A passive modified atmosphere preservation bag and a method for making the same - preservation of fruits and vegetables for tolerance to higher carbon dioxide atmosphere

Abstract

A passive modified atmosphere (MAP) bag and its preparation method are disclosed – used for preserving fruits and vegetables that can withstand high carbon dioxide atmospheres, belonging to the field of fruit and vegetable preservation technology. It is prepared using a three-layer co-extruded film and laser perforation. The first layer is a polyethylene-modified vinyl acetate copolymer resin film, composed of polyethylene and EVA; the second layer, the middle core layer, is a nanocellulose / EVA composite material, composed of nanocellulose and EVA; the third layer, the fruit and vegetable contact layer, is a modified EVA inner layer with moderate polarity and antibacterial function, prepared from LDPE, EVA, sodium bisulfite, and citric acid. The MAP of this invention has low carbon dioxide permeability, and laser perforation is performed on the film to improve oxygen permeability. The high carbon dioxide atmosphere established by this MAP significantly inhibits the respiration rate of fruits and vegetables, has suitable water vapor permeability, and inhibits mold growth.

Description

Technical Field

[0001] This invention belongs to the field of fruit and vegetable preservation technology, specifically relating to a passive modified atmosphere (MAP) bag and its preparation method, which is suitable for fruits and vegetables that can withstand high carbon dioxide atmospheres. Background Technology

[0002] Carbon dioxide is a metabolic byproduct of postharvest storage of fruits and vegetables. A high carbon dioxide atmosphere surrounding the stored fruits and vegetables lowers their metabolic rate compared to a zero-carbon dioxide atmosphere, thus extending their shelf life. This results in firmer fruits and vegetables, higher starch and polysaccharide content, and better quality preservation. Passive atmosphere modified storage (MAP) bags utilize this mechanism to extend the shelf life of harvested fruits and vegetables. Studies have shown that a high carbon dioxide atmosphere can cause carbon dioxide damage to some fruits, such as causing "black core" in apples and pears and sugaring in fresh dates. However, other fruits, such as lychees, durians, and mangosteens, can tolerate high carbon dioxide atmospheres, and a high carbon dioxide atmosphere is beneficial for their postharvest storage and preservation.

[0003] To achieve the purpose of using MAP to preserve fruits and vegetables, the fruits and vegetables are sealed in MAP preservation bags. In addition to the MAP passively establishing an atmosphere with a controllable carbon dioxide concentration, the MAP film (preservation bag) also needs to have a suitable water vapor transmission rate (WVTR). Through the film, some water vapor (produced by the metabolism of fruits and vegetables) is removed into the external environment, forming a dynamic balance of water vapor diffusion. This maintains a suitable relative humidity (RH) inside the MAP, preventing the water vapor inside the MAP from reaching a supersaturated state (condensation) and causing mold growth.

[0004] Currently, MAP fruit and vegetable preservation bags on the market are generally made of polyethylene or polypropylene and are used in open, perforated, or sealed forms. These MAP preservation bags can generally only create an atmosphere of low carbon dioxide (less than 5%) and high oxygen concentration (greater than 15%), which is not suitable for the preservation of fruits such as lychee, durian, and mangosteen that are tolerant of carbon dioxide gas. The preservation effect on these fruits will be poor.

[0005] Currently, the polyethylene or polypropylene headspace bags circulating in the market can only create a headspace atmosphere with a low carbon dioxide concentration after fruits and vegetables are packed in them. This is because polyethylene or polypropylene films have a very high carbon dioxide permeability, about four times that of oxygen. MAP materials also have a high oxygen permeability, so they can only create a headspace atmosphere with a high oxygen concentration and a low carbon dioxide concentration. This atmosphere is suitable for storing general fresh vegetables. However, the drawback is that this high oxygen concentration and low carbon dioxide concentration headspace atmosphere has a low ability to inhibit the respiration (metabolism) rate of vegetables and is prone to forming high RH inside the MAP, causing rot.

