest WVP, a 44.7% decrease compared to CTSS-0. This is because the incorporation of SiO2NPs into the polymer matrix increases the tortuous path for water vapor diffusion. Furthermore, as the SiO2NP concentration increases, the hydrophilicity of the nanocomposite film gradually increases, enhancing the attraction of water molecules to the film and thus accelerating its passage through the membrane.

[0044] Application of the CS-TEO / SA-SiO2NPs double-layer composite film prepared in the example in tomato preservation Based on the performance of the double-layer composite membranes with different SiO2NP additions, the composite membrane with 0.4 g / L SiO2NPs showed the best performance. Therefore, in the tomato preservation experiment, the CTSS-40 double-layer composite membrane treatment was compared with the untreated control group (CK) and the tomatoes sprayed with pesticides.

[0045] Experimental design Tomato fruit processing Tomatoes without external damage or disease were selected and disinfected with 0.05% NaClO, then washed with distilled water and air-dried. The fruits were randomly divided into four groups, each with 40 replicates. Each group was immersed in deionized water and CTSS-40 membrane solution (first immersed in CS-TEO membrane solution, removed and dried, and then immersed in SA-SiO2NPs membrane solution) for 5 minutes. Another group was sprayed with pesticides. The fruits were air-dried at room temperature for 1 hour and stored in a fresh-keeping container at 25°C for 20 days. The tomatoes from each treatment were then divided into two parts. One part, containing 15 fruits, was used to observe the decay index and weight loss rate, while the other part was used for sampling. The four indicators selected for measurement were weight loss rate, decay rate, titratable acid content, and vitamin C content. The measurement methods for these four indicators are as follows.

[0046] Decay Index The degree of fruit decay is expressed by the decay index, as shown in Table 4.

[0047] The degree of fruit decay is expressed by the decay index Fruit decay degree table

[0048] The calculation formula of tomato fruit rot index is as follows:

[0049] Weight loss rate Weight loss rate The weight loss of tomato fruits during storage was measured using a balance, and the percentage relative to the initial weight before storage was calculated.

[0050] Titratable acid (TA) Take 5 g of pulp sample, homogenize it with distilled water, and dilute it to 50 mL. Let it stand for 30 minutes, then centrifuge it. Take 20 mL of the supernatant and titrate it with a NaOH standard solution. The final result is expressed as the percentage of citric acid.

[0051]

[0052] Where: C is the concentration of NaOH solution (0.1 mol / L); V is the volume of NaOH solution consumed (mL); V 1 is the volume of the titrated supernatant (mL); V 2 is the total volume of the supernatant (mL); W The fresh weight of tomatoes (g) was used.

[0053] Vitamin C (VC) content 1 g of pulp tissue was homogenized with 2 mL of 5% (w / v) trichloroacetic acid (TCA) and centrifuged at 16,000 rpm for 15 minutes at 4°C. The supernatant was collected for testing. For the assay, 1 mL of supernatant was added, in that order, to 1 mL of 5% (w / v) TCA, 1 mL of anhydrous ethanol, 0.5 mL of 0.4% phosphoric acid, 1 mL of 0.5% (w / v) 4,7-diphenyl-1,10-phenanthroline, and 0.03% (w / v) FeCl₃ (dissolved in anhydrous ethanol). After incubation at 30°C in a water bath for 60 minutes, the absorbance was measured at 534 nm. The formula for calculating VC content is as follows:

[0054] in, m is the absorbance measured at 534 nm, W is the sample mass (g), V is the total volume of sample extract (mL), Vs is the volume of sample extract used for titration (mL).

[0055] Experimental results The decay index of tomatoes stored at 25±1℃ for 20 days is as follows: Figure 3 As shown, the decay index and weight loss rate are as follows Figure 4 、 5 As shown in the figure. On the 8th day, the surface of the tomatoes in the CK group first showed signs of decay, with black spots appearing, which then gradually intensified. However, the tomatoes sprayed with pesticides and coated with films largely maintained their initial appearance. On the 12th day of storage, the tomatoes sprayed with pesticides began to rot slightly, while the tomatoes coated with CTSS-40 films remained intact. By the 20th day of storage, nearly half of the tomatoes in the CK group had rotted, with a rot index and weight loss rate reaching 70.4% and 12.4% respectively. The tomatoes in the CTSS-40 coating group had a smaller surface rot area, a rot index that was 61.8% lower, and a weight loss rate that was 66.7% lower than those in the CK group, respectively. This indicates that double-layer coatings can effectively extend the storage life of tomatoes, with the double-layer composite film of CTSS-40 with SiO2 NPs showing the best effect.

[0056] The effects of different treatments on titratable acidity of tomatoes are shown in Figure 6 The TA content of all tomatoes showed an overall downward trend over storage time, with the control group showing a greater downward trend compared to the coated group. Compared to the initial level, the control group had decreased by 48.7% on the 20th day, while the CTSS-40 group had decreased by 33.2%. This is because the coating on the tomato surface reduced the tomato's respiration rate and slowed the transpiration rate.

[0057] VC content is one of the most important indicators for evaluating the nutritional value and freshness of fruits and vegetables, but it is easily oxidized and decomposed, and its physiological activity decreases during storage. Figure 7 As shown in the results, tomatoes in the film-coated group exhibited more moderate changes in VC, with VC content beginning to decline from day 12. The addition of SiO2 NPs to the bilayer film, due to its semipermeability to respiratory gases, reduced oxidation and TA loss, enhancing the ability of this biopolymer to prevent VC degradation in tomatoes. At the end of the storage period, VC content was lowest in the control group at 34.5 mg / 100 g, while the CTSS-40 film-coated group reached a highest of 48.76 mg / 100 g, a 41.3% increase compared to the control group. This demonstrates that the coating is effective in delaying VC degradation.

