Tea tree essential oil-thyme essential oil-chitosan composite membrane as well as preparation method and application thereof

Abstract

The invention discloses a tea tree essential oil-thyme essential oil-chitosan composite film as well as a preparation method and application thereof, and belongs to the technical field of preservative films. The preparation method of the tea tree essential oil-thyme essential oil-chitosan composite membrane comprises the following steps: dissolving polyvinyl alcohol in water to obtain a polyvinyl alcohol solution; the preparation method comprises the following steps: adding glacial acetic acid, chitosan, a plasticizer and an emulsifier into polyvinyl alcohol, uniformly mixing, adding tea tree essential oil and thyme essential oil, homogenizing, mixing, and degassing to obtain a film forming solution; and drying the film forming liquid to form a film, so as to obtain the tea tree essential oil-thyme essential oil-chitosan composite film. The prepared tea tree essential oil-thyme essential oil-chitosan composite film has an excellent antibacterial effect, and the preservation time of sweet cherries is prolonged.

Description

Technical Field

[0001] The present invention relates to the technical field of fresh-keeping films, and more particularly to a tea tree essential oil-thyme essential oil-chitosan composite film and a preparation method and application thereof. Background Art

[0002] Sweet cherries, also known as cherries and big cherries, belong to the genus Prunus in the Rosaceae family, Rosales. Sweet cherries are not only large, fleshy, delicious, and brightly colored, but also rich in iron, vitamins, organic acids, and other nutrients. Sweet cherries have an extremely thin skin, soft flesh, and are tender and juicy. Since they are harvested during the hot and rainy season, they are susceptible to microbial contamination after harvest, including Penicillium, Escherichia coli, and yeasts. This leads to dehydration, browning, and rot, reducing the nutritional value and quality of the cherries, shortening their shelf life and causing significant economic losses, which is detrimental to the development of the sweet cherry industry.

[0003] At present, polyethylene cling film is a widely used preservation material. Its main component is polyethylene, which has the advantages of being non-toxic, odorless, low-temperature resistant and chemically stable. However, low-density polyethylene has poor permeability to water vapor and air, and plants still undergo respiration, transpiration and enzyme activity after picking. Therefore, the respiration and transpiration of sweet cherries during storage will cause water mist to form on the inner surface of the polyethylene cling film, resulting in condensation, causing changes in the humidity inside the bag, thereby nourishing pathogens, causing excessive microbial reproduction and a high fruit rot rate, losing the preservation effect and shortening the shelf life. Summary of the Invention

[0004] To address the above problems, the present invention provides a tea tree essential oil-thyme essential oil-chitosan composite film, a preparation method, and an application thereof. The tea tree essential oil-thyme essential oil-chitosan composite film prepared by the present invention has an excellent antibacterial effect and improves the preservation time of sweet cherries.

[0005] The first object of the present invention is to provide a method for preparing a tea tree essential oil-thyme essential oil-chitosan composite film, comprising the following steps: Polyvinyl alcohol is dissolved in water to obtain a polyvinyl alcohol solution.

[0006] Glacial acetic acid, chitosan, a plasticizer and an emulsifier are added to polyvinyl alcohol and mixed evenly, and then tea tree essential oil and thyme essential oil are added, mixed evenly and then degassed to obtain a film-forming liquid.

[0007] The film-forming liquid is dried to form a film to obtain a tea tree essential oil-thyme essential oil-chitosan composite film.

[0008] It should be noted that when drying and film-forming, the drying temperature is mainly determined by the drying time and the volatility of the essential oil. If the drying temperature is too low, the drying effect and film-forming effect will be poor; if the drying temperature is too high, the essential oil will volatilize severely, affecting the antibacterial effect. Preferably, the drying film of the present invention is dried at 35°C for 4 hours.

[0009] In a preferred embodiment of the present invention, the ratio of chitosan to tea tree essential oil is 1 g:1 mL to 3 mL; for example, the ratio of chitosan to tea tree essential oil is 1 g:1 mL, 1 g:2 mL, 1 g:3 mL, etc.

[0010] In a preferred embodiment of the present invention, the volume ratio of tea tree essential oil to thyme essential oil is 1:1.

[0011] In a preferred embodiment of the present invention, the ratio of polyvinyl alcohol to glacial acetic acid is 3 g:1 mL, and the mass ratio of polyvinyl alcohol to chitosan is 6:1.

[0012] In a preferred embodiment of the present invention, the concentration of polyvinyl alcohol in the polyvinyl alcohol solution is 3%.

[0013] In a preferred embodiment of the present invention, the ratio of chitosan to plasticizer is 1 g:4 mL.

