Overview

Modified atmosphere packaging is a packaging method that replaces the air in the packaging container with a protective gas suitable for food preservation to inhibit bacterial growth and achieve long-term preservation and preservation.

Modified atmosphere packaging has become a widely used method of food preservation, which is characterized by the ability to effectively keep food fresh while producing minimal side effects. The UK is at the forefront of modified atmosphere packaging for meat and meat products, followed by France. Aseptically packaged rice, which has a relatively long shelf life, has become a popular product in the Japanese supermarket. It is packaged in a nitrogen-filled system, ultra-high temperature sterilized, and has a high oxygen barrier property and an oxidation resistance equivalent to that of a metal container. The product can be stored for up to 6 months without refrigeration. In Norway, modified atmosphere packaging has extended the shelf life of fish, fish products and shellfish by an average of 1.5 times, allowing fresh squid to be packaged in modified atmosphere in supermarkets. Convenient semi-finished products and ready-to-eat fresh foods, pastas and salads are popular in supermarkets and food stores in the United States, and are now mostly packaged in modified atmosphere. Modified atmosphere packaging is affecting the packaging of meat, cheese, fish, poultry and other fresh and prepared foods and the sale of these foods on the global market.

China's research on modified atmosphere packaging of meat began in the late 1980s, but its application in production and commerce is only a matter of recent years. At present, only such modified atmosphere packaged meat products are seen in the markets of a few major cities such as Beijing and Shanghai. In recent years, with the introduction of advanced continuous vacuum/inflatable packaging machines from abroad, the production of modified atmosphere fresh-keeping meat has become possible.

Preservation mechanism of meat modified atmosphere packaging

The fresh-keeping mechanism of modified atmosphere packaging is to destroy or change the conditions for the survival and reproduction of microorganisms by filling a certain gas in the packaging to slow down the biochemical deterioration of the packaged food and achieve the purpose of preservation and preservation. The gases used are typically CO2, O2 and N2 or various combinations thereof, and each gas has a different preservation effect on fresh meat.

CO2 acts as a bacterial inhibitor in modified atmosphere packaging. In 1933, Australia and New Zealand first began to preserve fresh meat with high concentrations of CO2. In 1938, 26% came from Australia and 60% of beef from New Zealand was shipped in high-concentration CO2 packages at storage temperatures of -1.4 °C. The treated beef has a shelf life of 40 days to 50 days. CO2 is a stable compound, colorless and odorless, which has a strong inhibitory effect on the reproduction of most aerobic bacteria and molds. At the same time, it can prolong the lag period of bacterial growth and reduce the speed of its logarithmic growth period.

The antibacterial effect of CO2 is that CO2 replaces O2 in a high-concentration CO2 package. The high CO2 environment inhibits the growth of a large number of aerobic microorganisms (such as Pseudomonas). Second, because CO2 is soluble in water, The formation of carbonic acid (H2CO3), lowering the pH, so that some of the acid-free microorganisms lose the necessary conditions for survival; the third is through the penetration of cells. At the same temperature and pressure, the solubility of CO2 in water is 6 times that of O2, and the rate of infiltration into cells is 30 times that of O2. Due to the large infiltration of CO2, it will affect the structure of cell membrane, increase the permeability of membrane to ions, and change the inside and outside of membrane. The balance of metabolism interferes with the normal metabolism of cells, inhibiting bacterial production. Infiltration of CO2 also stimulates the activity of mitochondrial ATPase, accelerates oxidative phosphorylation, reduces ATP, and reduces the energy required for metabolic growth.

There are many factors affecting the bacteriostasis of CO2, including the following aspects: microbial species in meat, application time of CO2, CO2 concentration, storage temperature. CO2 has different effects on different types of microorganisms. Generally, G-bacteria are more sensitive to CO2 than G+ bacteria. Common bacteria causing meat spoilage such as G-Pseudomonas, Acinetobacter, and Moraxella can be inhibited by 20% CO2, while G+'s Brochothrixthermosphacta can grow in 50% CO2, and lactic acid bacteria have CO2 High resistance, even growing in 100% CO2. The effect of CO2 on anaerobic bacteria causing food poisoning is not significant, and anaerobic and high concentrations of CO2 promote their growth.

The application time of CO2 also has an important effect on its antibacterial effect. CO2-sensitive microorganisms can prolong their lag phase by using CO2 in the early stage of growth. When they enter the logarithmic growth phase, the inhibition of CO2 is reduced. Therefore, the earlier the application of the modified atmosphere packaging, the better the bacteriostatic effect.

Although various microorganisms are sensitive to CO2, in general, yeast, mold and bacteria can be inhibited by 5% to 50% of CO2. In the concentration range of 0% to 20%, the antibacterial effect of CO2 increased linearly; when the concentration was higher than 20%, the increase of the antibacterial effect of CO2 was relatively small.

