A vegetable fresh-keeping modified atmosphere packaging film and preparation method thereof

Abstract

The present invention provides a vegetable atmosphere-controlled cling film and a preparation method thereof. Polylactic acid and polyethylene glycol are selected as raw materials, a solvent casting method is adopted to form a film, and a trace amount of cellulose nanocrystals are used to regulate microphase separation. The preparation method of the vegetable atmosphere-controlled cling film comprises the following steps: respectively dissolving polylactic acid and polyethylene glycol in a chloroform solvent; adding a trace amount of cellulose nanocrystals to the polyethylene glycol solution, uniformly dispersing, and mixing with the polylactic acid solution; and casting on a flat plate and drying at 40°C to form a film. The present invention can achieve microphase separation of polylactic acid and polyethylene glycol by adding a small amount of cellulose nanocrystals, and obtain a degradable film with good CO2 / O2 transmission selectivity, light transmittance and moisture permeability, which is suitable for preserving fresh vegetables, and its performance is far better than the mainstream PP cling film on the market.

Description

Technical Field

[0001] The invention belongs to the field of agricultural product preservation, and in particular relates to a vegetable fresh-keeping modified atmosphere packaging film and a preparation method thereof. Background Art

[0002] China is a major vegetable producer and consumer. In terms of vegetable varieties, there are regional differences in vegetable consumption preferences. Residents in the central and western regions like to eat lettuce, potatoes and radishes, while residents in Jiangsu and Zhejiang prefer green vegetables, lettuce, and leafy vegetables. Rational use of post-harvest vegetable preservation technology to reduce decay and loss and extend shelf life is of great significance to improving vegetable production efficiency and promoting industrial development.

[0003] Leafy vegetables have high water content and large surface area. They have vigorous transpiration and respiration after harvest. They are prone to dehydration, wilting, yellowing, and rotting, making them difficult to store. At present, vegetable markets mainly use water spraying and light avoidance to reduce leafy vegetable transpiration, but they cannot reduce the aerobic respiration of leafy vegetables, resulting in nutrient consumption and quality degradation. Supermarket terminals mostly use PP / PE packaging bags to reduce leafy vegetable transpiration, but condensation is very easy to occur, leading to anaerobic respiration and taste change. Although the perforation method can increase the permeability of packaging materials to water vapor and reduce condensation, the increased oxygen permeability and increased respiration rate will also lead to the loss of leafy vegetable nutrients. An effective way to solve this problem is to use controlled atmosphere preservation, develop controlled atmosphere fresh-keeping bags suitable for vegetables, control the gas composition in the bag, and create a weak aerobic respiration environment.

[0004] At present, there are four main types of vegetable modified atmosphere packaging developed at home and abroad, including spontaneous modified atmosphere packaging, mixed gas filled film packaging, high oxygen spontaneous modified atmosphere packaging and active packaging.

[0005] ① Modified atmosphere packaging

[0006] In a sealed environment, a suitable gas environment is formed through the respiration of vegetables themselves, achieving the purpose of spontaneous atmosphere control. A microenvironment with low oxygen and a certain volume fraction of CO2 is formed in the package, which inhibits the respiration of vegetables and the growth of some microorganisms while slowing down the loss of vegetables. This type of packaging is easy to use, low cost, and has a good preservation effect. Although the PE and PP cling films commonly used in the market have good barrier properties, they are not suitable for fresh vegetables and are prone to condensation and corruption. Silicon window modified atmosphere packaging has better permeability and permeability selectivity, and can effectively solve the problems of condensation and deterioration, but the production cost is high and it is not suitable for the application of low-value fast-moving consumer goods such as vegetables.

[0007] ②Gas-filled film packaging

[0008] According to the characteristics of vegetables, two or more mixed gases are selected to fill the packaging bag to adjust the gas composition during vegetable storage. Compared with spontaneous modified atmosphere packaging, filling the film with the minimum allowed O2 volume fraction and the maximum CO2 volume fraction can form a microenvironment more suitable for vegetable preservation, effectively inhibiting the physiological activity of vegetables and reducing their consumption. Zhou Chunmei found that the initial gas O2:CO2:N2 volume ratio was 0.4:13.0:86.6, which was more effective than 0.4∶7.0∶92.6 and the air control group to delay the respiratory peak of white jade mushroom. Fernández-León et al. found that broccoli can better maintain the quality of broccoli when filled with a volume fraction of 10% O2 and 5% CO2 under storage conditions of 1-2℃ and relative humidity of 85%-90%, and broccoli still has a high commercial value after storage for 9 days. D'Aquino et al. found that filling a mixed gas of 3kPa CO2 and 12kPa O2 at 20℃ and relative humidity below 90% can reduce the decay of cherry tomatoes. In recent years, some studies have found that O3 gas has great application value in modified atmosphere packaging, which is manifested in (a) blocking ethylene production and inhibiting vegetable aging; (b) having good disinfection and sterilization properties, preventing vegetable mold and rot; (c) closing stomata, reducing transpiration and dehydration. The biggest advantage of the O3 treatment method is that it is environmentally friendly and can effectively kill many harmful microorganisms on grapes and other fruits without leaving chemical residues. However, this type of operation will increase sales costs and is not economical for vegetable products.

