Preparation method and application of biodegradable food preservative film based on fruit and vegetable processing byproducts

Abstract

The invention discloses a preparation method and application of a biodegradable food preservative film based on fruit and vegetable processing byproducts. The preparation method comprises the following steps: (1) mixing fruit and vegetable processing byproduct powder with an ethanol water solution to extract polyphenol; (2) carrying out alkali treatment to remove lignin; (3) bleaching with hydrogen peroxide to obtain cellulose; and (4) compounding cellulose with the extracting solution, and adding sodium alginate and glycerol to form the film. The 24-hour bacteriostasis rate on escherichia coli and staphylococcus aureus is larger than or equal to 99%, the DPPH free radical scavenging rate is larger than or equal to 58%, and the 20-day degradation rate in soil is larger than or equal to 95%. According to the invention, high-value utilization of fruit and vegetable processing wastes is realized, and the obtained preservative film has excellent mechanical properties (the tensile strength is greater than or equal to 15MPa), oxidation resistance and rapid biodegradation characteristics, and can replace a traditional plastic preservative film to be used for preserving fruits such as Hami melons, grapes and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of food packaging materials, in particular to a preparation method and application of a biodegradable food preservative film based on fruit and vegetable processing by-products. BACKGROUND

[0002] With the rapid development of global food processing industry, the number of by-products such as fruit peels and pomace generated during fruit and vegetable processing has increased dramatically. According to statistics, more than 500,000 tons of loquat processing waste are generated in China every year. At present, these by-products are mainly disposed of by landfill or incineration, which not only wastes resources but also releases a large amount of greenhouse gases such as methane, exacerbating environmental burden. At the same time, traditional petroleum-based plastic preservative films have become one of the main sources of white pollution due to their non-biodegradable nature. According to data from the European Environment Agency, the recycling rate of food packaging plastics is less than 15%, and more than 80% of waste plastics eventually enter the natural environment, with a degradation period of hundreds of years.

[0003] In recent years, research on replacing traditional plastics with bio-based materials has gradually emerged. In the prior art, some studies attempt to directly use fruit and vegetable waste to prepare degradable film materials, but because the impurities such as lignin are not effectively removed, the light transmittance of the obtained film material is generally less than 75%, and the mechanical properties are poor, with a tensile strength of less than 5 MPa, which is difficult to meet the strength requirements of fresh fruit and vegetable packaging. In addition, such materials often need to add high-priced raw materials such as chitosan and nano titanium dioxide during processing to improve performance, resulting in an increase of more than 200% in production cost, and there is a risk of chemical additive migration exceeding the standard. For example, in the prior art, CN112125890A discloses a nano titanium dioxide reinforced film, which has a migration amount of 5.8 mg / kg due to particle agglomeration, which poses a safety hazard.

[0004] Other studies extract fruit and vegetable fibers through high-temperature acid hydrolysis or enzyme treatment processes, but such methods often result in a large loss of active polyphenol components. Experiments show that the polyphenol retention rate of conventional hot water extraction process is less than 60%, causing the film material to lose its natural antibacterial function and still need to add additional chemical preservatives. In addition, the degradation efficiency of existing biodegradable films is still low, and it takes more than 90 days to completely degrade in soil environment, which cannot meet the demand of the food packaging industry for rapid recycling. How to realize the efficient value-added utilization of waste, and at the same time solve the problems of mechanical performance defects, loss of antibacterial function, and long degradation period of film materials, has become a key technical bottleneck restricting the development of the industry.

[0005] Currently, although the prior art proposes a process route for hierarchical extraction of cellulose and polyphenols, it is mostly limited to laboratory scale, and has the defects of low extraction efficiency, high energy consumption, etc. Studies have shown that high-pressure steam pretreatment technology (such as steam explosion) can improve the purity of cellulose (Journal of Cleaner Production, 2021, 297: 126682), but its energy consumption is increased by more than 35% compared with the conventional process, which restricts industrial application. Therefore, developing a low-cost and high-efficiency fruit and vegetable by-product processing technology, and constructing a functional film material with high transparency, high mechanical strength and rapid degradation characteristics, is still a technical problem that needs to be broken through in the field. SUMMARY

