The hottest modified atmosphere packaging and the

2022-09-22
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Modified atmosphere packaging and permeability of materials

Abstract: This paper roughly introduces the packaging principle, fresh-keeping gas used and application field of modified atmosphere packaging (map), introduces in detail the reasons why different gases have different permeability to the same material, and gives some suggestions on the selection and detection of packaging materials combined with the permeability test characteristics in the field of modified atmosphere packaging

key words: modified atmosphere packaging, map, fresh-keeping gas, permeability, dynamic diameter

1, modified atmosphere packaging

also known as displaced gas packaging, internationally known as map packaging (i.e. modified atmosphere packaging), is a fresh-keeping packaging developed and improved on the basis of vacuum packaging and nitrogen filled packaging, which is mainly used for food fresh-keeping

the packaging principle of modified atmosphere packaging is to use modified atmosphere fresh-keeping gas (2 ~ 4 gases are mixed according to the characteristics of food), replace the air in the packaging box or packaging bag, change the external environment of the food in the box (bag), inhibit the growth and reproduction of bacteria (microorganisms), slow down the metabolism of new fresh fruits and vegetables, and thus extend the fresh-keeping period or shelf life of food. Take the preservation of fruits and vegetables as an example: fresh fruits and vegetables still breathe after picking, and consume O2. To sum up, it is the relevant introduction of the utilization scope and functional characteristics of metal tensile testing machine to produce CO2, gradually increase the content of CO2 in the environment and reduce the concentration of O2. Using high permeability plastic film can exchange gas with the atmosphere, supplement the consumed O2 and discharge CO2. When the gas permeation rate to the film is equal to the respiration rate of fruits and vegetables, the gas in the packaging bag reaches a certain equilibrium concentration, which can maintain the weak respiration rate of fruits and vegetables without anaerobic respiration, so as to delay the maturation of fruits and vegetables and get fresh preservation. Using modified atmosphere packaging can ensure the taste, nutrition and fresh-keeping period of food without preservatives and additives

the modified atmosphere fresh-keeping gas is generally composed of carbon dioxide (CO2), nitrogen (N2), oxygen (O2) and a small amount of special gases (NO2, SO2, AR, etc.). The functions of various gases are as follows: CO2 gas can inhibit the growth and reproduction of most spoilage bacteria and molds, and is the main bacteriostatic agent in fresh-keeping gases; O2 can inhibit the growth and reproduction of most anaerobic spoilage bacteria, maintain the color of fresh meat, and maintain the respiratory metabolism of fresh fruits and vegetables; N2 is an inert gas, which generally has no chemical reaction with food and is not absorbed by food. It is used as a filling gas in modified atmosphere packaging to prevent the packaging from collapsing due to the escape of CO2 and other gases from the packaging. For different foods, fruits and vegetables, the composition and proportion of fresh-keeping gas are also different. The key to the modified atmosphere packaging of crops is to keep the oxygen permeability of packaging materials consistent with the respiratory rate of products

the application of modified atmosphere packaging in some products has been relatively mature, such as small food packaging, coffee packaging, processed meat products, etc. now it is also increasingly used in refrigerated and pretreated convenience food, especially in the packaging of fresh sliced agricultural products, special cheese and prepackaged sandwiches

2. Permeability of different gases to materials

the design of modified atmosphere packaging system should consider many factors, among which the most important factor is the relative content of CO2 and O2 in the package, which is mainly determined by the gas concentration in the package and the permeability of the packaging material, that is, the proportion control accuracy of modified atmosphere fresh gas and the gas replacement rate of the packaging material. Unlike vacuum packaging or nitrogen filled packaging, modified atmosphere packaging materials are mostly low barrier materials with greater gas permeability

the difference in permeability of materials is related to the aggregation state (crystallinity) of polymer materials, the diffusion and solubility of polymer structure on gas, and the influence of additives. However, different gases have different permeability to the same material according to their frequency requirements. For the same material, generally, the permeability of N2 is the smallest, O2 is slightly larger, and CO2 is the largest, which is related to the size of gas molecules (dynamic diameter, see Table 1) and the shape of gas molecules. The smaller the kinetic diameter of the molecule, the easier the diffusion in the polymer and the larger the diffusion coefficient. However, the diameter of gas molecules is not the only factor that determines the permeability, because permeability is also related to the solubility of gas in polymers. In addition, the shape of molecules can also affect the permeability. Some studies have shown that the diffusion and permeability of long molecules are the strongest, and small changes in the shape of molecules will cause permeability. (2) because the experimental machine is used year-round, the permeability changes greatly

3. How to choose the appropriate modified atmosphere packaging materials

there is no doubt that to carry out modified atmosphere packaging for products, we must make a reasonable selection of the permeability of packaging materials according to the characteristics of products. Generally, the gas used for modified atmosphere packaging is the mixture of O2, CO2 and N2, or the mixture of O2 and CO2. Therefore, when selecting modified atmosphere packaging materials, it is necessary to accurately test the O2 permeability, CO2 permeability and N2 permeability of the materials. It is absolutely forbidden to detect all kinds of gases in a more or less way, or only detect the O2 transmittance of the materials and then convert it according to the empirical proportion, and it must be comprehensively tested. It is conceivable that if the selection of modified atmosphere packaging materials is lost due to the error of index detection or the failure of comprehensive detection, it will not only cause huge economic losses to enterprises, but also cause a serious waste of resources

at present, the commonly used permeability test methods in the world include differential pressure method and isobaric method. On the whole, the test object of isobaric method equipment is very single. At present, it can only detect the O2 permeability of materials (such as Labthink toy-c1 oxygen permeability meter) or CO2 permeability. So far, no company has been able to provide isobaric method equipment for detecting N2 permeability. For the differential pressure method equipment, it is completely different, because this method itself has no selectivity for the test gas. It can test the conventional gas of 249 steel structures such as O2, N2, CO2, etc., with low test cost, and it can also realize the self-control temperature of the test environment (such as Labthink vac-v1 gas permeameter). In addition, with the support of membrane technology theory, the vacuum differential pressure method equipment can not only detect the permeability of various common gases to the sample, but also give the diffusion coefficient and solubility coefficient of the test gas to the sample at the same time. Compared with the isobaric method equipment, the pressure difference method permeability test equipment is more suitable for the research institutions and manufacturers of modified atmosphere packaging materials to comprehensively test and analyze the materials

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