Over the past 30 years, the use of MAP technology has expanded to include raw and cooked meat products, fish, poultry, shellfish, fruits, sauces, chips, coffee, tea, vegetables, bread and more. At present, MAP has become the main packaging form of some products, and its market share has also been increasing. In recent years, modified atmosphere packaging MAP has become a research hotspot. Modified Atmosphere Packaging, or MAP for short, can be defined as "a technique for regulating the gaseous environment of foods in materials that prevent gas from entering and exiting." One of the importance of MAP is to store the gas components in the initial conditioning package to achieve suppressed decay and deterioration, to maintain the quality of the perishable food during normal shelf life or to extend its shelf life. Therefore, the modified atmosphere packaging is also called Active Air Conditioning Packaging (Activ eAtmospherePackaging). The status quo of modified atmosphere packaging The origins of modified atmosphere packaging date back to the 1830s. At that time, fresh beef and mutton were stored in large bags for storage and transportation, and the application in aquatic products began in 1930. But the large-scale commercial application of this technology began only after 1970. In 1979, Mark & ​​Spencer of the United Kingdom introduced MAP meat products, which were applied to fish and ham two years later. As for the study of fresh-keeping of fruits and vegetables, as early as 1955, Marcelan of the Plant Physiology Laboratory of the Gerhard National Research Center in the United States began to study various PE membranes for storing apples and pears, and for oxygen (O2) in the storage environment. A systematic study of carbon dioxide (CO2) changes was published in 1960 and is referred to as physiological packaging storage. China's modified atmosphere packaging started in the 1990s. In 1988, the National Agricultural Products Preservation Engineering Technology Center developed 24 kinds of PVC cling film for fruits and vegetables, from which 32 varieties were selected, and 47 specifications were used for garlic, cucumber, celery and grapes. Practical application of plastic wrap bags such as apples and pears. The composition of the modified atmosphere packaging system: There are two main influencing factors in the storage process of fruits and vegetables, namely aerobic bacteria and oxidation reaction, both of which require O2. Therefore, in order to extend the shelf life or maintain the quality of fruits and vegetables, it is necessary to reduce the O2 content of the environment. Tests have shown that the O2 content in the package is <1%. The growth of various bacteria rapidly decreased, and when it was reduced to 0.5%, its growth was inhibited and reproduction stopped. However, the decay of some fruits and vegetables is due to anaerobic/microaerobic microorganisms and non-oxidation reactions, which are actually difficult to effectively use by vacuum packaging alone, and the product is inevitably shrunk and is not suitable for many foods. MAP technology is specifically designed for problems in vacuum packaging to further inhibit microbial spoilage and product shrinkage. MAP is the same as vacuum packaging and the product is usually combined with refrigeration. The core is to adjust the gas around the fruits and vegetables to a gas containing low oxygen and high carbon dioxide compared with the normal atmosphere, with appropriate temperature conditions to extend the shelf life of fresh products. The conditioning gases of the MA P technology are oxygen, nitrogen and carbon dioxide. The ideal condition for oxygen and fruit packaging is to exclude O2. However, O2 is essential when packaging fresh fruits and vegetables. Because fruits and vegetables must undergo respiration after harvest (such as consumption of O2 and CO2 production), and if O2 is absent, anaerobic respiration will occur, which will accelerate the change of sensory quality and decay. Carbon dioxide CO2 can inhibit the growth and growth of bacteria and fungi, but the specific mechanism of action is not clear. To be sure, this depends on the gas diffusion within the package for the following reasons: (1) The inhibitory effect is directly related to the presence of CO2. Gill and Tan (1980) pointed out that the inhibition effect is linear with the concentration of CO2 until its concentration reaches 50% to 60% (atmospheric volume ratio), and further increase of concentration is not obvious for most microorganisms. Both s hay and Egan (1987) and Gill and Penney (1988) believe that more than 50% to 60% will spread to the product, which will achieve the best results. Therefore, the packaging volume and the breathability and surface area of ​​the packaging material should be considered. (2) The solubility of CO2 is inversely proportional to the storage temperature, so the low temperature has a synergistic effect. (3) When the CO2 concentration