[0006] In order to solve the problem of difficult development of low-cost and high-efficiency fruit and vegetable by-product processing technology, and construction of a functional film material with high transparency, high mechanical strength and rapid degradation characteristics, the present application provides a preparation method of a biodegradable food preservative film based on fruit and vegetable processing by-products, comprising the following steps: (1) Polyphenol extraction: mix fruit and vegetable processing by-product powder with a volume ratio of 1:1 ethanol aqueous solution, with a mass volume ratio of 3:50 g / ml, ultrasonic extraction for 4 hours, and separate the polyphenol-containing extract and residue after centrifugation; (2) Delignification: immerse the residue of step (1) in a mixed solution of NaOH and Na2CO3 with a molar ratio of 25:2, boil for 1 hour, then filter, and wash the filter residue with hot distilled water; (3) Preparation of bleached cellulose: place the filter residue of step (2) in a 30% H2O2 solution and boil until the yellow color disappears, then rinse with cold water to obtain bleached cellulose; (4) Film preparation: mix the bleached cellulose with the extract obtained in step (1), add 3 times the amount of sodium alginate and 2 times the amount of glycerol based on the mass of the fruit and vegetable processing by-product powder used in step (1), ultrasonic dispersion to form a slurry, cast into a film and dry.

[0007] Further, the fruit and vegetable processing by-products are at least one of loquat skin, grape skin, pomegranate skin or hairy bean pod shell, which is dried at 60°C for 8 hours and then ground through a 100 mesh sieve.

[0008] Further, the ultrasonic extraction in step (1) is carried out at 15-35°C.

[0009] Further, the centrifugation in step (1) is at 10000 rpm for 5 minutes.

[0010] Further, the ultrasonic dispersion in step (4) is carried out at 50-70°C.

[0011] Further, the slurry is cast into a film in step (4) and dried at room temperature for 48 hours.

[0012] The food preservation film prepared by the method has a thickness of 0.05-0.15 mm, a tensile strength of ≥15 MPa, and an elongation at break of ≥25%.

[0013] The food preservation film prepared by any one of the above methods is used for fresh fruit and vegetable preservation, and is used for extending the shelf life of melons and fruits, and has a preservation period of ≥7 days at room temperature. The barrier property is: water vapor transmission rate 42.5 g·mm / (m²·d·kPa) (22% lower than PE film). The shelf life of fresh-cut Hami melon at 10-25°C can be extended from 3 days to 7 days (sensory score ≥8 points, NY / T 939 standard). The mechanical property is: tensile strength 15.2-18.7 MPa (3-5 times higher than pure cellulose film) Compared with the prior art, the beneficial effects of the present application are: 1. Resource recycling and waste high value Using fruit and vegetable processing by-products (loquat skin, grape skin, etc.) as raw materials, the synergistic use of pectin, cellulose and polyphenol is realized by fractional extraction technology, and the utilization rate of raw materials is more than 90%. Each ton of waste fruit skin can produce about 120 kg of preservation film, which reduces carbon emissions by 95% compared with traditional landfill treatment, and solves the problem of resource waste caused by the accumulation of processing by-products. The ethanol recovery rate is ≥85%, and the production cost is reduced by 60% compared with chitosan-based film.

[0014] 2. Environment-friendly degradation characteristics The prepared preservation film has a biodegradation rate of ≥95% in natural soil environment within 20 days (ISO 17556 standard), and the degradation products are water, carbon dioxide and humus without microplastic residues. The degradation speed is 200 times that of traditional polyethylene film, which significantly reduces the risk of white pollution. The degradation of cellulose / pectin plastic film in soil mainly relies on the enzymes (such as cellulase and pectinase) secreted by microorganisms to break down polysaccharide chains, and then small molecule sugars are absorbed and utilized by microorganisms. Environmental humidity, temperature and material structure (such as crystallinity) will affect the degradation speed. Compared with traditional plastics, this kind of film degrades faster, has no microplastic residues, and is more environmentally friendly.

[0015] 3. Breakthrough improvement in mechanical properties Through the design of cellulose-pectin double network structure, the tensile strength of the film material reaches 15-18.7 MPa (GB / T1040.3), which is 3-5 times higher than that of single cellulose film, and the elongation at break is ≥25%, which can withstand the mechanical stress of fresh fruit and vegetable packaging. At the same time, the water vapor transmission rate is as low as 42.5 g·mm / (m²·d·kPa) (ISO 2528), and the moisture resistance performance is better than that of commercially available PE film.

[0016] 4. Self-antibacterial and antioxidant functions Retain natural polyphenols (quercetin, kaempferol, etc.) in the peel, endow the film material with excellent antibacterial activity, 24-hour bacterial inhibition rate ≥ 99.2% (GB / T 31402) for Escherichia coli (ATCC 25922) and Staphylococcus aureus (ATCC 25923), DPPH free radical scavenging rate ≥ 58.4% (GB / T 31740.3), and no need to add chemical preservatives to extend the shelf life of food.

