Application of a polylactic acid active packaging film in the preservation of Bacillus thuringiensis

Abstract

The invention discloses a use of a polylactic acid active packaging film in the preservation of B. cerana. The polylactic acid active packaging film is prepared by using an Ag@MOF carrier to load a natural antibacterial agent, which is then fully mixed with polylactic acid and then melt-extrusion blow molding. The packaging film is used to package the B. cerana to achieve active packaging of the B. cerana, inhibit the growth of microorganisms, and extend the shelf life of the B. cerana. The preservation method has a simple process and is easy to operate. The materials used are biodegradable, reducing environmental pollution caused by packaging. The natural antibacterial agent, Litsea cubeba essential oil, is added to the preservative film, which can effectively inhibit the growth and reproduction of major spoilage bacteria isolated and identified from the B. cerana. The Ag@MOF is used to load the Litsea cubeba essential oil, which can effectively control the release rate of the Litsea cubeba essential oil, achieving long-lasting antibacterial and antioxidant purposes. The preservation method can effectively extend the storage period of the B. cerana.

Description

Technical Field

[0001] The invention relates to application of a polylactic acid active packaging film in preserving Bacillus thuringiensis, and belongs to the field of food preservation. Background Art

[0002] Russula virescens, belonging to the order Agaricales, family Russlaceae, genus Russula, is a common edible and medicinal mushroom, also known as green red mushroom, green mushroom, blue bacillus, frog mushroom, green soup mushroom, green lid, and green bean mushroom. Its fruiting bodies primarily grow in true-leaf forests, broad-leaved forests, or mixed coniferous and broad-leaved forests, such as Masson pine and oak forests. They grow from June to September annually. Documented finds include Yunnan, Guizhou, Sichuan, Jiangxi, Guangdong, and Fujian. Russula virescens is delicious and rich in nutrients, including protein, carbohydrates, vitamins, and minerals. According to the Southern Yunnan Materia Medica, "Its flavor is sweet, slightly sour, and non-toxic. It is primarily used to treat blurred vision, relieve liver heat, dissipate heat, and relieve qi. It is particularly effective for women with qi stagnation." Therefore, Russula virescens has long been a popular health food in many parts of my country. However, fresh tricholoma still has vigorous physiological activities during storage and transportation after picking. Due to its soft tissue, high water content, high respiration intensity, it is easy to breed microorganisms such as spoilage bacteria during storage, and the mushroom body is prone to aging and fibrosis, resulting in deterioration phenomena such as opening and browning, which reduces the nutritional value and commercial value of edible fungi. As a result, tricholoma has problems such as short market shelf life, which seriously restricts the sales of tricholoma and reduces economic benefits. Therefore, how to extend the storage period of tricholoma after picking and improve the quality of tricholoma is of great significance to the tricholoma industry and consumers. Commonly used preservation methods such as refrigeration, radiation treatment, controlled atmosphere preservation, chemical treatment, etc. are mostly time-consuming and troublesome, and have certain safety risks. Therefore, there is an urgent need to develop a packaging system suitable for tricholoma.

[0003] In modern production processes, packaging not only protects food from harmful effects such as light, moisture, microorganisms, air, and dust, but also provides labels and essential information to consumers. Furthermore, packaging effectively ensures product quality, freshness, and safety. Active packaging is an innovative concept in the food packaging field that has emerged to meet consumer demand for safe, healthy, and high-quality food. Active packaging generally consists of three components: an active substance, a film-forming matrix, and the food. Adding active substances to the film-forming matrix not only improves the antioxidant and antimicrobial properties of the packaging material, extending the shelf life of the food, but also inhibits the growth of foodborne pathogens, ensuring food safety. Plant essential oils are natural active substances widely used in food packaging due to their excellent antioxidant and antimicrobial properties. Litsea cubeba essential oil, in particular, exhibits broad-spectrum antimicrobial activity, showing excellent inhibitory effects against pathogens that cause spoilage, such as Serratia marcescens, Escherichia coli, Acinetobacter baumannii, Candida albicans, and Schenckia spp. However, due to the common shortcomings of essential oils such as poor thermal stability and high volatility, adding Litsea cubeba essential oil directly to the film-forming matrix will cause a sudden release of the essential oil, resulting in unsatisfactory long-term antioxidant and antibacterial effects. In addition, the special flavor of Litsea cubeba essential oil may also affect the original sensory properties of packaged food. Summary of the Invention