[0014] In a preferred embodiment of the present invention, the plasticizer is propylene glycol, which is low in toxicity and inexpensive.

[0015] In a preferred embodiment of the present invention, the emulsifier is Tween 80. The Tween 80 used in the present invention has low toxicity and low price.

[0016] The second object of the present invention is to provide a tea tree essential oil-thyme essential oil-chitosan composite film prepared by the above preparation method.

[0017] The third object of the present invention is to provide the use of the tea tree essential oil-thyme essential oil-chitosan composite film in the preservation of sweet cherries.

[0018] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention uses glacial acetic acid as a solvent to dissolve chitosan, polyvinyl alcohol and chitosan as raw materials for preparing a composite film, tea tree essential oil and thyme essential oil cooperate with each other to inhibit pathogens, and a plasticizer and an emulsifier are added to assist the tea tree essential oil and thyme essential oil in mixing evenly with water, and the homogenous mixing makes them better emulsified, and finally dried to form a film to obtain a tea tree essential oil-thyme essential oil-chitosan composite film. The prepared tea tree essential oil-thyme essential oil-chitosan composite film reduces the oxygen concentration, increases the carbon dioxide concentration, inhibits plant respiration and enzyme activity, thereby inhibiting the loss of water in sweet cherries, delaying spoilage caused by bacteria, yeast and mold, inhibiting the intensity of respiration, slowing down the decline of soluble solids, measurable acid and vitamin C content in sweet cherries, maintaining their nutritional value, improving the preservation effect, and extending the storage time of sweet cherries.

[0019] (2) During the preparation process of the present invention, chitosan has good film-forming properties and high safety, but when used alone, it has poor fluidity and is expensive. The addition of polyvinyl alcohol helps to reduce costs, and the addition of tea tree essential oil and thyme essential oil can increase fluidity and enhance mechanical properties.

[0020] (3) When the volume ratio of tea tree essential oil to thyme essential oil is 1:1, the tensile strength and flexibility of the tea tree essential oil-thyme essential oil-chitosan composite film are appropriate, and it has a good preservation effect on sweet cherries. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The effect of chitosan composite film with different essential oil addition amounts on the total colony count of sweet cherry.

[0022] Figure 2 Effects of chitosan composite films with different essential oil addition amounts on the number of sweet cherry mold and yeast.

[0023] Figure 3 Effects of chitosan composite films with different essential oil addition amounts on soluble solids of sweet cherry.

[0024] Figure 4 Effects of chitosan composite films with different essential oil addition amounts on the measurable acidity content of sweet cherry.

[0025] Figure 5 Effects of chitosan composite films with different essential oil addition amounts on the vitamin C content of sweet cherry.

[0026] Figure 6 Effect of chitosan composite film with different essential oil addition amounts on the weight loss rate of sweet cherry.

[0027] Figure 7 Effects of chitosan composite films with different essential oil addition amounts on the decay rate of sweet cherries.

[0028] Figure 8The sensory change trend of sweet cherry in chitosan composite film with different essential oil addition amounts. DETAILED DESCRIPTION

[0029] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0030] The fresh sweet cherries used in this invention were obtained from the Yuanhua Planting Professional Cooperative in Hanyuan County, Ya'an City, Sichuan Province, and the variety was Black Pearl. Tea tree essential oil was purchased from Yunnan Jingyu Biological Co., Ltd.; thyme essential oil and chitosan were purchased from Shanghai MacLean Biochemical Technology Co., Ltd.; chitosan had a molecular weight of 150,000. Plate count agar and Bengal rose agar were purchased from Haibo Biotechnology Co., Ltd. in Qingdao High-Tech Industrial Park. Tween 80, sodium chloride, polyvinyl alcohol 1799, glacial acetic acid, and propylene glycol were all analytically pure and purchased from Chengdu Kelong Chemical Co., Ltd.; 2,6-dichloroindophenol was analytically pure and purchased from Tanmo Quality Inspection Technology Co., Ltd. Staphylococcus aureus ATCC 6538, batch number C0683KJ, was purchased from Guangdong Huankai Biotechnology Co., Ltd.; Pseudomonas aeruginosa ATCC 9027, batch number 230918S04, was purchased from Guangdong Huankai Biotechnology Co., Ltd.; and Candida albicans ATCC 10231, batch number J0070DX, was purchased from Guangdong Huankai Biotechnology Co., Ltd.