[0017] 5. Optimal optical performance and safety Through NaOH / Na2CO3 delignification and H2O2 bleaching processes, the light transmittance of the film material is increased to 88.5% (ASTM D1003), and the haze is ≤ 12%, meeting the transparency requirements of food packaging. The polyphenol migration amount is ≤ 0.5 mg / kg (GB 31604.8), meeting the GB 9685 safety standard for food contact materials.

[0018] 6. Low-energy green preparation process The whole process uses atmospheric pressure and low-temperature (≤ 100℃) treatment, saving 35% energy compared to the traditional high-pressure steam method. The ethanol solvent recovery rate is ≥ 85%, and no complex purification equipment is needed, reducing the production cost by more than 60% compared to chitosan-based films, and having potential for industrialization. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 For antioxidant performance test results, wherein: Figure 1 (a) Comparison of total phenol content extracted by different solvents column chart; Figure 1 (b) DPPH and ABTS free radical scavenging rate column chart; Figure 2 For the characterization of loquat peel preservation film material prepared in Example 1, wherein: Figure 2 (a) Scanning electron microscope image of the cross-section microstructure of the preservation film; Figure 2 (b) Scanning electron microscope image of the cross-section microstructure of the preservation film; Figure 3 X-ray diffraction pattern of the preservation film of Example 1; Figure 4 Infrared spectrum of the preservation film of Example 1; Figure 5 Thermogravimetric analysis curve of the preservation film of Example 1; Figure 6 Mechanical property test curve of the preservation film of Example 1, wherein: Figure 6(a) Effect of glycerol content on the mechanical properties of loquat peel film; Figure 6 (b) Effect of sodium alginate content on the mechanical properties of loquat peel film; Figure 7 Comparison chart of mechanical properties of loquat peel preservative film with and without delignification for Example 1; Figure 8 Test results of antibacterial properties of grape peel for Example 2 and pomegranate peel for Example 3, wherein: Figure 8 (a) Comparison chart of bacterial colony agar plates between the control group and the treatment group; Figure 8 (b) Colony inhibition rates of Escherichia coli and Staphylococcus aureus; Figure 9 Appearance chart of film formation of grape peel for Example 2, pomegranate peel for Example 3, and hairy bean shell for Example 4; Figure 10 Effect chart of antibacterial properties of loquat peel film for Example 1, wherein: Figure 10 (a) Comparison chart of bacterial colony agar plates between the control group and the treatment group, Figure 10 (b) Colony inhibition rates of Escherichia coli and Staphylococcus aureus; Figure 11 Comparison chart of biodegradability, wherein: Figure 11 (a) PE film and loquat peel film in the initial state, Figure 11 (b) After 20 days of soil burial, the loquat peel film is completely degraded, and the PE film remains intact; Figure 12 Effect chart of fresh-cut Hami melon preservation, wherein: Figure 12 (CK) Surface browning of the untreated group on the 3rd day; Figure 12 (PE) Mold spots appeared on the 5th day in the PE film group; Figure 12 (LPF) The loquat peel film group maintained bright color on the 7th day.

[0020] Figure 13 Effect chart of preservation of grapes for Example 1. DETAILED DESCRIPTION

[0021] The technical solutions of the present application will be described in detail below through examples, but the protection scope of the present application is not limited to the examples.

[0022] Example 1: Preparation of biodegradable food preservative film using loquat peel Raw material pretreatment: Take commercially available loquat peel, dry at 60°C for 8 hours, crush through a 100-mesh sieve, and weigh 3g of loquat peel powder.

[0023] Polyphenol extraction: Mix the loquat peel powder with 50 mL of 1:1 volume ratio ethanol aqueous solution at a mass volume ratio of 3:50 g / ml, and ultrasonically extract for 4 hours at 25°C. After centrifugation, separate the polyphenol-containing extract and residue. The ultrasonic extraction in step (1) is carried out at 15-35°C. The centrifugation in step (1) is carried out at 10000 rpm for 5 minutes. The 1:1 ethanol aqueous solution is used to ultrasonically extract at 15-35°C, realizing the synergistic dissolution of pectin (molecular weight 50-100 kDa) and polyphenols (quercetin, kaempferol), and the extraction rate reaches 82-85%.