[0006] MAP (Magnetic Headspace Modulation) films, capable of creating headspace atmospheres with lower oxygen concentrations and higher carbon dioxide concentrations, are made of materials different from ordinary polyethylene or polypropylene. Polyethylene and polypropylene films are non-polar films, in which non-polar oxygen and carbon dioxide have high solubility and high compatibility. Therefore, these gases have high permeability in polyethylene or polypropylene films, resulting in MAPs made from these materials only being able to create headspace atmospheres with higher oxygen concentrations and lower carbon dioxide concentrations. If a more polar film is chosen for MAP preparation, a headspace atmosphere with higher carbon dioxide concentrations can be formed. This is because the solubility of oxygen and carbon dioxide, two non-polar gases, is very low in polar films, meaning their compatibility is poor. Therefore, the permeability of oxygen and carbon dioxide in polar films is very low, and most of the carbon dioxide produced by fruit and vegetable metabolism cannot diffuse through the film to the outside, creating a carbon dioxide-rich environment. Furthermore, the polarity of polar film molecules is similar to that of water molecules, allowing a large portion of water vapor to diffuse through the film to the outside. The relative humidity inside a MAP prepared with a polar film is also much lower than that prepared with ordinary polyethylene (polypropylene), avoiding mold growth caused by excessively high RH. One of the drawbacks of MAPs prepared with polar thin films is their low oxygen transport capacity, which can easily lead to oxygen deficiency inside the MAP. Summary of the Invention

[0007] The present invention aims to provide a passive modified atmosphere packaging (MAP) bag, its preparation method and application. The MAP bag can effectively preserve fruits and vegetables with high respiration intensity and tolerance to high concentrations of carbon dioxide, thus maintaining their nutritional value and freshness.

[0008] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0009] A passive modified atmosphere packaging bag is characterized by being made of a film with a three-layer co-extruded structure (hereinafter referred to as three-layer co-extruded film) and prepared by laser perforation; the first layer, i.e., the outermost layer, is a polyethylene (LDPE) modified vinyl acetate copolymer (EVA) resin film, composed of polyethylene (LDPE) and EVA, with polyethylene accounting for 5-20% of the total weight of the outer layer, preferably 10%; the second layer, i.e., the middle core layer, is a highly polar nanocellulose / EVA composite material, composed of nanocellulose and EVA. Composition A comprises nanocellulose accounting for 30-45% (e.g., 30%, 40%, 45%) of the total weight of the core layer; the third layer of the film, namely the inner layer that contacts fruits and vegetables, is a modified EVA inner layer with moderate polarity and antibacterial function. The inner layer is prepared from LDPE, EVA, sodium bisulfite and citric acid, wherein LDPE accounts for 4-20% of the total weight of the inner layer, sodium bisulfite accounts for 0.5-3.5% of the total weight of the outer layer, citric acid accounts for 0.3-2.0% of the total weight of the outer layer, and the balance is EVA.

[0010] Furthermore, the mass ratio of sodium sulfite to citric acid powder (300-500 mesh) in the inner layer is 1.6:1.

[0011] Furthermore, in the inner layer, LDPE, sodium sulfite, and citric acid account for 9.5%, 3.08%, and 1.92% of the total weight of the inner layer, respectively.

[0012] The density of laser drilling is 1.0-4.0 cm (e.g., 1.0, 2.0, 3.0, 4.0 cm, etc.), and the hole diameter is 0.1-0.5 mm.

[0013] The thickness of the three-layer co-extruded film is 20-60 micrometers, preferably 30 micrometers, with the core layer, inner layer, and outer layer accounting for 2 / 3, 1 / 6, and 1 / 6 of the total film thickness, respectively. The nanocellulose in the core layer has higher polarity than EVA, therefore the core layer has the highest polarity in the three-layer co-extruded film and also accounts for the largest proportion of the three layers, contributing the most to the WVTR of the three-layer co-extruded film.

[0014] The above-mentioned method for preparing a passive modified atmosphere packaging (MAP) bag is characterized by comprising the following steps:

[0015] S1: Preparation of core material: Dilute nanocellulose with water, add EVA emulsion, stir at high speed until uniformly mixed, and obtain cellulose / EVA powder by spray drying.

[0016] S2. Preparation of LDPE modified EVA outer layer material: LDPE and EVA resin are melt-blended in a twin-screw extruder at 120-160℃ and pelletized to obtain modified EVA outer layer resin;

[0017] S3. Preparation of inner layer (fruit contact layer) material: LDPE, EVA, sodium bisulfite and citric acid are physically mixed and then melt-blended in a twin-screw extruder at 120-160℃, and then pelletized to obtain modified EVA inner layer resin.

[0018] S4. Add three modified resins to the hoppers of the inner, core, and outer screws of the three-screw co-extrusion equipment, set the screw temperature to 120-160℃, adjust the feeding speed of the three screws, and adjust the proportion of the inner, outer, and core layers to the total film thickness.

[0019] S5. Laser perforation is performed on the prepared three-layer co-extruded film to make bags.

[0020] This invention also provides a passive modified atmosphere packaging bag (MAP) prepared by the aforementioned preparation method, or the application of the aforementioned passive modified atmosphere packaging bag in fruit and vegetable preservation.

[0021] Preferably, the vegetables include cucumbers, tomatoes, eggplants, peppers, and zucchini, and the fruits include mangosteens, passion fruit, durians, grapes, and sweet corn.