[0004] In response to the problems existing in the prior art, the present invention provides a preparation method and use of a polylactic acid active packaging film. First, an Ag@MOF carrier is used to load the natural antibacterial agent Litsea cubeba essential oil to achieve controlled release of the Litsea cubeba essential oil. The oil is then fully and evenly mixed with polylactic acid, and a melt extrusion blow molding method is used to prepare a biodegradable polylactic acid active packaging film. The packaging film is used to package the Brussels Trichoderma lucidum to achieve active packaging of the Brussels Trichoderma lucidum, inhibit the growth of microorganisms for a long time, and thus extend the shelf life of the Brussels Trichoderma lucidum.

[0005] The above purpose is achieved by the following method:

[0006] (1) The Ag@MOF carrier and the natural antibacterial agent are mixed in a mass ratio of 5:6 to 12 to prepare a blend; the blend is added to dichloromethane, ultrasonically dispersed for 15 to 25 minutes, and then stirred at room temperature for 6 to 15 hours to fully evaporate the dichloromethane solvent, thereby obtaining an Ag@MOF composite carrier loaded with the natural antibacterial agent;

[0007] (2) Polylactic acid and Ag@MOF composite carrier loaded with natural antibacterial agent were mixed in a mass ratio of 100:1-6, vacuum dried at 80°C for 4-8 hours, and then poured into the hopper of LSJ-20 single-screw extruder. After melt extrusion, the sample was dried at 60°C for 4-8 hours and placed in LSC-120 film blowing machine for film blowing to obtain biodegradable polylactic acid active packaging film;

[0008] (3) Biodegradable polylactic acid active packaging film is made into fresh-keeping bags for the preservation of Bacillus subtilis.

[0009] The weight average molecular weight of the polylactic acid is 100,000 to 300,000.

[0010] The Ag@MOF support was prepared according to the method in “Aljohani, MM, Al-Qahtani, SD, Alshareef, M., El-Desouky, MG, El-Bindary, AA, El-Metwaly, NM, & El-Bindary, MA (2023). Highly efficient adsorption and removal of bio-staining dye from industrial wastewater onto mesoporous Ag-MOFs. Process Safety and Environmental Protection, 172, 395-407.10.1016 / j.psep.2023.02.036.”

[0011] The natural antibacterial agent is Litsea cubeba essential oil, which is prepared by conventional steam distillation or purchased from the market.

[0012] The biodegradable polylactic acid active packaging film is made into packaging bags, and the tricholoma is sorted and put into the packaging bags and sealed, with the amount of mushrooms filling being 75% of the volume of the packaging bags; after the packaging is completed, the bags are placed in a cold storage at 4±1°C.

[0013] Compared with the prior art, the present invention has the following advantages:

[0014] (1) The dichloromethane used in the preparation of the Bacillus thuringiensis fresh-keeping film of the present invention is completely volatilized, and the residual amount does not exceed 0.01%. The essential oil of Litsea cubeba used is processed from natural materials and is safe and non-toxic. The polylactic acid is safe, non-toxic and degradable. When used in food packaging, it can alleviate the "white pollution" caused by traditional plastics.

[0015] (2) The present invention's Bacillus thuringiensis fresh-keeping film uses Ag@MOF carrier to load Litsea cubeba essential oil. On the one hand, Ag@MOF has a certain antibacterial effect, and the fresh-keeping film can control the natural antibacterial agent Litsea cubeba essential oil to slowly release to the Bacillus thuringiensis in the fresh-keeping package through the pores of different structures in the Ag@MOF carrier, so that the Litsea cubeba essential oil is always maintained at a high level, thereby achieving the purpose of long-term antibacterial and antioxidant. On the other hand, the addition of Ag@MOF carrier can improve the water vapor permeability of the composite film, and has good air permeability, so that water vapor inside and outside the package can circulate at a certain rate, which can effectively inhibit the anaerobic respiration of Bacillus thuringiensis and allow water vapor to pass through effectively, thereby reducing the moisture content in the packaging bag, curbing the formation of accumulated water, and slowing down the mildew of Bacillus thuringiensis caused by accumulated water; avoiding the growth of microorganisms caused by water vapor accumulation; and the addition of Ag@MOF can also improve the mechanical properties of the composite film, such as tensile strength and elongation at break;