[0031] To investigate the antibacterial and bacteriostatic effects of tea tree and thyme essential oils, a total of 0.21 g of tea tree and thyme essential oils were weighed and tested at mass ratios of 1:1, 1:2, and 2:1 against Staphylococcus aureus, Pseudomonas aeruginosa, and Candida albicans. The specific steps are as follows.

[0032] Transfer the revived bacteria to a plate, then pick single colonies of Staphylococcus aureus, Pseudomonas aeruginosa, and Candida albicans from the plate to make bacterial suspensions, dilute them with sterile saline to 0.5 McFarland units of the test bacterial suspension, and contain a bacterial count equivalent to 1.0~1.5×108 CFU / mL for later use.

[0033] In order to investigate the minimum inhibitory concentration and minimum bactericidal concentration of thyme essential oil and tea tree essential oil against Staphylococcus aureus, Pseudomonas aeruginosa and Candida albicans, and to examine whether the two essential oils have synergistic effects on the inhibition and bactericidal effects of the three bacteria when mixed in a certain proportion.

[0034] The experiments were divided into three groups based on the bacteria being examined, each containing 50 sterilized test tubes. For the Staphylococcus aureus and Pseudomonas aeruginosa groups, 1 mL of brain-heart extract liquid culture medium was added to each tube. For the Candida albicans group, 1 mL of Sabouraud dextrose liquid culture medium was added to each tube. Each group of fifty test tubes was divided into groups of ten. To the first tube, 1 mL of the prepared essential oil emulsions of varying proportions was added and mixed. Then, 1 mL was pipetted from the first tube into the second tube and mixed. The dilution series was continued until the tenth tube was reached. 1 mL of the tenth tube was then discarded, forming a two-fold dilution series with concentrations of 11.6667 mg / mL, 5.8333 mg / mL, 2.9167 mg / mL, 1.4584 mg / mL, 0.7292 mg / mL, 0.3646 mg / mL, 0.1823 mg / mL, 0.09114 mg / mL, 0.04557 mg / mL, and 0.02279 mg / mL, numbered sequentially. Finally, 0.1 mL of the corresponding prepared bacterial suspension was added to each test tube. For example, 0.1 mL of S. aureus was added to the S. aureus group. The same procedure was performed for the P. aeruginosa and Candida albicans groups. Incubate the Staphylococcus aureus and Pseudomonas aeruginosa groups at 36°C for 24-48 hours, and the Candida albicans group at 26°C for 48-72 hours. Transfer the clarified tubes to the corresponding plates and observe for bacterial growth to determine the most inhibitory concentration and the minimum bactericidal concentration.

[0035] The antibacterial results against Staphylococcus aureus are as follows: in the experiments with adding tea tree essential oil alone or with a mass ratio of 2:1 of tea tree essential oil and thyme essential oil, the first to fifth branches were completely clear, and the sixth branch began to become turbid; in the experiments with adding thyme essential oil alone, with a mass ratio of 1:1 of tea tree essential oil and thyme essential oil, and with a mass ratio of 1:2 of tea tree essential oil and thyme essential oil, the first to seventh branches were completely clear, and the eighth branch began to become turbid.

[0036] The results showed that the minimum inhibitory concentration of tea tree essential oil alone or the mass ratio of tea tree essential oil to thyme essential oil of 2:1 against Staphylococcus aureus was 0.7292 mg / mL, and the minimum bactericidal concentration was 1.4584 mg / mL.

[0037] The minimum inhibitory concentration of thyme essential oil alone, tea tree essential oil and thyme essential oil in a mass ratio of 1:1, and tea tree essential oil and thyme essential oil in a mass ratio of 1:2 against Staphylococcus aureus was 0.1823 mg / mL, and the minimum bactericidal concentration was 0.3646 mg / mL.

[0038] Regarding the antibacterial results against Candida albicans, the strain was generally less sensitive to both essential oils than Staphylococcus aureus, and was relatively more sensitive to thyme oil. In the experiments with tea tree oil alone and a 2:1 ratio of tea tree oil to thyme oil, the first three tubes were completely clear, while the fourth tube began to become turbid.

[0039] In the experiments of adding thyme essential oil alone, tea tree essential oil and thyme essential oil in a mass ratio of 1:1, and tea tree essential oil and thyme essential oil in a mass ratio of 1:2, the first to fifth tubes were completely clear, and the sixth tube began to become turbid.

[0040] The results showed that the minimum inhibitory concentration (MIC) against Candida albicans for tea tree oil alone and a 2:1 ratio of tea tree oil to thyme oil was 2.9167 mg / mL, and the minimum bactericidal concentration was 5.8333 mg / mL. The minimum inhibitory concentration (MIC) against Candida albicans for thyme oil alone, a 1:1 ratio of tea tree oil to thyme oil, and a 1:2 ratio of tea tree oil to thyme oil was 0.7292 mg / mL, and the minimum bactericidal concentration was 1.4584 mg / mL.