[0022] Compared with the prior art, the present invention has the following advantages and technical effects:

[0023] Compared to ordinary polyethylene and polypropylene food storage bags, this three-layer co-extruded film uses highly polar EVA as the base material to improve the water vapor transmission rate (WVTR) of the three-layer co-extruded film. This highly polar film also has a high barrier to carbon dioxide, i.e., a low carbon dioxide transmission rate (CO2TR). Therefore, carbon dioxide produced by fruit and vegetable metabolism is blocked by the modified atmosphere (MAP) made from this material, accumulating in the MAP headspace and creating a high carbon dioxide concentration atmosphere. The oxygen transmission rate (OTR) of this highly polar three-layer co-extruded film is also low. To improve the OTR of the three-layer co-extruded film, it needs to be laser-perforated to increase the OTR of the MAP, establish a suitable oxygen concentration, and prevent anaerobic respiration of fruits and vegetables. This passive modified atmosphere storage bag can effectively preserve fruits and vegetables with high respiration rates and carbon dioxide tolerance. By adjusting the film composition, laser perforation density, and pore size, the oxygen concentration, carbon dioxide concentration, and relative humidity inside the MAP storage bag can be controlled, significantly extending the shelf life of fruits and vegetables in an environment of 85%–95% humidity.

[0024] The modified EVA resin has moderate water vapor permeability, while the EVA-modified nanocellulose (core layer) has a very high WVTR. Before laser perforation, the WVTR of this three-layer co-extruded film ranges from 15 to 50 g / m. 2 After laser drilling, the WVTR increases to 20-65 g / m². 2Due to the high polarity of nanocellulose (core layer), oxygen and carbon dioxide have very low solubility in the core layer (poor compatibility), resulting in a low OTR. Therefore, laser perforation is needed to improve the OTR of the film, achieving an OTR of 7000-19000 ml / m. 2 .day.atm.

[0025] This passive modified atmosphere packaging (MAP) bag creates an environment with a low oxygen concentration and a high carbon dioxide concentration, effectively slowing down the respiration of fruits and vegetables and reducing the metabolic breakdown of nutrients such as sugars and vitamins. Furthermore, by adjusting the proportion of polar resin (nanocellulose), the water vapor permeability of the three-layer co-extruded film can be controlled, effectively managing the humidity inside the bag and preventing mold growth and condensation caused by excessive humidity. The citric acid and sodium bisulfite components in the core resin provide antibacterial properties. When the humidity inside the MAP bag exceeds 85%, the dissolved water molecules in the inner layer create a humid environment where citric acid and sodium bisulfite react to generate sulfur dioxide gas, reducing mold growth on the vegetable surface. When the humidity is below 60%, the citric acid and sodium bisulfite in the inner layer remain relatively stable and do not react chemically.

[0026] Using the MAP method of this invention, fresh durian fruit was preserved at 20℃ for 12 days. The durian still had a fresh aroma, yellow-green skin, and no cracks. After being left in the air for 2 days after opening the bag, the skin cracked, indicating that the durian could ripen normally. The pulp maintained a high content of soluble solids (TSS) and vitamin C, which was significantly better than the control group that was left directly in the air.

[0027] Using the MAP of this invention, bunch tomatoes were preserved at 20℃ for 5 days. The fruit stems remained green, the petioles did not become moldy, the flesh was sweet and sour, and the flesh maintained a high content of soluble solids (TSS) and vitamin C, which was significantly better than bunch tomatoes preserved in ordinary CPP (cast polypropylene) perforated bags (4 holes of 0.5 cm).

[0028] Using the MAP method of this invention, bunch tomatoes and Kyoho grapes were preserved at 20°C for 6 days. The fruit stems remained green and did not turn brown. The pulp maintained a high content of soluble solids (TSS) and vitamin C, which was significantly better than grapes preserved in ordinary CPP (cast polypropylene) perforated bags (4 holes of 0.5 cm).

[0029] The MAP preservation bag of this invention provides an efficient and economical solution for the preservation of vegetables and fruits, helping to reduce the loss of agricultural products during transportation and storage, and maintaining their nutritional value and freshness. Detailed Implementation

[0030] The technical solution of the present invention will be further illustrated by the following embodiments.

[0031] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0032] In this invention, unless otherwise specified, all other test materials and instruments are conventional test materials in the field and can be purchased through commercial channels.