[0016] (3) The Bacillus preservative film of the present invention can effectively inhibit the growth of the main spoilage bacteria of Bacillus. During the storage of Bacillus, the main microbial phyla include Proteobacteria and Firmicutes at the level, and Lactococcus, Serratia, Enterococcus, Westerella and Acinetobacter at the genus level. Compared with Bacillus that were not treated with the composite film of Ag@MOF loaded with Litsea cubeba essential oil, the Bacillus bacteria packaged with Bacillus preservative film had a significant inhibitory effect on Proteobacteria at the phylum level, and the relative abundance at the genus level was lower, indicating that the preservative film can effectively inhibit the growth of the main spoilage bacteria of Bacillus.

[0017] (4) The preparation method of the fresh-keeping film of the present invention is simple in process, easy to operate, biodegradable, and environmentally friendly. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 is the relative abundance of the phylum Bryocytoplasma in the samples packaged with polylactic acid active packaging film;

[0019] Figure 2 is the relative abundance of the phylum Bletillaria in the composite film without the addition of Ag@MOF loaded with Litsea cubeba essential oil;

[0020] Figure 3 is the relative abundance of the genus Bacillus in the treatment with polylactic acid active packaging film;

[0021] Figure 4 The relative abundance of the genus Bacillus in the composite membrane treatment without the addition of Ag@MOF loaded with Litsea cubeba essential oil. DETAILED DESCRIPTION

[0022] The present invention will be further described in detail below with reference to the embodiments, but the scope of protection of the present invention is not limited to the contents described above;

[0023] Example 1:

[0024] (1) 0.2 g of Ag@MOF carrier and 0.44 g of Litsea cubeba essential oil, a natural antibacterial agent, were mixed to prepare a blend; the blend was added to 8 mL of dichloromethane, ultrasonically dispersed for 20 min, and stirred at room temperature for 12 h to fully evaporate the dichloromethane to obtain an Ag@MOF carrier loaded with Litsea cubeba essential oil;

[0025] (2) Polylactic acid (PLA) (PLA having a weight average molecular weight of 250,000) and Ag@MOF carrier loaded with Litsea cubeba essential oil were mixed in a mass ratio of 100:5, vacuum dried at 80°C for 7 hours, and then poured into the hopper of an LSJ-20 single-screw extruder. After melt extrusion, the extrudate was dried at 60°C for 7 hours and then placed in an LSC-120 film blowing machine for film blowing to obtain a biodegradable polylactic acid active packaging film.

[0026] At the same time, the composite membrane without adding Ag@MOF carrier was prepared according to the above method;

[0027] The polylactic acid active packaging film prepared in this example had a tensile strength of 12.8 MPa and an elongation at break of 32.3%, demonstrating good film ductility. Compared with the composite film without the addition of Ag@MOF-loaded Litsea cubeba essential oil, the tensile strength decreased by 60.7% and the elongation at break increased by 24.1%.

[0028] The water vapor permeability of polylactic acid active packaging film is 1.93g·mm / (m 2 ·h·kPa), showing good gas barrier properties. Compared with the composite film without Ag@MOF loaded with Litsea cubeba essential oil, the water vapor transmission rate increased by 57.2%; the release rate of Litsea cubeba essential oil on the first day was 7.7%, the cumulative release rate on the 5th day was 24.3%, the cumulative release rate on the 10th day was 37.3%, the cumulative release rate on the 20th day was 47.2%, the cumulative release rate on the 30th day was 62.7%, and the cumulative release rate on the 40th day was 99.8%. The sustained release time of Litsea cubeba essential oil was as long as 40 days, which was 24 days longer than that of the composite film without Ag@MOF loaded with Litsea cubeba essential oil.