[0041] Regarding the antibacterial effects on Pseudomonas aeruginosa, the two essential oils showed relatively weak antibacterial activity against the bacteria. In the experiments with tea tree oil alone and a 2:1 ratio of tea tree oil to thyme oil, the first three tubes were completely clear, but the fourth tube began to become cloudy.

[0042] In the experiments of adding thyme essential oil alone, tea tree essential oil and thyme essential oil in a mass ratio of 1:1, and tea tree essential oil and thyme essential oil in a mass ratio of 1:2, the first to second tubes were completely clear, and the third tube began to become turbid.

[0043] The results show that the minimum inhibitory concentration of tea tree essential oil alone and the mass ratio of tea tree essential oil to thyme essential oil of 2:1 against Pseudomonas aeruginosa is 2.9167 mg / mL, and the minimum bactericidal concentration is 5.8333 mg / mL. The minimum inhibitory concentration of thyme essential oil alone, the mass ratio of tea tree essential oil to thyme essential oil of 1:1, and the mass ratio of tea tree essential oil to thyme essential oil of 1:2 against Pseudomonas aeruginosa is 5.8333 mg / mL, and the minimum bactericidal concentration is 11.6667 mg / mL. Staphylococcus aureus is relatively sensitive to both essential oils, while Pseudomonas aeruginosa is relatively sensitive to tea tree essential oil, and Candida albicans is relatively sensitive to thyme essential oil. In summary, it can be seen that the tea tree essential oil and thyme essential oil selected by the present invention have excellent bactericidal and antibacterial effects. Adding tea tree essential oil and thyme essential oil in a 1:1 ratio into the composite film can effectively and more comprehensively ensure the antibacterial and bactericidal effect of the film.

[0044] It should be noted that, when carrying out bactericidal and antibacterial experiments, it was found in 5 groups of experiments that tea tree essential oil and thyme essential oil can be more comprehensively and collaboratively sterilized or antibacterial in a mass ratio of 1: 1. Considering that when preparing a composite film, the addition ratio of the two essential oils is either a mass ratio or a volume ratio, as long as they are added to the composite film in a ratio of 1: 1, when the two essential oils reach a certain addition concentration, an antibacterial effect can be achieved. Based on this, the present invention, when subsequently preparing tea tree essential oil-thyme essential oil-chitosan composite films, adds them in a volume ratio of 1: 1.

[0045] Example 1 Weigh 3g polyvinyl alcohol 1799, add boiling water 100mL, 80 DEG C of water baths are stirred until polyvinyl alcohol 1799 is completely dissolved, add 1mL glacial acetic acid, 0.5g chitosan, stirring and dissolving, add propylene glycol 2mL, Tween 80 1mL, add the tea tree essential oil of 0.5mL and the thyme essential oil of 0.5mL of volume, stir and mix 5min at homogenizer 900r / min, deaerate 1h, make film-forming liquid.Take 50mL film-forming liquid and pour in 900mm film-forming plate, be placed in 35 DEG C of drying ovens and dry 4h, after film is cooled to room temperature, uncover film, obtain tea tree essential oil-thyme essential oil-chitosan composite film, and be placed in normal temperature desiccant for standby use.The tea tree essential oil-thyme essential oil-chitosan composite film prepared by the present embodiment is designated as 1%TTO-TEO.

[0046] Example 2 Weigh 3g polyvinyl alcohol 1799, add boiling water 100mL, 80 DEG C of water baths are stirred until polyvinyl alcohol 1799 is completely dissolved, add 1mL glacial acetic acid, 0.5g chitosan, stirring and dissolving, add propylene glycol 2mL, Tween 80 2mL, add the tea tree essential oil of 1.0mL and the thyme essential oil of 1.0mL of volume, stir and mix 5min at homogenizer 900r / min, deaerate 1h, make film-forming liquid. Take 50mL film-forming liquid and pour in 900mm film forming plate, be placed in 35 DEG C of drying ovens and dry 4h, after film is cooled to room temperature, uncover film, obtain tea tree essential oil-thyme essential oil-chitosan composite film, and be placed in normal temperature desiccator for standby use. The tea tree essential oil-thyme essential oil-chitosan composite film prepared by the present embodiment is designated as 2%TTO-TEO.