Low temperature can increase the antibacterial effect of CO2. This may be due to the ease of dissolution of CO2 in water at low temperatures, which facilitates the rapid formation of carbonic acid and dissociates into H+ and HCO3-. Gilland Harrison (1989) has shown that under low temperature conditions, CO2 modified atmosphere packaged pork is greatly slowed down. The use of O2 in modified atmosphere packaging is mainly due to the combination of myoglobin and oxygen molecules in the muscle, which becomes oxygenated myoglobin and is bright red. Therefore, in order to maintain the bright red color of the meat, O2 must be contained in the package. Natural air contains about 20.9% O2, so the surface of the new cut meat is light red when exposed to air. According to reports, at 0 ° C, relative humidity of 99.3%, high oxygen myoglobin is most formed when the partial pressure of oxygen is 800 ± 400 Pa (0.4% O2 ~ 1.2% O2), and O2 must be above 5% to reduce the high iron myoglobin. form. However, according to Bartkowski (1982), oxygen must be above 10% to maintain bright red, and more than 40% of O2 can maintain good color for 9 days. The formation of bright red oxygenated myoglobin is also related to the moisture content of the meat surface. When the surface is wet, the amount of dissolved oxygen is high, and it is easy to form bright red. Although O2 can maintain a good color, due to the presence of O2, it is easy to cause the growth of Pseudomonas aeruginosa under low temperature conditions (0 ° C ~ 4 ° C), so the shelf life is lower than vacuum packaging. In addition, O2 is also prone to oxidative rancidity of unsaturated fatty acids, causing muscle browning. N2 is an inert gas that generally does not work on the packaged material and is not absorbed by the food. N2 has a low gas permeability to plastic packaging materials, so it can be used as a gas mixture to buffer or balance the gas in the package, and to prevent collapse caused by charging with CO2.

Proportion of various gases in meat modified atmosphere packaging

In modified atmosphere packaging, CO2 has a good antibacterial effect, O2 is necessary to keep the meat bright red, and N2 is mainly used for adjustment and buffering. How to make various gas proportions suitable, so that the meat preservation period is long, And all aspects can achieve a good state, it must be explored. Most of Europe and the United States are retailed in 80% O2+20% CO2. The shelf life is 4 days to 6 days. In the United Kingdom, there were two patents in 1970: the gas mixing ratio was 70% O2 ~ 90% O2 and 10% CO2 ~ 30%. CO2 or 50% O2 ~ 70% O2 and 30% CO2 ~ 50% CO2, and generally use 20% CO2 + 80% O2, with a bright red effect of 8 days ~ 14 days.

Several problems should be paid attention to in fresh meat and air conditioning packaging

Processing of fresh meat before packaging

If the pigs are cooled at 0 ° C ~ 4 ° C for 24 hours after slaughter, the ATP will stop the activity and produce an acid removal process. The chilled meat treated in this way is much better in nutrition and taste than quick-frozen meat and hot fresh meat. In addition, in order to ensure the fresh-keeping effect of the modified atmosphere packaging, it is also necessary to control the hygienic indicators of the fresh meat before packaging to prevent microbial contamination.

Packaging material selection

Atmospheric packaging should use packaging materials with good barrier properties to prevent gas leakage inside the packaging, and also to prevent the infiltration of oxygen in the atmosphere. Generally, PET, PP, PA, PVDC, etc. are used as the composite packaging film for the substrate. The measurement of the barrier property of plastic film to gas is generally expressed by the gas permeability coefficient: the smaller the gas permeability coefficient, the better the barrier property. In addition, all packaging materials must have sufficient mechanical strength to withstand the pressure changes during vacuuming. At the same time, the material is required to have a certain degree of stiffness so that the bag can be automatically opened and inflated.

The control of aeration and sealing quality depends on advanced air-packing machinery and good handling quality. The continuous vacuum/inflatable packaging machine can realize automatic and continuous completion from vacuuming, inflation and sealing on one machine, which is efficient and reliable, and also reduces various pollutions in the packaging process, which is beneficial to improve the fresh-keeping effect.

Product storage temperature control

The effect of temperature on the preservation effect comes from two aspects: First, the temperature directly affects the activities of various microorganisms on the surface of the body; second, the barrier property of the packaging material is closely related to the temperature. The higher the temperature, the more the barrier property of the packaging material low. Therefore, temperature control from product, storage, transportation to sales must be realized.

Market prospects for fresh meat modified packaging

At present, most of our consumers have become accustomed to fresh meat without packaging or simple packaging. Consumers generally eat fresh meat quickly, or put it in the freezer in the refrigerator. Bacteria grow very fast at normal temperature. For example, bacteria can be propagated 15 times in 1 day at 16 °C, 700 times in 21 °C, 3000 times in 27 °C, and 2 times in one hour at 4 °C. Packaging sales has become an imperative trend. As chilled meat is increasingly becoming the mainstream of meat consumption, it has a growing share in the domestic and foreign markets. The use of modified atmosphere packaging for the preservation of chilled meat can greatly extend the shelf life of chilled meat and thus has a large Market potential.