[0024] Delignification: immerse the residue of step (1) in 100 mL of boiling mixed solution of 2.5 mol / L NaOH and 0.4 mol / L Na2CO3 (molar ratio 25:2) for 1 hour, and then rinse with hot distilled water for 3 times after suction filtration. The boiling treatment for 1 h can remove more than 90% of lignin, and the transparency of the film is increased to 88.5%.

[0025] Preparation of bleached cellulose: immerse the residue of step (2) in 30% H2O2 solution for boiling treatment for 1 hour until the yellow color disappears, and then rinse with cold water to obtain bleached cellulose; the 30% H2O2 treatment makes the whiteness of the cellulose reach 82.3 (CIE LAB value), the crystallinity decreases from 63% to 51%, and the interfacial bonding is enhanced.

[0026] Film formation: mix the bleached cellulose with the extract obtained in step (1), and add 1:3 sodium alginate (binder) and 1:2 glycerol (plasticizer) according to the mass ratio of loquat peel powder, ultrasonically treat at 60°C for 1 h to form a homogeneous slurry, cast on a 14 cm culture dish, and dry at room temperature for 48 h to form a film.

[0027] Example 2: Preparation of biodegradable food preservation film using grape skin Each of the grape skins is dried at 60°C for 8 hours, and then ground through a 100-mesh sieve. 3 g of powder is weighed in each group, and the other steps are operated according to the preparation process of Example 1 (polyphenol extraction→delignification→bleaching→film formation).

[0028] Example 3: Preparation of biodegradable food preservation film using pomegranate peel Each of the pomegranate peels is dried at 60°C for 8 hours, and then ground through a 100-mesh sieve. 3 g of powder is weighed in each group, and the other steps are operated according to the preparation process of Example 1 (polyphenol extraction→delignification→bleaching→film formation).

[0029] Example 4: Preparation of biodegradable food preservation film using hairy bean pod shell The shells of green soybeans were dried at 60°C for 8 hours, and each was pulverized through a 100-mesh screen. 3 g of each powder was weighed, and the other steps were performed according to the preparation procedure of Example 1 (polyphenol extraction → delignification → bleaching → film formation).

[0030] The specific steps are as follows: the ethanol aqueous solution in step (1) of Example 1 was replaced with 10 mL of pure ethanol, and the bioactive compounds of loquat peel powder (0.6 g) were ultrasonically extracted at room temperature for 4 hours. After centrifugation at 10,000 rpm for 10 minutes using a high-speed centrifuge, the extract was filtered, and the extract solution was stored at 2-8°C until testing.

[0031] A gallic acid standard curve was prepared: 0, 0.2, 0.4, 0.6, 0.8, 1, 1.2, and 1.4 mL of a gallic acid standard solution with a concentration of 0.10 mg / mL were taken and placed in 10 mL volumetric flasks, 3 mL of deionized water and 0.5 mL of Folin-Ciocalteu reagent were added to each, and 2 mL of 1.0 mol / L Na2CO3 solution was added to each, which was shaken and allowed to stand for 60 min, and then diluted to the calibration mark with deionized water, and the absorbance was measured at 760 nm. 1 mL of the sample solution was measured by the same method (repeated 3 times). The results were calculated in terms of gallic acid equivalent (GAE) (mg GAE / g Dry Weight).

[0032] The antioxidant activity of the composite film was evaluated by the DPPH radical scavenging assay. Briefly, 1 mL of the ethanol extract of the loquat peel film was mixed thoroughly with 0.5 mL of a 0.1 mM DPPH ethanol solution. The reactants were then sealed and left in the dark for 1 hour. After the reaction, the absorbance of the solution was measured at 517 nm using a UV-vis spectrophotometer. In addition, the absorbance of the blank DPPH solution was measured, and the percentage of DPPH radical scavenging activity was calculated using the following equation:

[0033] where A1 and A0 are the absorbances of the blank and sample, respectively.

[0034] The antioxidant activity of the LPF was estimated using the ABTS radical scavenging assay. 1 g of loquat peel film was placed in a conical flask containing 20 mL of an ethanol solution, which was shaken at 25°C (100 rpm) for 1 hour. Then, 0.1 mL of the extract was mixed thoroughly with 1 mL of an ABTS solution (0.2 mM) and reacted in the dark for 1 hour. Subsequently, the absorbance of the sample was measured at 734 nm. For the control, 0.1 mL of ethanol was used instead of the extract and mixed thoroughly with 2 mL of the ABTS working solution. The formula calculation is as follows:

[0035] Where A1 and A0 are the absorbance of the blank and sample, respectively.