[0029] (3) The biodegradable polylactic acid active packaging film in step (2) was heat-sealed to prepare a fresh-keeping bag for preserving the fresh-keeping mushroom; 250 g of the fresh-keeping mushroom was weighed and placed in the bag, and fresh air was introduced to fill the fresh-keeping bag. After sealing, the bagged fresh-keeping mushroom was stored in a refrigerator at 4±1°C. After 12 days of storage, compared with the composite film without the addition of Ag@MOF loaded with Litsea cubeba essential oil, the browning index of the fresh-keeping mushroom treated with the polylactic acid active packaging film was reduced by 36.3%, the hardness was increased by 1.17 times, and the electrolyte permeability was reduced by 41.2%. The antibacterial effect of the fresh-keeping film was verified by analyzing the changes in microorganisms at the phylum and genus levels during the storage of the fresh-keeping mushroom. The results showed that the bacteria on the surface of the fresh-keeping mushroom packaged with the fresh-keeping film had a significant inhibitory effect on the Proteobacteria at the phylum level: the relative abundance of Proteobacteria in the fresh-keeping mushroom was lower on the 12th day (51.6%) than on the 0th day (78.6%). Figure 1 、 2 The relative abundance of the genus of the Bacillus thuringiensis treated with the polylactic acid active packaging film was lower: on the 12th day of storage, the genus Lactococcus accounted for 33.5%, the genus Serratia accounted for 4.02%, and no Acinetobacter and Westerella were detected in the Bacillus thuringiensis treated with the polylactic acid active packaging film; the genus Acinetobacter accounted for 2.05%, the genus Lactococcus accounted for 52.6%, the genus Serratia accounted for 3.84%, and no Westerella was detected in the Bacillus thuringiensis treated with the composite film without the addition of Ag@MOF loaded with Litsea cubeba essential oil. Figure 3 、 4 The above-mentioned preservation method effectively extends the shelf life of the Brussels sprouts.

[0030] Example 2

[0031] (1) 0.2 g of Ag@MOF carrier and 0.24 g of Litsea cubeba essential oil were mixed to prepare a blend; the blend was added to 3 mL of dichloromethane, ultrasonically dispersed for 15 min, and stirred at room temperature for 6 h to fully evaporate the dichloromethane solvent to obtain an Ag@MOF carrier loaded with Litsea cubeba essential oil;

[0032] (2) Polylactic acid (PLA) (PLA having a weight average molecular weight of 100,000) and Ag@MOF carrier loaded with Litsea cubeba essential oil were mixed in a mass ratio of 100:1, vacuum dried at 80°C for 4 hours, and then poured into the hopper of an LSJ-20 single-screw extruder. After melt extrusion, the extrudate was dried at 60°C for 4 hours and then placed in an LSC-120 film blowing machine for film blowing to obtain a biodegradable polylactic acid active packaging film.

[0033] At the same time, the composite membrane without adding Ag@MOF carrier was prepared according to the above method;

[0034] The depolymerized lactic acid active packaging film prepared in this example has a tensile strength of 13.4 MPa and an elongation at break of 32.1%, showing good film ductility. Compared with the composite film without the addition of Ag@MOF loaded with Litsea cubeba essential oil, the tensile strength is reduced by 57.1% and the elongation at break is increased by 22.3%. Its water vapor permeability is 0.89 g·mm / (m 2 ·h·KPa), showing good gas barrier properties. Compared with the composite film without Ag@MOF loaded with Litsea cubeba essential oil, the water vapor transmission rate increased by 13.8%; the release rate of Litsea cubeba essential oil on the first day was 7.4%, the cumulative release rate on the 5th day was 22.2%, the cumulative release rate on the 10th day was 40.4%, the cumulative release rate on the 20th day was 44.3%, the cumulative release rate on the 30th day was 57.9%, and the cumulative release rate on the 41st day was 99.7%. The sustained release time of Litsea cubeba essential oil was as long as 41 days, which was 25 days longer than that of the composite film without Ag@MOF loaded with Litsea cubeba essential oil.

[0035] (3) The biodegradable polylactic acid active packaging film in step (2) is heat-sealed to make a fresh-keeping bag for preserving the Bacillus cirrhosa. About 250 g of Bacillus cirrhosa is weighed and put into the bag, and fresh air is injected into the bag. The fresh-keeping bag is filled up and sealed. The bagged Bacillus cirrhosa is stored in a refrigerator at 4±1°C. After 12 days of storage, compared with the composite film without the addition of Ag@MOF loaded with Litsea cubeba essential oil, the browning index of the Bacillus cirrhosa treated with the polylactic acid active packaging film is reduced by 20.6%, the hardness is increased by 0.54 times, and the electrolyte permeability is reduced by 17.4%. The antibacterial effect of the polylactic acid active packaging film was verified by analyzing the changes in microbial abundance at the phylum and genus levels during storage of B. edulis. The results showed that the surface bacteria of B. edulis wrapped in the polylactic acid active packaging film had a significant inhibitory effect on Proteobacteria at the phylum level: the relative abundance of Proteobacteria in B. edulis on day 12 (52.6%) was lower than that on day 0 (81.7%). The relative abundance of B. edulis treated with the polylactic acid active packaging film was even lower at the genus level: on day 12, Lactococcus accounted for 47.9% and Serratia for 7.92%, with no Acinetobacter or Westerella detected. In contrast, in B. edulis treated with the composite film without the addition of Ag@MOF-loaded Litsea cubeba essential oil, Acinetobacter accounted for 2.17%, Lactococcus accounted for 51.6%, Serratia accounted for 6.41%, and no Westerella was detected. This preservation method effectively extended the shelf life of B. edulis.