[0009] ③High oxygen spontaneous atmosphere packaging film

[0010] It is generally believed that a high concentration of O2 environment will accelerate the respiration rate of vegetables and is not conducive to vegetable storage. However, some studies have shown unexpected results. Compared with the air storage environment, a 50% or 90% O2 environment can significantly reduce the browning of fresh-cut lettuce, reduce odor generation and product quality loss during storage. At the same time, a high concentration of O2 environment can also reduce product hardness and total soluble solid loss. Studies have found that the use of film packaging with an O2 volume fraction of 21% to 100% can reduce vegetable respiration, inhibit bacterial and fungal reproduction, reduce decay rate, and slow down tissue browning. Jacxsens found that compared with low oxygen modified packaging, high oxygen modified packaging can effectively inhibit the increase in mushroom polyphenol oxidase activity and reduce the browning rate. Liang Xiaoling used 92% O2 to treat fresh-cut potatoes and found that at 4°C, fresh-cut potatoes can maintain good quality within 14 days. However, in the field of fresh vegetable preservation applications, this type of method is too costly and is not suitable for promotion.

[0011] ④Active packaging

[0012] Adding gas (ethylene, CO2) absorbents or gas (SO2, ClO2 and other antibacterial gases) releasers to the packaging materials or packaging systems, and replenishing O2 in time, can maintain a suitable gas environment in the packaging bag to preserve fresh-cut vegetables, delay aging and maintain their nutritional flavor. Ethylene gas can promote fruit ripening. Adding substances that reduce ethylene content, such as ethylene inhibitors, ethylene absorbents or ethylene removers, to the packaging bag can delay vegetable aging. Some researchers used 3μL / L of ethylene inhibitor 1-methylcyclopropene (1-methylcyclopropene, 1-MCP) to treat rapeseed and found that the treated product had better appearance and flavor than the control group. Ag-TiO2 / activated carbon fiber photocatalytic film can also significantly accelerate ethylene degradation. In addition, adding CO2 removers such as Ca(OH)2, NaOH, KOH, CaO, etc. to the packaging bag can reduce the CO2 content in the package and reduce anaerobic respiration of vegetables. Overall, this type of method has high application costs and is not suitable for fresh vegetable preservation.

[0013] Vegetables are disposable low-value consumer goods, and spontaneous modified atmosphere packaging is one of the most suitable packaging forms for vegetable preservation. Among the common film materials, polylactic acid has moderate gas and water vapor permeability, and is not easy to form condensation. In addition, biodegradability is also one of the advantages of polylactic acid. In recent years, the research on polylactic acid films has attracted a lot of attention. However, since the gas permeation selectivity of pure polylactic acid film is low (only about 3), it is not easy to form a low-oxygen breathing environment relying solely on polylactic acid. It is necessary to add materials with high gas permeation selectivity to the film to solve this problem. At present, there have been reports on films made of polylactic acid / polyethylene glycol / polylactic acid triblock copolymers and polylactic acid / polybutylene terephthalate-adipate-butylene terephthalate. These films have good CO2 / O2 permeation selectivity and can be used for modified atmosphere preservation of strawberries, onions and tomatoes. However, this method has a high production cost and is not suitable for the preservation of low-value fresh vegetables (leafy vegetables), and it is difficult to promote. Summary of the invention

[0014] The technical problem to be solved by the present invention is to provide a vegetable modified atmosphere cling film and a preparation method thereof. The vegetable modified atmosphere cling film has the characteristics of being easy to degrade, permeable to moisture and light, having good CO2 / O2 transmission selectivity, having mechanical properties that meet application requirements and having low application cost.

[0015] To solve the above technical problems, an embodiment of the present invention provides a vegetable fresh-keeping modified atmosphere packaging film, the preparation raw materials of which include polylactic acid, polyethylene glycol and cellulose nanocrystals, and the microphase separation of polylactic acid and polyethylene glycol is regulated by cellulose nanocrystals.