[0036] According to Comparative Example 1, pure ethanol is replaced by ethanol-water with a volume ratio of 1:1. Other steps remain unchanged.

[0037] According to Comparative Example 1, pure ethanol is replaced by ethanol-water with a volume ratio of 3:1. Other steps remain unchanged.

[0038] According to Comparative Example 1, pure ethanol is replaced by ethanol-water with a volume ratio of 1:3. Other steps remain unchanged.

[0039] According to Comparative Example 1, pure ethanol is replaced by pure water. Other steps remain unchanged.

[0040] By comparing Examples 1-5, the effects of water, ethanol, and ethanol-water as three different extraction solvents on the extraction of total phenol content in loquat peel are compared, and the scavenging effects of loquat peel film on ABTS and DPPH free radicals are determined to evaluate its antioxidant activity.

[0041] The test results are shown in Figure 1 As shown in (a), the antioxidant activity of phenolic compounds is attributed to their ability to donate H + ions (hydroxyl groups) and delocalize free electrons. Gallic acid, one of the main phenolic components found in loquat peel, was used as a standard for measuring total phenol content in the test. The results show that alcohol-water extraction > alcohol extraction > water extraction.

[0042] The test results of DPPH and ABTS free radical scavenging activity are shown in Figure 1 (b). The DPPH free radical scavenging rate of loquat peel film is 58.46±0.28%, and the ABTS scavenging rate is 64.72%±1.49%, indicating that the composite film has strong antioxidant properties and can effectively reduce oxidative damage to food. Therefore, loquat peel film can be used as a preservative film to improve the shelf life of food. Folin-Ciocalteu method for measuring total phenol content: Prepare a gallic acid standard solution of 0-0.14 mg / mL, measure the absorbance at 760 nm, and establish a regression equation (y = absorbance, x = concentration). Take 1 mL of supernatant, add 3 mL of deionized water, 0.5 mL of Folin-Ciocalteu reagent, and 2 mL of 10% Na2CO3, and react in the dark for 60 minutes. Measure the absorbance at 760 nm, and calculate the total phenol content according to the standard curve (expressed in gallic acid equivalent GAE, unit mg GAE / g dry weight).

[0043] The final results of each ratio are as follows in Table 1: Ethanol:water volume ratio Total phenol content (mg GAE / g) 1:0 (pure ethanol) 36.8±0.51 3:1 33.04±0.12 1:1 44.12±0.38 1:3 29.38±0.15 0:1 (pure water) 27.5±0.36 Table 1 From the results of Comparative Examples 1-5, it can be seen that different ethanol-water volume ratios have an effect on the total phenols in fruit and vegetable processing by-products (loquat peel). The ethanol concentration gradient experiment shows that 50% ethanol (1:1 volume ratio) has the best synergistic extraction effect on pectin and polyphenols, with a total phenol content of 44.12 mg GAE / g. High concentration of ethanol (>75%) leads to incomplete precipitation of pectin, while pure water extraction efficiency is low.

[0044] Experiment 1: Micro-morphology observation The fresh-keeping film obtained in Example 1 was subjected to scanning electron microscopy analysis test, and the results are shown in Figure 2 . As shown in Figure 2 (a), the microstructure of the fresh-keeping film can be seen, and the surface of the fresh-keeping film presents a smooth, uniform and continuous microstructure. This is due to the high content of soluble biopolymers in loquat peel, pectin and protein. These biopolymers can form a well-dispersed film-forming solution after high-speed homogenization, thereby forming a uniform and stable film structure. As shown in Figure 2 (b), the cross-section of the fresh-keeping film has no cracks and no pores, and the structure is dense. The applicant believes that this is due to the interaction between the high molecular weight polysaccharides in loquat peel and the added glycerol and sodium alginate. This interaction further forms a compact network structure, effectively preventing the penetration of water molecules and oxygen.

[0045] Experiment 2: Structure analysis test The fresh-keeping film obtained in Example 1 was subjected to X-ray diffraction analysis test, and the results are shown in Figure 3 . The structure of the fresh-keeping film can be seen, and the amorphous nature of the fresh-keeping film, which has good compatibility between its components.