[0036] Example 3:

[0037] (1) 0.2 g of Ag@MOF carrier and 0.32 g of Litsea cubeba essential oil were mixed to prepare a blend; the blend was added to 4 mL of dichloromethane, ultrasonically dispersed for 17 min, and stirred at room temperature for 8 h to fully evaporate the dichloromethane solvent to obtain an Ag@MOF carrier loaded with Litsea cubeba essential oil;

[0038] (2) Polylactic acid (PLA) (PLA having a weight average molecular weight of 150,000) and Ag@MOF carrier loaded with Litsea cubeba essential oil were mixed in a mass ratio of 100:2, vacuum dried at 80°C for 5 hours, and then poured into the hopper of an LSJ-20 single-screw extruder. After melt extrusion, the extrudate was dried at 60°C for 5 hours and then placed in an LSC-120 film blowing machine for film blowing to obtain a biodegradable polylactic acid active packaging film.

[0039] At the same time, the composite membrane without adding Ag@MOF carrier was prepared according to the above method;

[0040] The polylactic acid active packaging film prepared in this example has a tensile strength of 13.1 MPa and an elongation at break of 31.9%, showing good film ductility. Compared with the composite film without the addition of Ag@MOF loaded with Litsea cubeba essential oil, the tensile strength is reduced by 55.6% and the elongation at break is increased by 23.9%. Its water vapor permeability is 1.02 g·mm / (m 2 ·h·KPa), showing good gas barrier properties. Compared with the composite film without Ag@MOF loaded with Litsea cubeba essential oil, the water vapor transmission rate increased by 22.8%; the release rate of Litsea cubeba essential oil on the first day was 6.7%, the cumulative release rate on the 5th day was 28.7%, the cumulative release rate on the 10th day was 36.6%, the cumulative release rate on the 20th day was 58.9%, the cumulative release rate on the 30th day was 78.7%, and the cumulative release rate on the 35th day was 99.7%. The sustained release time of Litsea cubeba essential oil was as long as 35 days, which was 19 days longer than that of the composite film without Ag@MOF loaded with Litsea cubeba essential oil.

[0041] (3) The biodegradable polylactic acid active packaging film in step (2) is heat-sealed to make a fresh-keeping bag for preserving the tricholoma. About 250 g of tricholoma is weighed and put into the bag, and fresh air is injected into the bag. The fresh-keeping bag is sealed and the bagged tricholoma is stored in a refrigerator at 4±1°C. After 12 days of storage, compared with the tricholoma to which the composite film of Ag@MOF loaded with litsea cubeba essential oil is not added, the browning index of the tricholoma treated with the polylactic acid active packaging film is reduced by 26.8%, the hardness is increased by 0.87 times, and the electrolyte permeability is reduced by 28.8%. The antibacterial effect of the plastic wrap was verified by analyzing the changes in microbial abundance at the phylum and genus levels during storage of B. cerevisiae. The results showed that bacteria on the surface of B. cerevisiae wrapped with polylactic acid active packaging film had a significant inhibitory effect on Proteobacteria at the phylum level: the relative abundance of Proteobacteria in B. cerevisiae on day 12 (48.9%) was lower than that on day 0 (80.7%). The relative abundance of B. cerevisiae treated with the polylactic acid active packaging film was even lower at the genus level: on day 12, Lactococcus accounted for 42.7% and Serratia for 5.61%, with no detection of Acinetobacter or Westerella. In B. cerevisiae treated with the plastic wrap without the addition of the Ag@MOF-loaded Litsea cubeba essential oil composite film, Acinetobacter accounted for 2.21%, Lactococcus accounted for 47.1%, Serratia accounted for 11.6%, and no detection of Westerella. The above preservation method effectively extended the shelf life of B. cerevisiae.