[0016] The mass ratio of the polylactic acid to the polyethylene glycol is 98:2-92:8, and the mass of the cellulose nanocrystals is 0.01-0.03% of the mass of the polylactic acid.

[0017] Preferably, the weight average molecular weight of the polylactic acid is greater than 110,000.

[0018] The number average molecular weight of the polyethylene glycol is greater than 20,000.

[0019] The present invention also provides a method for preparing a vegetable fresh-keeping modified atmosphere packaging film, comprising the following steps:

[0020] S1, taking polylactic acid, dissolving it in chloroform solvent to obtain a polylactic acid solution;

[0021] S2, taking polyethylene glycol, dissolving it in chloroform solvent to obtain a polyethylene glycol solution;

[0022] S3, adding cellulose nanocrystals to the polyethylene glycol solution prepared in step S2, and uniformly dispersing by ultrasonication to obtain a polyethylene glycol / cellulose nanocrystal dispersion;

[0023] S4, mixing the polylactic acid solution prepared in step S1 and the polyethylene glycol / cellulose nanocrystal dispersion prepared in step S3 to obtain a polylactic acid / polyethylene glycol / cellulose nanocrystal mixed solution;

[0024] S5, pouring the polylactic acid / polyethylene glycol / cellulose nanocrystal mixed solution obtained in step S4 onto a flat plate;

[0025] S6. Dry the plate in an oven at 40° C. for at least 4 hours until the solvent is completely evaporated to obtain a polylactic acid / polyethylene glycol / cellulose nanocrystal composite film, i.e., a vegetable fresh-keeping modified atmosphere packaging film.

[0026] The cellulose nanocrystals used in step S3 have a length range of 100-200 nm and a diameter range of 10-20 nm.

[0027] The thickness of the vegetable fresh-keeping modified atmosphere packaging film prepared in step S6 is 25-35 μm.

[0028] The beneficial effects of the above technical solution of the present invention are as follows:

[0029] 1. The present invention uses degradable polylactic acid and a small amount of polyethylene glycol with a high CO2 / O2 transmittance ratio as the main raw materials. At the same time, a trace amount of cellulose nanocrystals is added to promote microphase separation of polylactic acid and polyethylene glycol in the film, thereby improving the CO2 / O2 transmittance ratio of the composite film. A fresh-keeping film material with good light and moisture permeability, high CO2 / O2 transmittance ratio, and strength that meets application requirements is prepared to increase the shelf life of fresh vegetables.

[0030] 2. The CO2 / O2 permeability ratio of the 30 μm thick packaging film prepared by the preparation method provided by the present invention can reach 6-9, which is much higher than 2.5 of the mainstream commercially available PE film and 3.2 of the pure polylactic acid film. This advantage is mainly due to the high gas permeability selectivity of the polyethylene glycol component in the film, and the cellulose nanocrystals promote microphase separation. In addition, the moisture permeability of the polylactic acid component in the fresh-keeping film is higher than that of the PE material, and the polyethylene glycol and cellulose nanocrystals in the film are both hydrophilic substances, which are conducive to water vapor permeation. Therefore, the water vapor permeability of the fresh-keeping film prepared by the present invention can reach 100 times that of the commercially available PE film, which is 15% higher than that of the polylactic acid / polyethylene glycol film of the same thickness, and can effectively reduce condensation. In addition, the cellulose nanocrystals in the film also play a reinforcing role, which can compensate for the decrease in film strength caused by polyethylene glycol to a certain extent. Finally, the tensile strength of the polylactic acid / polyethylene glycol / cellulose nanocrystal film can reach 20N, which is 57% higher than the polylactic acid / polyethylene glycol film control sample. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 is a preparation flow chart of the present invention;

[0032] Figure 2 Schematic diagram of adding cellulose nanocrystals to regulate film microphase separation in the present invention. DETAILED DESCRIPTION

[0033] In order to make the technical problems, technical solutions and advantages to be solved by the present invention more clear, a detailed description will be given below with reference to the accompanying drawings and specific embodiments.

[0034] The invention provides a vegetable fresh-keeping modified atmosphere packaging film, the preparation raw materials of which include polylactic acid, polyethylene glycol and cellulose nanocrystals, and the microphase separation of polylactic acid and polyethylene glycol is regulated by cellulose nanocrystals.

[0035] The mass ratio of the polylactic acid to the polyethylene glycol is 98:2-92:8. The weight average molecular weight of the polylactic acid is greater than 110000. The number average molecular weight of the polyethylene glycol is greater than 20000. The mass of the cellulose nanocrystals is 0.01-0.03% of the mass of the polylactic acid.