[0058] As described above, in the double-layer composite membrane performance research experiment, parameters such as the composite membrane's thickness, water permeability, and mechanical properties were measured. The results showed that when the SiO2NPs addition level was 0.4 g / L, the double-layer composite membrane's performance was relatively good. Analysis of the double-layer composite membrane performance results revealed that the double-layer composite membrane CTSS-40, with a SiO2NPs addition level of 0.4 g / L, was ultimately selected for the tomato fruit preservation study. Analysis of tomato appearance, decay index, weight loss rate, and TA and VC content revealed that the addition of SiO2NPs to the double-layer composite membrane effectively delayed tomato decay, reduced weight loss, and the loss of nutrients such as TA and VC. Therefore, the CS-TEO / SA-SiO2NPs double-layer composite membrane can effectively extend the shelf life of tomatoes.

[0059] The above is only an embodiment of the present invention. Common knowledge such as the known specific structures and characteristics in the scheme is not described in detail here. Ordinary technicians in the field are aware of all common technical knowledge in the technical field of the invention before the application date or priority date, can obtain all existing technologies in the field, and have the ability to apply conventional experimental means before that date. Ordinary technicians in the field can improve and implement this scheme in combination with their own abilities under the inspiration given by this application. Some typical known structures or known methods should not become obstacles for ordinary technicians in the field to implement this application. It should be pointed out that for those skilled in the art, without departing from the structure of the present invention, several variations and improvements can be made, which should also be regarded as the scope of protection of the present invention. These will not affect the effect of the implementation of the present invention and the practicality of the patent. The scope of protection required by this application shall be based on the content of its claims, and the specific implementation methods and other records in the specification can be used to interpret the content of the claims.

Claims

1. A method for preparing a double-layer composite membrane, comprising the following steps: Green synthesis of S1 and SiO2 NPs S1-1, mixing the roxburghii powder with deionized water, heating and stirring, cooling to room temperature, centrifuging, filtering the obtained liquid to obtain the roxburghii extract, and refrigerating; S1-2, mixing the roxburghii extract with a sodium metasilicate solution, adjusting the pH, heating and stirring, centrifuging the resulting precipitate, washing, drying, and calcining to obtain SiO2 NPs; S2. TEO essential oil MIC and MFC screening After the TEO essential oil is screened, it is ready for use; S3. Preparation of CS-TEO / SA-SiO2 NPs double-layer composite membrane S3-1. Preparation of CS-TEO membrane CS and glycerol were mixed with acetic acid aqueous solution and stirred to obtain chitosan solution CS, and then TEO with the selected minimum bactericidal concentration was added to the chitosan solution and named CS-TEO; S3-2. Preparation of SA-SiO2 NPs membrane SA was placed in ionized water, glycerol was added, and then SiO2 NPs were added and stirred to obtain SA-SiO2 NPs solution; S4. Preparation of double-layer composite membrane The CS-TEO emulsion was poured into a Petri dish and dried to form a CS-TEO film. The SA-SiO2 NPs solution was then cast on top of the CS-TEO film. After drying, the formed film was peeled off from the Petri dish to obtain a double-layer composite film.

2. The method for preparing a double-layer composite membrane according to claim 1, wherein: In the step S1-1, the material-liquid ratio of the roxburghii powder to the deionized water is 1:15-30 (g / mL), the heating temperature is 55-60°C, and the refrigeration temperature is 3-5°C.

3. The method for preparing a double-layer composite membrane according to claim 1, wherein: In step S1-2, the volume ratio of the roxburghii extract to the sodium metasilicate solution is 1:2-3, and the concentration of the sodium metasilicate solution is 0.1 mol / L.

4. The method for preparing a double-layer composite membrane according to claim 1, wherein: In step S1-2, washing is performed by alternating deionized water and ethanol solution for 3 to 6 times, the drying temperature is 70 to 90° C., and the calcination condition is calcination in a tube furnace at 450 to 550° C. for 3 to 5 hours.

5. The method for preparing a double-layer composite membrane according to claim 1, wherein: In step S2, the MIC and MFC screening of TEO essential oils was carried out by preparing TEO of different concentrations using a two-fold serial broth dilution method, adding the above TEO and 1×10 6 indivual A. alternata Spores∙mL -1 The suspension was then cultured in a constant temperature shaker. The MIC value was determined by visual observation using the lowest concentration at which no fungal growth was observed in potato dextrose broth. After the MIC experiment, the culture liquid was taken and inoculated onto PDA medium. The culture was observed for colony formation and the concentration at which no colony growth was observed was used as the MFC.

6. The method for preparing a double-layer composite membrane according to claim 1, wherein: In step S3-1, the feed ratio of CS and glycerol to the acetic acid aqueous solution is 1.5-2.5 g:1.5-2 ml:100 ml, the concentration of the acetic acid aqueous solution is 2% (v / v), and the amount of TEO added at the minimum bactericidal concentration is 0.3-0.4 μL / mL based on the chitosan solution.

7. The method for preparing a double-layer composite membrane according to claim 1, wherein: In step S3-2, the material-liquid ratio of SA to deionized water is 1-3:100 (g / ml), the amount of glycerol added is 2-3% (v / v), and the mass ratio of SA to SiO2 NPs is 75:1-40:

1.

8. The method for preparing a double-layer composite membrane according to claim 1, wherein: In step S4, the mass ratio of CS-TEO to SA-SiO2 NPs is 1-2:1-2.

9. A double-layer composite membrane obtained according to the preparation method according to any one of claims 1 to 8.

10. An application of the double-layer composite membrane according to claim 9, characterized in that: Used to keep fruits fresh.

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