[0042] Example 4:

[0043] (1) 0.2 g of Ag@MOF carrier and 0.36 g of Litsea cubeba essential oil were mixed to prepare a blend; the blend was added to 6 mL of dichloromethane, ultrasonically dispersed for 18 min, and stirred at room temperature for 10 h to fully evaporate the dichloromethane solvent to obtain an Ag@MOF carrier loaded with Litsea cubeba essential oil;

[0044] (2) Polylactic acid (PLA) (PLA having a weight average molecular weight of 200,000) and Ag@MOF carrier loaded with Litsea cubeba essential oil were mixed in a mass ratio of 100:3, vacuum dried at 80°C for 6 hours, and then poured into the hopper of an LSJ-20 single-screw extruder. After melt extrusion, the extrudate was dried at 60°C for 6 hours and then placed in an LSC-120 film blowing machine for film blowing to obtain a biodegradable polylactic acid active packaging film.

[0045] At the same time, the composite membrane without adding Ag@MOF carrier was prepared according to the above method;

[0046] The polylactic acid active packaging film prepared in this example has a tensile strength of 14.1 MPa and an elongation at break of 30.7%, showing good film ductility. Compared with the composite film without the addition of Ag@MOF loaded with Litsea cubeba essential oil, the tensile strength is reduced by 55.7% and the elongation at break is increased by 22.6%. The water vapor permeability is 1.41 g·mm / (m 2 ·h·KPa), showing good gas barrier properties. Compared with the composite film without Ag@MOF loaded with Litsea cubeba essential oil, the water vapor transmission rate increased by 44.8%; the release rate of Litsea cubeba essential oil on the first day was 5.2%, the cumulative release rate on the 5th day was 28.9%, the cumulative release rate on the 10th day was 45.9%, the cumulative release rate on the 20th day was 55.7%, the cumulative release rate on the 30th day was 72.9%, and the cumulative release rate on the 36th day was 99.9%. The sustained release time of Litsea cubeba essential oil was as long as 36 days; compared with the composite film without Ag@MOF loaded with Litsea cubeba essential oil, the sustained release time was increased by 20 days;

[0047] (3) The biodegradable polylactic acid active packaging film in step (2) is heat-sealed to make a fresh-keeping bag for preserving the Bacillus cirrhosa. Approximately 250 g of Bacillus cirrhosa is weighed and placed in the bag, and fresh air is introduced into the bag. The bag is filled and sealed, and the bagged Bacillus cirrhosa is stored in a refrigerator at 4±1°C. After 12 days of storage, compared with the composite film without the addition of Ag@MOF loaded with Litsea cubeba essential oil, the browning index of the Bacillus cirrhosa treated with the polylactic acid active packaging film is reduced by 30.7%, the hardness is increased by 0.91 times, and the electrolyte permeability is reduced by 31.5%. The antibacterial effect of the plastic wrap was verified by analyzing the changes in microbial abundance at the phylum and genus levels during storage of B. cerevisiae. The results showed that bacteria on the surface of B. cerevisiae wrapped with the polylactic acid active packaging film had a significant inhibitory effect on Proteobacteria at the phylum level: the relative abundance of Proteobacteria in B. cerevisiae on day 12 (46.7%) was lower than that on day 0 (79.1%). B. cerevisiae treated with the polylactic acid active packaging film had even lower relative abundance at the genus level: on day 12, Lactococcus accounted for 39.4% and Serratia for 7.81%, with no detection of Acinetobacter or Westerella. In B. cerevisiae treated with the polylactic acid active packaging film without the addition of the Ag@MOF-loaded Litsea cubeba essential oil composite film, Acinetobacter accounted for 1.89%, Lactococcus accounted for 48.4%, Serratia accounted for 11.6%, and no detection of Westerella. The above preservation method effectively extended the shelf life of B. cerevisiae.