[0036] The present invention also provides a method for preparing a vegetable fresh-keeping modified atmosphere packaging film, comprising the following steps:

[0037] S1, taking polylactic acid, dissolving it in chloroform solvent to obtain a polylactic acid solution;

[0038] S2, taking polyethylene glycol, dissolving it in chloroform solvent to obtain a polyethylene glycol solution;

[0039] S3, adding cellulose nanocrystals to the polyethylene glycol solution prepared in step S2, and uniformly dispersing them by ultrasonication to obtain a polyethylene glycol / cellulose nanocrystal dispersion; wherein the cellulose nanocrystals have a length range of 100-200 nm and a diameter range of 10-20 nm.

[0040] S4, mixing the polylactic acid solution prepared in step S1 and the polyethylene glycol / cellulose nanocrystal dispersion prepared in step S3 to obtain a polylactic acid / polyethylene glycol / cellulose nanocrystal mixed solution;

[0041] S5, pouring the polylactic acid / polyethylene glycol / cellulose nanocrystal mixed solution obtained in step S4 onto a flat plate;

[0042] S6. Place the plate in an oven at 40°C and dry for at least 4 hours until the solvent is completely evaporated to obtain a polylactic acid / polyethylene glycol / cellulose nanocrystal composite film, i.e., a vegetable fresh-keeping modified atmosphere packaging film, with a thickness of 25-35 μm.

[0043] The present invention utilizes cellulose nanocrystals to regulate the microphase separation of polylactic acid and polyethylene glycol in the film, and prepares a composite film for vegetable preservation with high CO2 / O2 permeability and good moisture and light permeability. The theoretical basis is:

[0044] Polylactic acid material is biodegradable and has moderate moisture permeability, but its CO2 / O2 permeability ratio is low, which is not conducive to the controlled atmosphere preservation of vegetables. At present, there have been reports of using block copolymerization to introduce polyethylene glycol, a substance with high gas permeability selectivity, to improve the CO2 / O2 permeability of the film. However, this method is extremely expensive and not conducive to industrialization. The cost of preparing films by blending polylactic acid and polyethylene glycol is lower, but when the polyethylene glycol content exceeds 8wt%, the two phases are obviously separated, resulting in a significant decrease in the light transmittance of the film, affecting its application. In addition, due to the strong water solubility of polyethylene glycol, low molecular weight polyethylene glycol or high content polyethylene glycol can also pose potential risks to food safety.

[0045] The present invention uses polyethylene glycol with a number average molecular weight of more than 20,000 and polylactic acid with a weight average molecular weight of more than 110,000 as raw materials, and the ratio of polyethylene glycol to polylactic acid is in the range of 2:98-8:92. On the one hand, it reduces the food safety risk caused by the dissolution of polyethylene glycol, and on the other hand, it improves the gas permeability selectivity of the membrane material. At the same time, the present invention also adds a trace amount of cellulose nanocrystals, uses hydrophilic cellulose nanocrystals to promote the aggregation of polyethylene glycol, and adjusts the microphase separation of the membrane material. Figure 2 The figure shows the schematic diagram of adding cellulose nanocrystals (CNC) to adjust the microphase separation of the film, where: Figure 2 a is the macromolecular aggregation state without CNC. Figure 2 b and Figure 2 c shows the macromolecular aggregation state at the beginning of adding CNC and after adding CNC, respectively.

[0046] The gas permeability selectivity of the film can be improved by film microphase separation. In the present invention, the use of a relatively low content (such as 0.02%) of cellulose nanocrystals can achieve a microphase separation effect, so that the CO2 / O2 permeability of the film reaches 6-9. However, when the content of cellulose nanocrystals is relatively high (>0.1wt%), the CO2 / O2 permeability of the film will drop to below 2.5, which is close to the commercially available PP film. This is because the excess cellulose nanocrystals act as a barrier in the film.

[0047] The technical solution of the present invention is further described below in conjunction with several specific embodiments.

[0048] Example 1

[0049] Weigh 0.17g of polyethylene glycol with a number average molecular weight of 20,000 and dissolve it in 5ml of chloroform. After the polyethylene glycol is dissolved, add 0.0008g of cellulose nanocrystals (length 100nm, diameter 20nm) to the solution and disperse it evenly at 600W ultrasonic. Then weigh 4g of polylactic acid with a weight average molecular weight of 110,000 and dissolve it in 25ml of chloroform. Mix the polyethylene glycol solution and the polylactic acid solution evenly and pour them on a flat plate. After drying in an oven at 40°C for 4.5h, obtain a polylactic acid / polyethylene glycol / cellulose nanocrystal composite film, i.e., a modified atmosphere packaging film for vegetable preservation.