[0047] Example 3 Weigh 3g polyvinyl alcohol 1799, add boiling water 100 mL, 80 DEG C of water baths are stirred until polyvinyl alcohol 1799 is completely dissolved, add 1mL glacial acetic acid, 0.5 g chitosan, stirring and dissolving, add propylene glycol 2mL, Tween 80 3mL, add the tea tree essential oil of 1.5mL and the thyme essential oil of 1.5mL of volume respectively, stir 900r / min mixing 5min in homogenizer, deaerate 1h, make film-forming liquid. Take 50mL film-forming liquid and pour into 900mm film-forming plate, be placed in 35 DEG C of drying ovens and dry 4 h, after film is cooled to room temperature, uncover film, obtain tea tree essential oil-thyme essential oil-chitosan composite film, and be placed in normal temperature desiccator for standby use. The tea tree essential oil-thyme essential oil-chitosan composite film prepared by the present embodiment is designated as 3%TTO-TEO.

[0048] The tea tree essential oil-thyme essential oil-chitosan composite film prepared in the example is characterized and tested below.

[0049] Mechanical Properties: Tensile strength and nominal strain at break were determined according to the methods for determining the tensile properties of plastics specified in GB / T 1040.3-2006, "Determination of Tensile Properties of Plastics - Part 3: Test Conditions for Thin Plastics and Sheets." The composite film was mounted on a universal testing machine with an initial gauge length of 50 mm and a test rate of 50 mm / min. The tensile load and nominal strain at break were measured. Five sets of tests were performed and the average value was calculated.

[0050] Fresh-keeping performance: Fresh sweet cherries were picked, and those with plump fruits, uniform size and intact appearance were selected. They were placed in food-grade open polyester fresh-keeping boxes, each box weighing about 250g, and randomly divided into five groups, each with 57 boxes. Among them, Group 1 used the 1% TTO-TEO sealed fresh-keeping box prepared in Example 1, Group 2 used the 2% TTO-TEO sealed fresh-keeping box prepared in Example 2, Group 3 used the 3% TTO-TEO sealed fresh-keeping box prepared in Example 3, Group 4 used a food-grade polyethylene fresh-keeping film sealed fresh-keeping box, recorded as polyethylene fresh-keeping, and Group 5 did not use a composite film sealed fresh-keeping box, recorded as a blank control group. The sweet cherries in each group were stored in the refrigerator at 4±1°C, and the microbial count, physical and chemical properties and sensory quality were measured on the 0th, 2nd, 4th, 6th, 8th, 10th, 12th, 14th and 16th days of storage.

[0051] Determination of total colony count: According to GB4789.2-2022 "National Food Safety Standard - Microbiological Examination of Food - Determination of Total Colony Count", 25.0 g of sample was randomly weighed from each group of samples and added to 225 mL of normal saline. After mixing, 1.00 mL was aspirated onto two plates respectively, and about 20 mL of plate count agar at about 45°C was added. After cooling and solidification, the plates were inverted and incubated in an incubator for 48 hours before counting.

[0052] Determination of mold and yeast: According to GB4789.15-2016 "National Food Safety Standard - Microbiological Examination of Food - Count of Molds and Yeasts", 25.0 g of sample was randomly weighed from each group and added to 225 mL of normal saline. After mixing, 1.00 mL was pipetted onto two plates, and about 20 mL of Bengal red agar at about 45°C was added. After cooling and solidification, the plates were inverted and incubated at 28°C in an incubator for 5 days before counting.

[0053] Determination of soluble solids content: Soluble solids were determined based on the study "Effects of Cutting Methods on Storage Quality and Microbial Content of Fresh-cut Cucumbers" by Wang Haidan et al., published in Northern Horticulture, Issue 10, 2022. Specifically, pitted and pulped sweet cherries were filtered through four layers of gauze, and the filtrate was used to determine the soluble solids content using a digital Abbe refractometer. Three parallel experiments were performed for each group, and the average value was calculated.

[0054] Determination of quantifiable acid content: Remove the pits of sweet cherries, crush them with a tissue masher, and mix thoroughly. Weigh 5 g of the cherry pulp using an electronic balance and place it in a 150 mL Erlenmeyer flask. Add 10 mL of 80°C CO2-free water and mix thoroughly. Boil in a boiling water bath for 30 min, shaking three times to dissolve all the organic acids. Remove the cherries, cool to room temperature, and dilute to 50 mL with CO2-free water. Filter through fast filter paper and collect the filtrate. Accurately pipette 10.0 mL of the filtrate and add 30 mL of CO2-free water. Start the fully automatic potentiometric titrator with a stirrer and rapidly titrate with 0.09938 mol / L NaOH standard titrant until the pH reaches 8.2. Record the volume of NaOH standard titrant consumed. Take three replicate samples for each control group, and perform a blank test simultaneously. Quantifiable acid is expressed as the percentage of malic acid. For details, refer to GB 12456-2021 National Food Safety Standard, Determination of Total Acids in Foods.