[0046] Experiment 3: Functional group interaction test The fresh-keeping film obtained in Example 1 was subjected to infrared analysis test to observe the interaction of each functional group, and the results are shown in Figure 4 . The peak at 3288 cm -1 is attributed to the stretching vibration of hydroxyl and amino groups in biopolymers (pectin, protein, cellulose, hemicellulose, lignin and sodium alginate) and the hydrogen bond interaction between film components. The peaks observed at 2919 cm -1 and 2850 cm -1 are related to the stretching vibration of C-H groups. The peaks at 1602 cm -1 and 1412 cm -1 are related to the C-C stretching vibration of aromatic compounds such as polyphenols. The peaks in the range of 921-1098 cm -1 are attributed to the C-O-H and C-O-C vibration of polysaccharide structure.

[0047] Experiment 4: Thermal stability test The preservative film obtained in Example 1 was subjected to thermogravimetric analysis test, and the thermal stability results are shown in Figure 5 The weight of the preservative film decreases with the increase of temperature, which can be divided into three decomposition stages. The first decomposition stage below 200℃ is mainly due to water loss, resulting in a weight loss of 23%, which is consistent with the high water content of loquat film. The second decomposition stage, with the fastest decomposition rate, occurs between 200-300℃, representing the main stage of mass loss, corresponding to the decomposition of key film components such as glycerol, protein, pectin, cellulose, and sodium alginate. The third decomposition stage above 300℃ is related to the gradual oxidative degradation of residual carbon.

[0048] Experiment 5: Mechanical property test The preservative film obtained in Example 1 was subjected to mechanical property test, and the results are shown in Figure 6 and Figure 7 and Table 2: Sample Opacity (mm -1 )]]> LPF 1.06±0.06 N-LPF 2.74±0.15 Table 2 As shown in Figure 6 , the mechanical property test curve of the preservative film is shown, wherein Figure 6 (a) the effect of glycerol content on the mechanical properties of loquat peel film, Figure 6 (b) the effect of sodium alginate content on the mechanical properties of loquat peel film. As shown in Figure 7 , LPF refers to delignified loquat peel film, and N-LPF refers to non-delignified loquat peel film.

[0049] The mechanical properties of the film were tested according to ASTM D882 standard, and the tensile strength was measured to be 16.3±0.7 MPa, and the elongation at break was 28.1±1.5%, which was significantly better than the commercially available PE film (10.2 MPa, 150%).

[0050] This experiment verified the feasibility of the fractional extraction process described in the present application. Through the continuous treatment of ultrasonic extraction, alkaline delignification, and hydrogen peroxide bleaching in ethanol aqueous solution, high-purity cellulose was successfully separated from loquat peel. The tensile strength of the obtained preservative film reached 16.3 MPa, the opacity value was 1.06±0.06, and the soil degradation rate reached 95.1% in 20 days, proving that the fractional extraction process described in the present application can effectively realize the high-value utilization of fruit and vegetable waste, while meeting the mechanical and environmental requirements of food packaging.

[0051] Experiment 6: Comparison test of properties of preservative films prepared from different fruit and vegetable processing by-products Biodegradable food preservative films of Examples 1-4 were taken, 0.1 g of dried bleached cellulose sample was added to 10 mL of 72% sulfuric acid, and hydrolyzed at 30℃ for 1 hour. Diluted to 4% sulfuric acid concentration, treated in an autoclave (121℃) for 1 hour, and after centrifugation, the glucose content was measured (DNS method). The cellulose content calculation formula: (Note: 0.9 is the conversion coefficient of cellulose into glucose) The cling film was cut into 10 cm x 1 cm strip samples. A universal material testing machine was used, with a tensile rate of 50 mm / min, to record the tensile strength (MPa) and elongation at break (%). The standard is GB / T 1040.3.

[0052] Escherichia coli (ATCC 25922) was selected as the indicator strain. The cling film samples (1 cm x 1 cm) were co-cultured with the bacterial suspension (1 x 10 6 CFU / mL) for 24 hours, and the plate count method was used to calculate the inhibition rate. The standard is GB / T 31402. The results are shown in Figure 8

[0053] The 2 cm x 2 cm film samples were buried in standard soil (humidity 60%, 25°C), and samples were taken every 5 days. After drying and weighing, the mass loss rate was calculated, and the final degradation rate was recorded after 20 days. The comprehensive results are shown in Table 3: Raw material Cellulose content (%) Tensile strength (MPa) Bacteriostatic rate (%) 20-day degradation rate (%) Loquat peel 38.2 16.3 99.2 95.1 Grape skin 34.7 15.8 98.7 93.6 Pomegranate peel 35.9 17.2 99.5 96.3 Hairy bean pod shell 41.5 13.7 97.4 91.2 Table 3 Grape skin contains more tannin, and the bleaching time needs to be extended to 1.5 hours to remove the pigment. The hairy bean pod shell is recommended to be ground to 200 mesh to improve the uniformity of film formation. If acid-resistant bacteria such as Listeria are tested, the medium pH needs to be adjusted to 6.0-7.0. The soil humidity needs to be stable at 60% ± 5%, avoiding dryness leading to a decrease in degradation rate.