[0050] Table 1 Properties of polylactic acid / polyethylene glycol / cellulose nanocrystal (PLA / PEG / CNC) composite films

[0051]

[0052] Note: The mass ratio of the three in the PLA / PEG / CNC composite film is 96:4:0.02; the content ratio of PLA / PEG in the PLA / PEG film is 96:4. The thickness of all films is 30 μm. The shelf life days are the days that fresh lettuce can be kept fresh on supermarket shelves.

[0053] Example 2

[0054] Weigh 0.1867g of polyethylene glycol with a number average molecular weight of 20,000 and dissolve it in 5ml of chloroform. After the polyethylene glycol is dissolved, add 0.0005g of cellulose nanocrystals (length 120nm, diameter 15nm) to the solution and disperse it evenly at 600W ultrasonic. Then weigh 2.147g of polylactic acid with a weight average molecular weight of 110,000 and dissolve it in 20ml of chloroform. Mix the polyethylene glycol solution and the polylactic acid solution evenly and pour them on a flat plate. After drying in an oven at 40°C for 4.5h, obtain a polylactic acid / polyethylene glycol / cellulose nanocrystal composite film, i.e., a modified atmosphere packaging film for vegetable preservation.

[0055] Table 2 Properties of polylactic acid / polyethylene glycol / cellulose nanocrystal (PLA / PEG / CNC) composite films

[0056]

[0057] Note: The mass ratio of the three in the PLA / PEG / CNC composite film is 92:8:0.02; the content ratio of PLA / PEG in the PLA / PEG film is 92:8. The thickness of all films is 30 μm. The shelf life days are the days that fresh lettuce can be kept fresh on supermarket shelves.

[0058] The water vapor permeability of the fresh-keeping film prepared by the present invention can reach 100-200 times that of the commercially available PE film, which is 8-15% higher than that of the polylactic acid / polyethylene glycol film of the same thickness, and can effectively reduce the condensation phenomenon. In addition, the cellulose nanocrystals in the film also play a reinforcing role, which can compensate for the decrease in film strength caused by polyethylene glycol to a certain extent. Finally, the tensile strength of the polylactic acid / polyethylene glycol / cellulose nanocrystal film can reach 20N, which is 57% higher than that of the polylactic acid / polyethylene glycol film control sample.

[0059] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A vegetable fresh-keeping modified atmosphere packaging film, characterized in that: The raw materials for its preparation include polylactic acid, polyethylene glycol and cellulose nanocrystals, and the polylactic acid and polyethylene glycol are regulated to separate into microphases through cellulose nanocrystals; The mass ratio of the polylactic acid to the polyethylene glycol is 98:2-92:8, and the mass of the cellulose nanocrystals is 0.01-0.03% of the mass of the polylactic acid.

2. The vegetable fresh-keeping modified atmosphere packaging film according to claim 1, characterized in that: The weight average molecular weight of the polylactic acid is greater than 110,000.

3. The vegetable fresh-keeping modified atmosphere packaging film according to claim 1, characterized in that: The number average molecular weight of the polyethylene glycol is greater than 20,000.

4. A method for preparing a vegetable fresh-keeping modified atmosphere packaging film, characterized in that: The steps include: S1, taking polylactic acid, dissolving it in chloroform solvent to obtain a polylactic acid solution; S2, taking polyethylene glycol, dissolving it in chloroform solvent to obtain a polyethylene glycol solution; S3, adding cellulose nanocrystals to the polyethylene glycol solution prepared in step S2, and uniformly dispersing by ultrasonication to obtain a polyethylene glycol / cellulose nanocrystal dispersion; S4, mixing the polylactic acid solution prepared in step S1 and the polyethylene glycol / cellulose nanocrystal dispersion prepared in step S3 to obtain a polylactic acid / polyethylene glycol / cellulose nanocrystal mixed solution; S5, pouring the polylactic acid / polyethylene glycol / cellulose nanocrystal mixed solution obtained in step S4 onto a flat plate; S6. Dry the plate in an oven at 40° C. for at least 4 hours until the solvent is completely evaporated to obtain a polylactic acid / polyethylene glycol / cellulose nanocrystal composite film, i.e., a vegetable fresh-keeping modified atmosphere packaging film.

5. The method for preparing the vegetable fresh-keeping modified atmosphere packaging film according to claim 4, characterized in that: The cellulose nanocrystals used in step S3 have a length range of 100-200 nm and a diameter range of 10-20 nm.

6. The method for preparing the vegetable fresh-keeping modified atmosphere packaging film according to claim 4, characterized in that: The thickness of the vegetable fresh-keeping modified atmosphere packaging film prepared in step S6 is 25-35 μm.

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