[0055] Vitamin C content was determined using the 2,6-dichlorophenol indophenol titration method, as described in "Guide to Postharvest Physiology and Biochemistry of Fruits and Vegetables" published by Cao Jiankang et al. in 2007. In this experiment, vitamin C content was determined only in sweet cherry pulp. Three replicates were performed for each treatment group, and the average results were reported as mg / 100g.

[0056] Determination of weight loss rate: The weight loss rate of sweet cherries during storage was determined by weighing method and calculated according to formula (1).

[0057] (1).

[0058] Determination of rot rate: During the storage of sweet cherries, the number of rotten fruits was regularly observed and counted, and calculated according to formula (2).

[0059] (2).

[0060] Sensory evaluation: Sensory evaluators were recruited and trained according to the methods outlined in GB / T 16291.1-2012, "General Guidelines for the Selection, Training, and Management of Sensory Analysis Evaluators - Part 1: Selection of Evaluators." A final sensory evaluation panel of 10 individuals of varying genders and ages was selected. A sensory evaluation was conducted on each group of sweet cherries during storage using a 100-point scale and the sensory evaluation criteria in Table 1. The higher the score, the better the quality of the sweet cherries. A score below 60 points is considered to have lost commercial value, while a score below 40 points is considered to have lost edible value.

[0061] Table 1 Sensory evaluation criteria for sweet cherries Mechanical properties are essential for all packaging films. Tensile strength and nominal strain at break are important criteria for evaluating the mechanical properties of packaging films, reflecting the tensile strength and flexibility of the composite film. Using the tensile strength and nominal strain at break of the composite films as indicators, the composite film preparation process was evaluated to obtain the composite film with the best overall performance. The mechanical property analysis results of the three tea tree oil-thyme oil-chitosan composite films prepared in Examples 1-3 with different essential oil addition levels are shown in Table 2.

[0062] Table 2 shows that the tensile strength of the composite film increases and then decreases with increasing essential oil addition, reaching its maximum tensile strength at 2% essential oil. This may be because an appropriate amount of essential oil increases fluidity and improves the ductility of the film. However, an excessive amount of essential oil may increase intermolecular repulsion, resulting in larger intermolecular gaps and a decrease in tensile strength. The nominal fracture strength of the composite film should gradually decrease with increasing essential oil addition, reaching its maximum at 1%. This may be due to the hydrophobicity of the essential oil, which alters the intermolecular forces and molecular arrangement. Mechanical property analysis shows that the composite film exhibits a certain degree of ductility and flexibility.

[0063] Table 2 Mechanical properties of chitosan composite films with different essential oil addition amounts The total number of colonies is a sign of the degree of food contamination. Observing the reproduction dynamics of bacteria during food storage can provide a basis for food hygiene evaluation. Figure 1As shown in the data, the initial total colony count of sweet cherries during storage was 3.92 log CFU / g. With increasing storage time, the total colony count increased, but decreased in the later stages of storage. This is because spoilage leads to nutrient loss and water loss, which are not conducive to bacterial growth and reproduction. The blank control group had the fastest colony growth and reproduction, reaching a total colony count of 7.20 log CFU / g on the 12th day. All film-sealed treatments demonstrated a moderate antibacterial effect, with the composite film outperforming standard polyethylene cling film. The 2% and 3% TTO-TEO groups showed the greatest antibacterial and preservation effects, effectively extending the storage life of sweet cherries. This may be due to the antimicrobial activity of the plant essential oils and chitosan in the composite films.

[0064] Yeast and mold are widely present in the natural environment and are one of the causes of food spoilage. When yeast and mold contaminate food, they not only change the color, aroma, taste, and shape of the food, but also cause the loss of nutrients, reduce its edible value, and even produce toxins that endanger human health. Therefore, yeast and mold are indicator bacteria for evaluating food hygiene quality. Figure 2 As shown, the initial total mold and yeast count of sweet cherries was 0.69 log CFU / g. With extended storage, the total number of molds and yeasts initially increased and then decreased. This is likely due to the loss of nutrients and water, as well as changes in the environmental pH caused by metabolites from previous growth, which were not conducive to continued yeast and mold growth. On the 12th day, the total number of molds and yeasts reached its peak, reaching 4.3 log CFU / g in the blank control group and the standard polyethylene plastic wrap group, 3.3 log CFU / g in the 1% TTO-TEO group, 2.5 log CFU / g in the 2% TTO-TEO group, and 2.6 log CFU / g in the 3% TTO-TEO group. The addition of tea tree and thyme essential oils effectively inhibited mold and yeast growth, with the 2% TTO-TEO group showing the strongest inhibitory effect and the best preservation effect.