[0054] The hairy bean pod shell has the highest cellulose content (41.5%) due to its dense fiber structure. The pomegranate peel film has a tensile strength of 17.2 MPa, as its cellulose crystallinity is moderate (51%), forming a dense network with pectin. The degradation rate of loquat peel and pomegranate peel is >95%, attributed to the high pectin content (8-12%) promoting microbial decomposition.

[0055] The pomegranate peel has a low lignin content (<8%) and a high cellulose yield (35.9%), with a film tensile strength of 17.2 MPa. The hairy bean pod shell needs to be ground to 200 mesh to improve the uniformity of film formation. The performance of films made from different raw materials is ranked as follows: pomegranate peel > loquat peel > grape skin > hairy bean pod shell, verifying the adaptability of the process to various fruit and vegetable by-products. The appearance of the films after formation is shown in Figure 9

[0056] The pomegranate peel and grape skin interacted with Staphylococcus aureus and Escherichia coli for 24 hours, and the plate maps of the control and experimental groups are shown in Figure 8 (a), and their inhibition rates reached 100%, as shown in Figure 8 (b).

[0057] ​​The sample group and the control group were placed in a full-temperature shaking flask cabinet (37°C, 120 r / min) for 12 h, 24 h of shaking to allow the sample to be in full contact with the bacterial solution and to be wetted. After the irradiation, 100 μL of the bacterial solution was taken from the ep tube and diluted, and 100 μL of the diluted bacterial solution was added dropwise to the agar medium and evenly coated, and placed in a constant-temperature incubator for 24 h of culture. The antibacterial effect was evaluated according to the number of bacterial colonies in the culture dish.

[0058] The results are shown in Figure 10 (a) and (b), respectively. As shown in (a), many bacterial colonies appeared on the agar plate in the control, whereas few colonies were observed on the plate to which the persimmon peel film was added, and the inhibition rates of both bacteria were >99%, as shown in Figure 10 (b). The results show that the preservative film prepared in Example 1 significantly inhibited the survival of bacteria and had good antibacterial performance. The applicant believes that this is because the persimmon peel contains a large amount of phenolic compounds, and phenol can inhibit bacteria by affecting the cell wall.

[0059] Experiment 7: Soil degradation performance The biodegradability of the persimmon peel preservative film was evaluated by burying commercially available polyethylene (PE) plastic and the preservative film prepared in Example 1 in the same location (about 5 cm deep) in the soil and observing the changes in appearance over time. The 2 cm x 2 cm preservative film sample was weighed (initial mass m0), buried in the soil 5 cm deep, and sampled every 7 days. The sample was taken out, washed with distilled water, and dried at 60°C to a constant weight (m1). The degradation rate (%) = (m0 - m1) / m0 x 100%. The results are shown in Figure 11 (b). It can be observed that the persimmon peel preservative film was almost completely degraded after 20 days. This is attributed to the direct attack and digestion of the pectin and cellulose in the bioplastic by microorganisms (such as bacteria and fungi) in the soil. As shown in Figure 11 (a), in contrast, the PE maintained its original form without any degradation throughout the observation period. This indicates that the persimmon peel preservative film does not put pressure on the environment.

[0060] Experiment 8: Preservative effect on different melons and fruits The Hami melons were peeled and cut, and randomly placed in plastic boxes. The samples were divided into three groups: (1) a blank group (untreated, control group), (2) a control group (wrapped with a PE film), and (3) an experimental group (wrapped with the preservative film prepared in Example 1). All groups were stored at room temperature indoors. The appearance of the Hami melons was measured and recorded every day. The quality evaluation criteria were: weight loss rate: daily weighing, calculation of (initial mass - current mass) / initial mass x 100%.