[0033] Materials and Instruments

[0034] Materials: Fresh durian fruit, purchased from the official Taobao store "Liu Xiao Xun", weighing 1.3-1.5 kg / fruit, AAA grade; ordinary CPP self-sealing bags (50 microns thick), purchased from the Taobao store "Sheng Jiang Flagship Store"; nanocellulose TL-010-2, Nanjing Tianlu Nanotechnology Co., Ltd.; EVA emulsion, 40% concentration, Wuhan Jiyesheng Chemical Co., Ltd.; EVA resin, Elvax® 265A, Dow Chemical Co., Ltd.

[0035] Instruments: HY-D laser perforation equipment, Zhejiang Zhoushan Archipelago New Area Ziqi Donglai Intelligent Packaging Co., Ltd.; GH-B600 bag making machine, Zhejiang Zhoushan Archipelago New Area Ziqi Donglai Intelligent Packaging Co., Ltd.; FY-11 electronic hygrometer, Shenzhen Youxin Electronic Technology Co., Ltd.; digital display refractometer, Japan Aituo PAL-1; fruit firmness tester GY-2, Beijing Wowei Technology Co., Ltd.; oxygen and moisture permeability tester, 2 / 22H, Mocon, USA; headspace atmosphere analyzer Dansenso® CheckPoint 4, Mocon, USA.

[0036] Preservation performance and nutritional composition test

[0037] CO2 and O2 concentrations in MAP

[0038] The CO2 and O2 concentrations in the headspace of each MAP group were monitored using a gas analyzer.

[0039] Moisture loss

[0040] After measuring the CO2 and O2 concentrations in the packaging, the fruit was removed and weighed. The weight was compared with the initial weight on day 0 to calculate the cumulative water loss rate of the fruit.

[0041] Browning (Mold) Index of Fruit Stalk

[0042] For grapes or bunches of tomatoes, remove all the grapes, leaving only the stems. Assuming the total stem length is L0 cm and the length of the stem that has turned brown (moldy) is L1 cm, then the browning (moldy) index is L1 / L0×100%.

[0043] hardness

[0044] Using an FHM-5 fruit firmness analyzer, the test head has a diameter of 2mm and a height of 10mm; for each bead, a 1cm section is cut off with a fruit knife along the equator. 2 The test head was inserted 10 mm into the pulp at a speed of 20 mm / min, and the fruit firmness value (kgf) was recorded.

[0045] Soluble solids (TSS)

[0046] Collect the fruit berries together and grind them at high speed for 1 minute. Filter the pulp through gauze to obtain a clear liquid. Use a refractometer to measure the TSS (Brix) of the fruit berries.

[0047] VC

[0048] Take 20 ml of the above juice clear liquid into an Erlenmeyer flask, add 0.5 g of 1% amylose solution as an indicator, and 50 ml of distilled water as a diluent. Titrate with 0.01 M iodine-potassium iodide standard solution until the solution turns deep blue and does not fade within 15 seconds. Calculate the total ascorbic acid (Vc) concentration in the sample (mg / 100g juice).

[0049] Titratable acid (TA)

[0050] The percentage of titratable acid (TA) was determined using the method of Sivakumar and Korsten. 20 ml of the above juice supernatant was placed in an Erlenmeyer flask, 2 drops of phenolphthalein indicator were added, and titrated with 0.1 M sodium hydroxide standard solution until the solution turned deep red. TA was expressed as citric acid.

[0051] Example 1

[0052] S1. Preparation of the core layer (cellulose / EVA composite material): 67 parts (by weight, 2%) of nanocellulose (2%) were diluted with 50 parts of water, and 5 parts of EVA emulsion (40wt%) were added. The mixture was stirred at high speed until homogeneous, and then spray-dried to obtain cellulose / EVA powder; {2%×67 / (2%×67+5×40%)=40%}

[0053] S2. Preparation of LDPE modified EVA outer layer material: 10 parts of LDPE and 90 parts of EVA resin are melt-blended in a twin-screw extruder at 120-160℃ and pelletized to obtain modified EVA outer layer resin.

[0054] S3. Preparation of inner layer (fruit contact layer) resin: 9.5 parts LDPE, 85.5 parts EVA, 3.08 parts sodium bisulfite and 1.92 parts citric acid are physically mixed and then melt-blended in a twin-screw extruder at 120-160℃, and pelletized to obtain modified EVA inner layer resin.

[0055] S4. Add three modified resins to the hoppers of the inner, core, and outer screws of the three-screw co-extrusion equipment, respectively. Set the screw temperature to 120-160℃ and adjust the feeding speed of the three screws so that the thickness of the inner and outer layers of the co-extruded film accounts for 1 / 6 of the total thickness of the film; and obtain a co-extruded film with a thickness of 60 micrometers.