[0048] Example 5

[0049] (1) 0.2 g of Ag@MOF carrier and 0.4 g of Litsea cubeba essential oil were mixed to prepare a blend; the blend was added to 10 mL of dichloromethane, ultrasonically dispersed for 25 min, and stirred at room temperature for 15 h to fully evaporate the dichloromethane solvent to obtain an Ag@MOF carrier loaded with Litsea cubeba essential oil;

[0050] (2) Polylactic acid (PLA) (PLA having a weight average molecular weight of 300,000) and Ag@MOF carrier loaded with Litsea cubeba essential oil were mixed in a mass ratio of 100:3, vacuum dried at 80°C for 8 hours, and then poured into the hopper of an LSJ-20 single-screw extruder. After melt extrusion, the extrudate was dried at 60°C for 7 hours and then placed in an LSC-120 film blowing machine for film blowing to obtain a biodegradable polylactic acid active packaging film.

[0051] At the same time, the composite membrane without adding Ag@MOF carrier was prepared according to the above method;

[0052] The polylactic acid active packaging film prepared in this example has a tensile strength of 13.6 MPa and an elongation at break of 31.4%, showing good film ductility. Compared with the composite film without the addition of Ag@MOF loaded with Litsea cubeba essential oil, the tensile strength is reduced by 52.1% and the elongation at break is increased by 25.1%. The water vapor permeability is 2.66 g·mm / (m 2 ·h·KPa), showing good gas barrier properties. Compared with the composite film without Ag@MOF loaded with Litsea cubeba essential oil, the water vapor transmission rate increased by 68.2%. The release rate of Litsea cubeba essential oil was 8.9% on the first day, 26.7% on the fifth day, 39.2% on the tenth day, 50.1% on the 20th day, 64.9% on the 30th day, and 99.8% on the 39th day. The sustained release time of Litsea cubeba essential oil was as long as 40 days. Compared with the composite film without Ag@MOF loaded with Litsea cubeba essential oil, the sustained release time was increased by 23 days.

[0053] (3) The biodegradable polylactic acid active packaging film in step (2) is heat-sealed to make a fresh-keeping bag for preserving the Bacillus cirrhosa. Approximately 250 g of Bacillus cirrhosa is weighed and placed in the bag, and fresh air is introduced into the bag. The bag is sealed and the bagged Bacillus cirrhosa is stored in a refrigerator at 4±1°C. After 12 days of storage, compared with the composite film without the addition of Ag@MOF loaded with Litsea cubeba essential oil, the browning index of the Bacillus cirrhosa treated with the fresh-keeping film is reduced by 33%, the hardness is increased by 1.05 times, and the electrolyte permeability is reduced by 37.2%. The antibacterial effect of the plastic wrap was verified by analyzing the changes in microorganisms at the phylum and genus levels during storage. The results showed that the surface bacteria of B. cerevisiae packaged with the polylactic acid active packaging film had a significant inhibitory effect on Proteobacteria at the phylum level: the relative abundance of Proteobacteria in B. cerevisiae on day 12 (42.3%) was lower than that on day 0 (76.9%). The relative abundance of B. cerevisiae treated with the polylactic acid active packaging film was even lower at the genus level: on day 12, Lactococcus accounted for 36.2% and Serratia for 4.67%, with no detection of Acinetobacter and Weissella. In B. cerevisiae treated with the polylactic acid active packaging film without the addition of the Ag@MOF-loaded Litsea cubeba essential oil composite film, Acinetobacter accounted for 2.31%, Lactococcus accounted for 47.2%, Serratia accounted for 10.71%, and no detection of Weissella. The above preservation method effectively extended the shelf life of B. cerevisiae.

Claims

1. Application of a polylactic acid active packaging film in the preservation of Bacillus thuringiensis; The polylactic acid active packaging film is prepared by using an Ag@MOF carrier to load a natural antibacterial agent, which is then fully mixed with polylactic acid and then melt extrusion blow molding. The weight average molecular weight of the polylactic acid is 100,000 to 300,000, and the natural antibacterial agent is Litsea cubeba essential oil; The Ag@MOF carrier and the natural antibacterial agent are mixed in a mass ratio of 5:6~12, the blend is added to dichloromethane, ultrasonically dispersed for 15~25 minutes, and then stirred at room temperature to fully evaporate the dichloromethane to obtain Ag@MOF nanoparticles loaded with the natural antibacterial agent.

2. The use according to claim 1, characterized in that: Polylactic acid and Ag@MOF carrier loaded with natural antibacterial agent were mixed in a mass ratio of 100:1~6.

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