[0061] The results are shown in Figure 12 It can be observed that all groups of Hami melons showed similar degrees of spoilage after the preservation treatment.Figure 12 As shown in (PE), the cantaloupes in the PE group developed mold and bacterial infections after 5 days of storage, while the uncovered group did not spoil, but showed significant surface moisture loss. However, as... Figure 12 As shown in the (LPF) data, the cantaloupes in the loquat peel preservation film group maintained their bright color. Notably, after 7 days of storage, the cantaloupes treated with the loquat peel preservation film exhibited excellent color retention and fruit integrity throughout the storage period. These results demonstrate that the loquat peel preservation film has excellent preservation capabilities for fresh-cut cantaloupes, which the applicant attributes to the antibacterial and antioxidant properties of the loquat peel preservation film observed in previous studies.

[0062] The results are as follows Figure 13 As shown, the overall sensory indicators of packaged grapes were significantly better than those of unpackaged grapes, with grapes packaged with loquat skin film receiving the best sensory evaluation. The images visually depict grape decay; grapes in the unpackaged group showed obvious wrinkling and mold spots, while grapes in the PE film group began to show mold spots on the surface on the fifth day. In contrast, the grapes in the loquat skin film group had intact surfaces without visible shrinkage or wrinkles. Compared to the unpackaged and PE film control groups, the loquat skin film significantly delayed post-harvest quality deterioration, indicating that the loquat skin film also possesses antibacterial activity, effectively isolating bacterial infection from the external environment and thus extending the shelf life of grapes.

[0063] The specific results are shown in Table 4 below: Fruit type Fresh-keeping index Uncoated film PE group Film of the present application Hami melon Weight loss rate (7 days) 25.71%±0.19% 20.36±0.53% 18.38±0.56% Grape Weight loss rate (7 days) 7.83±1.15% 4.83±0.59% 2.40±0.16% Table 4 The loquat peel film extended the shelf life of fresh-cut cantaloupe from 3 days to 7 days, with a lower weight loss rate (18.38%) compared to PE film (20.36%). Preservation experiments on grapes showed that its antioxidant and antibacterial functions are universally applicable, achieving a sensory score of 7.8 out of 9, meeting the commercial preservation needs of various fresh fruits and vegetables.

[0064] Unless otherwise specified, all technologies mentioned above refer to existing technologies.

[0065] As described above, although the invention has been shown and described with reference to specific preferred embodiments, it should not be construed as limiting the invention itself. Various changes in form and detail may be made without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

1. A method for preparing a biodegradable food preservation film based on fruit and vegetable processing by-products, characterized in that, Includes the following steps: (1) Polyphenol extraction: The powder of fruit and vegetable processing by-products was mixed with an ethanol aqueous solution with a volume ratio of 1:1 at a mass-volume ratio of 3:50 g / ml, and ultrasonically extracted for 4 hours. After centrifugation, the extract containing polyphenols and the residue were separated. (2) Removal of lignin: Immerse the residue from step (1) in a mixed solution of NaOH and Na2CO3 with a molar ratio of 25:2, boil for 1 hour, filter, and wash the filter residue with hot distilled water. (3) Preparation of bleached cellulose: The filter residue from step (2) was boiled in 30% H2O2 solution until the yellow color disappeared, and then rinsed with cold water to obtain bleached cellulose; (4) Film preparation: Bleached cellulose is mixed with the extract obtained in step (1). Based on the mass of the fruit and vegetable processing by-product powder used in step (1), 3 times the amount of sodium alginate and 2 times the amount of glycerol are added, ultrasonically dispersed to form a slurry, cast into a film and dried.

2. The preparation method according to claim 1, characterized in that, The fruit and vegetable processing by-products are at least one of loquat peel, grape peel, pomegranate peel, or edamame pod shells, which are dried at 60°C for 8 hours and then ground through a 100-mesh sieve.

3. The preparation method according to claim 1, characterized in that, In step (1), ultrasonic extraction is performed at 15-35℃.

4. The preparation method according to claim 1, characterized in that, In step (1), the centrifugation conditions are 10,000 rpm for 5 minutes.

5. The preparation method according to claim 1, characterized in that, In step (4), ultrasonic dispersion is carried out at 50-70℃.

6. The preparation method according to claim 1, characterized in that, In step (4), the slurry is cast into a film and then dried at room temperature for 48 hours.

7. The food preservation film prepared by the method according to any one of claims 1-6, characterized in that, The plastic wrap has a thickness of 0.05-0.15 mm, a tensile strength ≥15 MPa, and an elongation at break ≥25%.

8. The application of the food preservation film prepared by the method according to any one of claims 1-6 in the preservation of fresh fruits and vegetables, characterized in that, Used to extend the shelf life of fruits and vegetables, with a freshness period of ≥7 days at room temperature.

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