[0056] S5. Laser perforation was performed on the prepared three-layer co-extruded film with a spacing of 4.0 cm and a pore size of 0.1 mm; the resulting film had an OTR of 6500 ml / m. 2 The CO2 TR was 3520 ml / m². 2 .day.atm, WVTR is 30 g / m 2 .day.

[0057] The durian was preserved using a laser-perforated film with three-sided sealing. The MAP environment was 85% RH, with a headspace oxygen concentration of 5.1% and a carbon dioxide concentration maintained at 14.9%. After being preserved at 20°C for 12 days, the durian was taken out and exposed to air for 2 days. The shell began to crack, and the pulp was removed and tested for quality.

[0058] Comparative Example 1

[0059] Fresh durian fruit was left exposed in the air at 20℃ (RH 45%). After 2 days, the fruit shell began to crack. The pulp was then removed and its quality was tested.

[0060] Example 2

[0061] S1. Preparation of the core layer (cellulose / EVA composite material): 82 parts (by weight, same below) of 2% nanocellulose were diluted with 50 parts of water, and then 5 parts of EVA emulsion (40wt%) were added. The mixture was stirred at high speed until homogeneous, and then spray-dried to obtain cellulose / EVA powder; {2%×82 / (2%×82+5×40%)=45%}

[0062] S2. Preparation of LDPE modified EVA outer layer material: 5 parts LDPE and 95 parts EVA resin are melt-blended in a twin-screw extruder at 120-160℃, and then pelletized to obtain modified EVA outer layer resin.

[0063] S3. Preparation of inner layer (fruit contact layer) resin: 4.95 parts LDPE, 94.05 parts EVA, 0.615 parts sodium bisulfite and 0.385 parts citric acid are physically mixed and then melt-blended in a twin-screw extruder at 120-160℃, and pelletized to obtain modified EVA inner layer resin.

[0064] S4. Add three modified resins to the hoppers of the inner, core, and outer screws of the three-screw co-extrusion equipment, respectively. Set the screw temperature to 120-160℃ and adjust the feeding speed of the three screws so that the thickness of the inner and outer layers of the co-extruded film accounts for 1 / 6 of the total thickness of the film; a co-extruded film with a thickness of 30 micrometers is obtained.

[0065] S5. Laser perforation was performed on the prepared three-layer co-extruded film with a spacing of 1.0 cm and a pore size of 0.5 mm; the resulting film had an OTR of 240,000 ml / m³. 2 The day-atm CO2 TR is 18500 ml / m 2 .day.atm, WVTR is 61g / m 2 .day.

[0066] Tomatoes were prepared using a MAP (Modified Absorbent Material) preservation method by sealing the three sides of the laser-perforated film. The MAP established a RH environment of 95%, with an oxygen concentration of 17.5% in the headspace and a carbon dioxide concentration of 2.5%. After being preserved at 20°C for 5 days, the tomatoes were removed and their quality was tested.

[0067] Comparative Example 2

[0068] Tomato bunches were preserved in a regular CPP bag with four 0.5 cm holes. The oxygen and carbon dioxide concentrations in the headspace were the same as in the air, and the relative humidity (RH) was 99%. After being preserved at 20°C for 5 days, the tomatoes were removed and their quality was tested.

[0069] Table 2. Quality Comparison of Cluster Tomatoes

[0070]

[0071] Post-harvest bunch tomatoes showed deterioration primarily in the appearance of moldy sepals and fruit drop. Bunches of tomatoes preserved in perforated CPP bags experienced increased respiration rates due to the large pores, leading to rapid metabolism and fruit drop. Fruits began to detach from the abscission layer after 3 days, and the drop rate reached 38% after 5 days (Table 2). Although perforated CPP bags have a large pore structure, this structure contributes little to water vapor permeability. The relative humidity in areas far from the pores still reached 99%, causing mold adhering to the fruit to multiply rapidly in this high-humidity environment, resulting in mold growth on 57% of the sepals.

[0072] The three-layer co-extruded film with a total thickness of 30 micrometers has a 0.5 mm hole every 1 cm, resulting in a large pore size and the highest pore density. MAP has high OTR and CO2TR. MAP establishes an atmosphere with a concentration of 17.5% oxygen and 2.5% carbon dioxide. For tomatoes, the carbon dioxide concentration in the more suitable storage environment should not be too high. An atmosphere with a carbon dioxide concentration greater than 5% will affect the activity of various metabolic molds in tomatoes.

[0073] The core layer has the highest cellulose content, the thinnest film, and the largest pore size and pore density, resulting in a WVTR of 61 g / m² for this MAP film. 2 On day, an 85% RH environment was established to eliminate the fruit splattering problem caused by the high humidity environment of traditional CPP. Although the water loss rate was slightly higher, the firmness of the fruit flesh, TSS and vitamin C of the bunch tomatoes preserved by MAP for 5 days were higher than those of the CPP control group.

