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Application of plastic packaging in fresh-keeping packaging of agricultural products

Plastic packaging materials and containers are the most used materials and containers for fresh-keeping packaging of agricultural products. Fresh agricultural products are accompanied by the respiration of tissues and the metabolism of enzymes Generally speaking, metabolism slows down due to the decline of temperature, and increases proportionally with the life of goods. However, the vitality of fresh agricultural products is related to their own characteristics and environmental factors. This related factor and characteristic is that plastic packaging materials can be kept fresh by applying certain technical treatment. There are many kinds of fresh-keeping technologies for plastic packaging materials

1. moisturizing and fresh keeping

any fresh agricultural products will have water evaporation after picking, which will affect their quality. Therefore, many plastic packaging films can prevent or reduce water evaporation to achieve fresh-keeping effect. Plastic films of different materials and thicknesses have different moisturizing effects. Polyvinylidene chloride is the plastic packaging film with the best moisturizing and sealing effect. Generally, the plastic packaging film used for fruit or vegetable packaging is polyethylene. This is a kind of fresh-keeping material for fresh agricultural products that was first accepted and popularized

2. Anti atomization, condensation and fresh keeping

the moisture preservation mentioned above depends on the sealing and barrier properties of plastic packaging materials, but the water content of fresh agricultural products is very high, and the sealed package is in a high humidity water vapor supersaturation state. The inner surface of plastic packaging film is easy to condense into water droplets. This condensation or atomization not only reduces the transparency, but also accelerates the decay and deterioration of agricultural products. In order to avoid this phenomenon, when developing packaging plastic materials, we should change its process and add a variety of components, such as glycerol, polyoxide, etc., so as to achieve anti atomization and condensation

an existing anti fog and anti condensation "ethylene vinyl alcohol copolymer (EVOH) packaging film" can achieve the purpose of anti fog and anti condensation and fresh-keeping. After two days of packaging leeks and spinach with this material, it will not shrink, its color will not change, it will not decay, and the inner wall of the film will not condense

3. porous and breathable fresh-keeping

many agricultural products can be sealed to extend their fresh-keeping period, but after the packaging is sealed, fresh agricultural products will produce a lot of respiratory gas, which will make the packaging swell and burst after reaching a certain degree. The porous and breathable polyolefin film can achieve breathable and fresh-keeping. After the package is sealed, the permeability of oxygen or air is increased, while the permeability of water and C02 is reduced. This porous breathable film is partially perforated on its material by discharge method, and the specific perforation is made by high-energy electronic light. The number of holes per square meter can reach 1.1 × 106. Any fresh-keeping packaging of raw fish and other agricultural products has a problem of excluding a certain amount of oxygen or air. It is hoped that the packaging can be made with breathable film. Therefore, the value of work consumed in the impact sample can be read directly after impact, and the internal and external air pressure is relatively balanced. In particular, the retention of carbon dioxide gas is more conducive to preservation

4. Water permeable and impermeable film preservation

many agricultural products need to be compatible with a certain amount of water in the fresh-keeping packaging, but in the storage process after packaging, gas will be generated to form water vapor, which will expand the packaging bag. At this time, water permeable steaming is required until the conditioning is qualified. Gas impermeable film preservation

the moisture permeable and impermeable fresh-keeping film is usually made by using polyethylene as the base material and adding different components through special processes. The principle of water vapor impermeability is that liquid water has a strong intermolecular hydrogen bond, so the molecular formula of its associated molecule is close to (H20) 8 at room temperature, while the gaseous water structure is smaller, and its molecular formula is H20, but it cannot pass through the larger liquid associated molecule (H20) 8. The water vapor transmission rate of this plastic packaging film is 1.5E/100cra2 h。

5. Automatic sealing heat shrinkable film preservation

in the process of plastic film manufacturing, different methods (extrusion, compounding, etc.) are used to produce different effects on different layers. Among them, there are two layers: automatic sealing and layering and automatic sealing of barrier layer, and the heat shrinkable packaging film, which is also necessary for the safe start of product experiment, has a certain fresh-keeping effect. When this kind of film is heated to a certain temperature, it can realize automatic sealing by relying on the shrinkage of the film and the adhesion of the inner and outer films. Because the barrier layer is added when compounding, it can produce good barrier to the atmosphere and water vapor, and tighten the loose agricultural product packaging materials, reduce the evaporation area, so as to achieve fresh-keeping. This kind of film is mainly used for the packaging of fresh meat materials

6. Self adhesive stretch film preservation

the biggest feature of this kind of packaging film is that it can be white sticky and stretch tight. In addition, it also has the transparency, barrier and other functions of other films. Therefore, it is widely used in the fresh-keeping packaging of small agricultural products, vegetables and fresh meat in modern supermarkets. Self adhesion is that it only has adhesion to its own surface. After packing, wrap it for a few more circles, and then cut it off. It can wrap the finished product well. It is also convenient to use, and the contents can be opened by gently pulling away the winding end. This is the most commonly used packaging material in clean vegetable agricultural products at present

7. gas selective preservation

this kind of preservation uses the principle of molecular sieve in the manufacture of plastic film, so that the pore diameter (material molecular gap) of the film is larger than the diameter of some gas molecules, and the gas substances with smaller molecular diameter can pass through. As fresh-keeping, it is mainly to let the gases that are not conducive to fresh-keeping or harmful to fresh-keeping pass through and leave the beneficial gases, such as the silicon window packaging material (silicon window plastic film) mentioned above. It is the application of this gas selective breathable fresh-keeping technology. A variety of gas selection methods can also be designed and manufactured according to the fresh-keeping conditions of different items and the required ambient air conditioning for fresh-keeping. In fact, this is also an example of modified atmosphere packaging

8. Deodorization film preservation

in addition to O2, CO2 and other gases, agricultural products will also produce other gases in the fresh-keeping packaging, which will also cause the agricultural products to change flavor or produce odor. These odors mainly include ammonia odor and other odors. Now, the deodorizing and fresh-keeping film can be obtained by adding ingredients that can absorb these odors to the raw materials (resins) for manufacturing plastic film, such as ferrous sulfate, trimellitic acid, citric acid, etc

9. Film preservation to maintain water balance

this is a kind of packaging film made by adding water absorbent resin to the plastic resin of the base material. This packaging film has the functions of moisture absorption and dehumidification of material properties, so as to maintain the environment in the package to achieve water balance (ensure a certain proportion of water content). So as to achieve the purpose of keeping agricultural products fresh. The water absorbent (resin) can also be sealed into the sealed packing towel, which also has the same function

the above nine aspects of plastic packaging fresh-keeping methods are the main methods to develop new plastic packaging fresh-keeping materials. With the application of new technologies and materials, there will be more and more methods in this field, and more and better new plastic fresh-keeping packaging materials will be produced