[0065] Figure 3 The effect of chitosan composite films with different essential oil addition amounts on the soluble solids of sweet cherry. Figure 3It can be seen that the soluble solids content of sweet cherries showed a downward trend during storage. This is because some soluble solids participate in the body's respiration. Cherries are non-climacteric fruits and produce very little ethylene, but the fruit stalks are climacteric. As storage time increases, ethylene production is primarily generated by the fruit stalks, promoting respiration and leading to an overall decrease in the soluble solids content of sweet cherries. The loss of soluble solids content in the composite film group was slower than that in the blank control group and the conventional polyethylene cling film group. The soluble solids content of sweet cherries sealed with 2% TTO-TEO showed the slowest loss, reaching 17.60% on the 16th day, an increase of 4.60 percentage points and 3.60 percentage points compared to the blank control group and the conventional polyethylene cling film group, respectively. This may be because the composite film can effectively reduce oxygen concentration and increase carbon dioxide concentration, inhibiting plant respiration, reducing soluble solids loss, and extending shelf life.

[0066] Figure 4 The effect of different essential oil addition amounts of chitosan composite films on the measurable acid content of sweet cherry. Figure 4 As can be seen, the detectable acid content decreased with extended storage time, with the composite film-sealed group maintaining a relatively high level of detectable acid. On the eighth day, the difference in detectable acid content began to increase. The blank control group experienced the most significant decrease in detectable acid, from 0.87% to 0.51%. The 2% TTO-TEO group showed the most gradual decrease in detectable acid throughout storage, from 0.87% to 0.74%. This is because the composite film-sealed treatment inhibited cherry respiration and metabolism, reducing the consumption of organic acids and maintaining the sweet cherry flavor during storage.

[0067] Vitamin C is one of the most important nutrients and antioxidants in fruits and vegetables. It plays a variety of biological functions in the human body and can eliminate free radicals produced in it. The effects of chitosan composite films with different essential oil additions on the vitamin C content in sweet cherries are shown in Figure 2. Figure 5 As shown, vitamin C content in sweet cherries shows a downward trend during postharvest storage. This is because organic acid vitamin C is often used as a primary substrate for respiration and other metabolic processes. During storage, the vitamin C content in each experimental group was significantly higher than that in the blank control group, indicating that the composite film sealing treatment inhibits the decomposition of vitamin C in sweet cherries. This is because the composite film's oxygen barrier properties reduce the oxygen concentration within the fruit, reducing its respiration rate and related enzyme activity, slowing the fruit's own water transpiration and metabolism, thereby reducing vitamin C decomposition. The tea tree oil-thyme oil-chitosan composite film exhibits superior gas and moisture barrier properties compared to conventional polyethylene cling film, further improving the storage quality of sweet cherries. The 2% TTO-TEO group exhibited the lowest vitamin C loss and the best preservation effect.

[0068] Figure 6 The effect of different essential oil addition amounts on the weight loss rate of sweet cherry. Figure 6 It can be seen that during the storage of sweet cherries, the weight loss rate of each group showed an upward trend. The blank control group had the highest weight loss rate, with a weight loss rate of 28.29% on the 16th day. Compared with the blank control group, the composite film covering treatment effectively reduced weight loss, and the composite film's water retention effect was better than that of ordinary polyethylene cling film. The 2% TTO-TEO group had a weight loss rate of 10.21% on the 16th day, a decrease of 18.08 percentage points compared to the blank control group, and had the best effect in inhibiting the loss of water and other substances in sweet cherries. The chitosan in the composite film has multiple hydroxyl and amino groups and has certain water-retention properties; the plant essential oil strengthens the hydrophobic properties of the composite film, further isolating it from external air and reducing water loss.

[0069] The changes in the rot rate of sweet cherries in each test group during storage are shown in Figure 7 As can be seen, the blank control group had the fastest decay rate, with the decay rate increasing rapidly in the later stages of storage, reaching 49.10% on the 16th day. This may be due to direct contact between the sweet cherries in this group and environmental microorganisms. The conventional polyethylene plastic wrap group had a faster decay rate, reaching 41.61% on the 16th day. Compared with the blank control group and the conventional polyethylene plastic wrap group, the composite film group was more effective in inhibiting the decay rate of the sweet cherries. Among them, the 2% TTO-TEO group had the best preservation effect, with a decay rate of 3.50% on the 16th day. The 1% TTO-TEO group had a poorer preservation effect, with a decay rate of 13.37% on the 16th day. This is because thyme and tea tree essential oils are rich in antibacterial active substances such as alkenes and phenols, which can effectively destroy the structure of microbial cell membranes, thereby reducing the decay rate of sweet cherries.