[0074] Example 3

[0075] S1. Preparation of core layer (cellulose / EVA composite material): 43 parts of 2% nanocellulose (by weight, the same below) are diluted with 50 parts of water, and then 5 parts of EVA emulsion (40wt% concentration) are added {2%×43 / (2%×43+5×40%)=30%}. The mixture is stirred at high speed until it is uniformly mixed, and cellulose / EVA powder is obtained by spray drying.

[0076] S2. Preparation of LDPE modified EVA outer layer material: 20 parts of LDPE and 80 parts of EVA resin are melt-blended in a twin-screw extruder at 120-160℃ and pelletized to obtain modified EVA outer layer resin.

[0077] S3. Preparation of inner layer (fruit contact layer) resin: 19.4 parts LDPE, 77.6 parts EVA, 1.85 parts sodium bisulfite and 1.15 parts citric acid are physically mixed and then melt-blended in a twin-screw extruder at 120-160℃, and pelletized to obtain modified EVA inner layer resin.

[0078] S4. Add three modified resins to the hoppers of the inner, core, and outer screws of the three-screw co-extrusion equipment, respectively. Set the screw temperature to 120-160℃ and adjust the feeding speed of the three screws so that the thickness of the inner and outer layers of the co-extruded film accounts for 1 / 6 of the total thickness of the film; a co-extruded film with a thickness of 40 micrometers is obtained.

[0079] S5. Laser perforation was performed on the prepared three-layer co-extruded film with a spacing of 4.0 cm and a pore size of 0.1 mm; the resulting film had an OTR of 3200 ml / m. 2 The day-atm CO2 TR is 1700 ml / m 2.day.atm, WVTR is 22 g / m 2 .day.

[0080] The laser-perforated film was used to create a three-sided sealed MAP (Modified Absolute Motion) environment for preserving Kyoho grapes. The MAP environment was RH 90%, with an oxygen concentration of 11.4% in the headspace and a carbon dioxide concentration maintained at 8.6%. After being preserved at 20°C for 6 days, the grapes were taken out and their quality was tested.

[0081] Comparative Example 3

[0082] Kyoho grapes were preserved in ordinary CPP bags with four 0.5 cm holes. The oxygen and carbon dioxide concentrations in the headspace were the same as those in the air, and the relative humidity (RH) was 92%. After being preserved at 20°C for 6 days, the grapes were taken out and their quality was tested.

[0083] The following experiments were conducted to verify the effectiveness of the food preservation bags provided in Examples 1-3 and Comparative Examples 1-3 (control group).

[0084] Table 1. Quality Comparison of Durian

[0085]

[0086] Durian has a respiration rate of more than 220 mlCO2 / Kg.hr at 20℃. Therefore, durians left exposed to air will ripen within 2 days under such a high metabolic rate. The 9.3% water loss causes the shell to crack, and the shelf life is only 2 days (Table 1).

[0087] The three-layer co-extruded film, with a total thickness of 60 micrometers, exhibits good puncture resistance, preventing durian spikes from piercing the MAP. A 0.1mm pore is punched every 4 centimeters, resulting in a small pore size and lowest density. The MAP has low OTR and CO2TR, creating a 5.1% oxygen concentration atmosphere that keeps the durian in a dormant state, maximally limiting its metabolic rate. Furthermore, the ultra-high 14.9% carbon dioxide concentration not only helps inhibit durian metabolism but also, in turn, inhibits mold growth. The MAP film has a thickness of 30 g / m³. 2 The WVTR (Wastewater Transmission Rate) caused a daily water loss of 0.6%, and after 12 days, the durian shell had not cracked, remained yellow-green, and had a fragrant aroma. After opening the bag, the durian continued to ripen normally for two days. The durian preserved by MAP for 12 days had higher flesh firmness, TSS (Total Water Surface Saturation), and Vitamin C levels than the untreated control group.

[0088] Table 2. Quality Comparison of Cluster Tomatoes

[0089]

[0090] Post-harvest bunch tomatoes showed deterioration primarily in the appearance of moldy sepals and fruit drop. Bunches of tomatoes preserved in perforated CPP bags experienced increased respiration rates due to the large pores, leading to rapid metabolism and fruit drop. Fruits began to detach from the abscission layer after 3 days, and the drop rate reached 38% after 5 days (Table 2). Although perforated CPP bags have a large pore structure, this structure contributes little to water vapor permeability. The relative humidity in areas far from the pores still reached 99%, causing mold adhering to the fruit to multiply rapidly in this high-humidity environment, resulting in mold growth on 57% of the sepals.

[0091] A three-layer co-extruded film with a total thickness of 30 micrometers features 0.5mm pores spaced 1 cm apart, resulting in large pore size and maximum pore density. MAP exhibits high OTR and CO2TR. MAP establishes an atmosphere with 17.5% oxygen and 2.5% carbon dioxide concentration. For tomatoes, a suitable storage environment requires a low carbon dioxide concentration; an atmosphere exceeding 5% carbon dioxide can negatively impact the activity of various metabolic fungi within the tomato. Cluster tomatoes are typically grown in greenhouses. During the summer rainy season, increased humidity in greenhouses allows mold spores to spread in the air, contaminating the fruit and sepals. The sepals, in particular, are most susceptible to mold growth during storage and transportation, affecting marketability. The sodium bisulfite and citric acid in the inner layer of the MAP film effectively kill some mold, preserving the green color of the sepals. The core layer has the highest cellulose content, the thinnest film, and the largest pore size and density, resulting in a WVTR of 61 g / m² for this MAP film. 2 On day, an 85% RH environment was established to eliminate the fruit splattering problem caused by the high humidity environment of traditional CPP. Although the water loss rate was slightly higher, the firmness of the fruit flesh, TSS and vitamin C of the bunch tomatoes preserved by MAP for 5 days were higher than those of the CPP control group.

[0092] Table 3 Comparison of Grape Quality

[0093]

[0094] Post-harvest grapes show deterioration primarily in the form of browning of the stems, berry drop, and mold growth at the detached berries. When Kyoho grapes are preserved in perforated CPP bags, the large pores of the bags allow for oxygen and carbon dioxide concentrations in the headspace to be almost identical to those in the air. This prevents the grapes from respiring, and the stems, with their higher respiration rate than the berries, exhibit rapid metabolic activity. Within two days, the stems begin to age, lose water, and gradually turn brown (Table 3). As the stems harden and the connection to the berries weakens, the berries fall off automatically or due to vibrations during transport. After six days, the berry drop rate reaches 66%. High-sugar sap oozes from the detached berries, promoting mold growth. The wounds on the berries turn white and deteriorate, reducing their marketability.

[0095] A three-layer co-extruded film with a total thickness of 40 micrometers was prepared, with a 0.1 mm pore spaced every 4 cm. The small pore size and low pore density resulted in a low OTR and CO2TR for the MAP (Modified Aeration Membrane) film, ideal for preserving grapes and other fruits and vegetables with low respiration rates. The MAP established an atmosphere of 11.4% oxygen and 8.6% carbon dioxide. For grapes, the optimal carbon dioxide concentration should not exceed 15%, otherwise it will cause carbon dioxide damage. The 8.6% carbon dioxide concentration established by the MAP significantly reduced the respiration rate of grape berries and stems, reducing stem browning to 5%, keeping the stems green and supple, preventing a decrease in the bond between the stems and berries, and thus preventing berry detachment. Because grapes have a low respiration rate and produce less water vapor, the WVTR of the film prepared in this example was only 22 g / m³. 2 A 92% RH environment was established. Similar to tomatoes, grapes are prone to bursting (the skin absorbs water, swells, and cracks) in high humidity environments, while low humidity accelerates the browning and dehydration of the stem. Therefore, mold growth is a difficult problem to solve. In this example, a higher proportion of sodium bisulfite and citric acid were added to enhance the bactericidal ability of MAP. Grapes preserved with MAP for 6 days showed higher pulp firmness, TSS (total salinity), and vitamin C levels than the CPP control group.

[0096] The film of this invention has a moderately polar modified EVA (ethylene-vinyl acetate copolymer) outer layer, a high-polarity core layer, and a moderately polar modified EVA inner layer with antibacterial properties. Compared to ordinary polyethylene and polypropylene food storage bags, this three-layer co-extruded film uses high-polarity EVA as the substrate to improve the water vapor transmission rate (WVTR) of the three-layer co-extruded film. Furthermore, this high-polarity film has high carbon dioxide barrier properties, i.e., a low carbon dioxide transmission rate (CO2TR). Therefore, the carbon dioxide produced by fruit and vegetable metabolism will be absorbed by the MA film made from this material. P is blocked and accumulates in the headspace of the MAP, forming a high carbon dioxide concentration atmosphere. The oxygen transmission rate (OTR) of this high polarity three-layer co-extruded film is also low. In order to improve the oxygen transmission rate of the three-layer co-extruded film, the film needs to be laser-perforated to increase the OTR of the MAP, establish a suitable oxygen concentration, and prevent anaerobic respiration of fruits and vegetables. A high polarity nanocellulose / EVA composite material is prepared as the core layer. The core layer has the highest thickness. The polarity of nanocellulose is higher than that of EVA. Therefore, the polarity of the core layer is the highest in the three-layer co-extruded film and contributes the most to the WVTR of the three-layer co-extruded film.