[0070] Figure 8The sensory change trends of sweet cherries in the chitosan composite film group with different essential oil additions, the ordinary polyethylene cling film group and the blank control group. During the entire storage period, the sensory changes of sweet cherries in the blank control group and the ordinary polyethylene cling film group were the greatest. On the 6th day, the sensory evaluation scores were both below 60 points, and they lost their commercial value. On the 8th and 10th days, the fruits showed slight browning and softening, with a mild odor, and lost their edible value. The sensory scores of the 2% TTO-TEO group and the 3% TTO-TEO group were 71 and 67 points, respectively, on the 16th day. The color of the fruit became slightly darker, the flesh tissue softened slightly, and there was no odor, and it still had a certain commercial value. On the 12th day, the color of the fruit in the 1% TTO-TEO group became significantly darker, the width of the softened flesh tissue increased slightly, and it lost its commercial value. The good barrier properties of the composite film prepared by the present invention can adjust the concentration of carbon dioxide and oxygen in the package, inhibit the respiration of sweet cherries, reduce the consumption of nutrients, and maintain their nutritional value. The oils in the composite film form a hydrophobic surface layer, reducing contact between water vapor and the composite film, forcing it to remain around the sweet cherries, reducing water loss, making the fruit plump and lustrous, and maintaining its sensory appearance. Thyme essential oil, tea tree essential oil, and chitosan exhibit a certain antibacterial effect, not only inhibiting the growth and reproduction of microorganisms in the composite film itself, but also mitigating the decay of the sweet cherries through the volatility and diffusivity of the active substances. This demonstrates that the composite film prepared by this invention has a significant freshness-preserving effect on sweet cherries, with the 2% TTO-TEO group being the most effective.

[0071] Although the preferred embodiments of the present invention have been described, those skilled in the art may make additional changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention.

[0072] Obviously, those skilled in the art may make various modifications and variations to the present invention without departing from the spirit and scope of the present invention. Thus, if such modifications and variations fall within the scope of the claims and their equivalents, the present invention is intended to include such modifications and variations.

Claims

1. A method for preparing a tea tree essential oil-thyme essential oil-chitosan composite film, characterized in that: The following steps are involved: dissolving polyvinyl alcohol in water to obtain a polyvinyl alcohol solution; Adding glacial acetic acid, chitosan, a plasticizer and an emulsifier to polyvinyl alcohol and mixing them uniformly, then adding tea tree essential oil and thyme essential oil, mixing them uniformly and degassing to obtain a film-forming liquid; The film-forming liquid is dried to form a film to obtain a tea tree essential oil-thyme essential oil-chitosan composite film.

2. The method for preparing a tea tree essential oil-thyme essential oil-chitosan composite film according to claim 1, wherein The ratio of chitosan and tea tree essential oil is 1g:1mL~3mL.

3. The method for preparing a tea tree essential oil-thyme essential oil-chitosan composite film according to claim 1, wherein The volume ratio of tea tree essential oil to thyme essential oil is 1:

1.

4. The method for preparing a tea tree essential oil-thyme essential oil-chitosan composite film according to claim 1, wherein The ratio of polyvinyl alcohol to glacial acetic acid is 3 g:1 mL, and the mass ratio of polyvinyl alcohol to chitosan is 6:

1.

5. The method for preparing a tea tree essential oil-thyme essential oil-chitosan composite film according to claim 1, wherein The concentration of polyvinyl alcohol in the polyvinyl alcohol solution is 3%.

6. The method for preparing a tea tree essential oil-thyme essential oil-chitosan composite film according to claim 1, wherein The ratio of chitosan to plasticizer was 1 g:4 mL.

7. The method for preparing a tea tree essential oil-thyme essential oil-chitosan composite film according to claim 1, wherein The plasticizer is propylene glycol.

8. The method for preparing a tea tree essential oil-thyme essential oil-chitosan composite film according to claim 1, wherein The emulsifier is Tween 80.

9. A tea tree essential oil-thyme essential oil-chitosan composite film prepared by the preparation method according to any one of claims 1 to 8.

10. Use of the tea tree essential oil-thyme essential oil-chitosan composite film according to claim 9 in preserving sweet cherries.

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