[0097] The purpose of adding polyethylene (LDPE) modified EVA resin as an outer layer material is to improve the mechanical properties (modulus and tensile strength) of EVA, improve the film processing performance (film bubble stability), reduce the stickiness of EVA film, reduce the friction coefficient between the film and the working platform (metal plate) of the bag making machine, and improve the opening performance of MAP food storage bags.

[0098] In summary, the MAP prepared by this invention, compared with ordinary CPP material food preservation bags, has adjustable oxygen, carbon dioxide concentration and humidity, and also has antibacterial function. It can establish a headspace atmosphere with low oxygen concentration, high carbon dioxide concentration and 80%-95% relative humidity environment, thus maximizing the preservation of the nutritional quality of carbon dioxide-tolerant fruits and vegetables.

[0099] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A passive modified atmosphere packaging bag, characterized in that, The film is prepared by laser perforation using a three-layer co-extruded structure (referred to as three-layer co-extruded film). The first layer, the outermost layer, is a polyethylene (LDPE) modified vinyl acetate copolymer (EVA) resin film, composed of polyethylene (LDPE) and EVA, with polyethylene accounting for 5-20% of the total weight of the outer layer, preferably 10%. The second layer, the middle core layer, is a high-polarity nanocellulose / EVA composite material, composed of nanocellulose and EVA, with nanocellulose accounting for 30-45% of the total weight of the core layer. The third layer, the inner layer for contacting fruits and vegetables, is a modified EVA inner layer with medium polarity and antibacterial function, prepared from LDPE, EVA, sodium bisulfite, and citric acid, with LDPE accounting for 4-20% of the total weight of the inner layer, sodium bisulfite accounting for 0.5-3.5% of the total weight of the outer layer, citric acid accounting for 0.3-2.0% of the total weight of the outer layer, and the remainder being EVA. The thickness of the three-layer co-extruded film is 20-60 micrometers.

2. A passive modified atmosphere packaging bag according to claim 1, characterized in that, The mass ratio of sodium sulfite to citric acid powder (300-500 mesh) in the inner layer is 1.6:

1.

3. A passive modified atmosphere packaging bag according to claim 1, characterized in that, In the inner layer, LDPE, sodium sulfite, and citric acid account for 9.5%, 3.08%, and 1.92% of the total weight of the inner layer, respectively.

4. A passive modified atmosphere packaging bag according to claim 1, characterized in that, The density of laser drilling is 1.0-4.0 cm (e.g., 1.0, 2.0, 3.0, 4.0 cm, etc.), and the hole diameter is 0.1-0.5 mm.

5. A passive modified atmosphere packaging bag according to claim 1, characterized in that, The thickness of the three-layer co-extruded film is 30 micrometers.

6. A passive modified atmosphere packaging bag according to claim 1, characterized in that, The core layer, inner layer, and outer layer account for 2 / 3, 1 / 6, and 1 / 6 of the total film thickness, respectively.

7. A method for preparing a passive modified atmosphere packaging bag according to any one of claims 1-6, characterized in that, Includes the following steps: S1: Preparation of core material: Dilute nanocellulose with water, add EVA emulsion, stir at high speed until uniformly mixed, and obtain cellulose / EVA powder by spray drying. S2. Preparation of LDPE modified EVA outer layer material: LDPE and EVA resin are melt-blended in a twin-screw extruder at 120-160℃ and pelletized to obtain modified EVA outer layer resin; S3. Preparation of inner layer (fruit contact layer) material: LDPE, EVA, sodium bisulfite and citric acid are physically mixed and then melt-blended in a twin-screw extruder at 120-160℃, and then pelletized to obtain modified EVA inner layer resin. S4. Add three modified resins to the hoppers of the inner, core, and outer screws of the three-screw co-extrusion equipment, set the screw temperature to 120-160℃, adjust the feeding speed of the three screws, and adjust the proportion of the inner, outer, and core layers to the total film thickness. S5. Laser perforation is performed on the prepared three-layer co-extruded film to make bags.

8. The application of a passive modified atmosphere packaging bag as described in any one of claims 1-6, in the preservation of fruits and vegetables.

9. The application according to claim 8, wherein the vegetable includes one of cucumber, tomato, eggplant, pepper and zucchini, and the fruit includes one of mangosteen, passion fruit, durian, grape and sweet corn.

10. The application according to claim 8 extends the shelf life of fruits and vegetables in an environment with 85% to 95% humidity.

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