is high, if there is some decomposing gas containing carbonic acid, a sour taste will be produced. (4) The absorption of gas by the product will reduce the volume of the gas, so this will cause the product to collapse, which will cause slight changes, sometimes mistaken for packaging defects and packaging materials. In addition, the bacteriostatic effect of CO2 is also dependent on the stage of growth of the microorganisms present. CO2 can increase the delay period and reduce the reproductive efficiency in the logarithmic growth phase; however, the former has a more obvious effect, so the inhibition effect will be weakened when the bacteria transition from the delayed phase to the logarithmic growth phase. In this way, CO2 will be more effective in the early stages of aerated packaging. It should be noted that the references cited above for the effect of CO2 inhibition are based on experimental methodologies that are still questionable (eg, using unbuffered media, changing several variables in an experiment, etc.), and there are certainly some potential errors as a result. In summary, the design of the modified atmosphere packaging system should consider a variety of factors, 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. Fruit and vegetable modified atmosphere packaging application and development Fruits and vegetables undergo aerobic respiration during normal oxygen supply, and anaerobic respiration during hypoxia. Too fast aerobic or anaerobic respiration will cause the fruits and vegetables to age or rot. The degree of two breaths produced by fruits and vegetables is proportional to the concentration of oxygen in the environment. Controlling the oxygen concentration in the environment allows the fruits and vegetables to produce only weak aerobic respiration without anaerobic respiration. Fruits and vegetables packed in plastic bags consume O2 and produce CO2 under respiration, gradually increase the concentration of CO2 in the environment and reduce the concentration of CO2. The highly transparent plastic film can exchange gas with the atmosphere to supplement the O2 consumed. Exclude CO2. When the speed of penetration of the body from the film is equal to the breathing speed of the fruit and vegetable, the gas in the bag reaches a certain equilibrium concentration, so that the fruit and vegetable maintain a weak breathing speed without anaerobic respiration, thereby delaying the ripening of the fruit and vegetable to obtain freshness. There are many factors that affect the concentration of gas in the package, such as the respiratory rate of the fruit and vegetable, the air permeability of the film to O2 and CO2, and the storage temperature. There are two kinds of packaging methods for fruit and vegetable modified atmosphere packaging, one is called passive air conditioning, that is, the plastic membrane exchange gas supplied by the supermarket is used to regulate O2 and CO2 content; one is called active air conditioning, and the breathing intensity is low according to the fruit and vegetable. The gas mixture of oxygen and carbon dioxide is exchanged with different gas permeable membranes to balance the gas exchange in the package as soon as possible. Obviously, the active air-conditioning mode is superior to the passive air-conditioning mode. However, the technology is complicated and limited by the variety of highly gas permeable films. The shelf life of fresh fruits and vegetables depends on the physiological aging and water loss of fruits and vegetables. Modified atmosphere packaging can effectively control the above influencing factors and extend the shelf life of some important commodities. When sugar and acid are decomposed during storage, the sensory quality of fruits and vegetables begins to change, and the rate of change is determined by its respiration. Therefore, shelf life is inevitably closely related to respiration. MAP is to reduce the rate of anaerobic respiration by reducing the amount of oxygen. Because the packaging system must allow gas exchange for respiration. Vacuum packaging does not meet this requirement. Although inflatable packaging sometimes uses natural packaging methods, it is most commonly used to design a packaging system that achieves a suitable gas atmosphere that balances breathability and package breathability. In such a series, the permeability of the packaging film (such as O2 entry and CO2 removal) should be selected according to the product's respiration rate (such as O2 consumption and CO2 production), in order to store gas components during storage. Adjusted from 20.95% O2 and 0.03% CO2 to 2% to 5% and 3% to 8%. However, in fact, MAP applications are complex because the respiration and the permeability of the packaging film are constantly changing. The rate of respiration depends on many factors, such as the type of vegetables and the different varieties of fruits and vegetables (eg different lettuce varieties, respiratory rate) It is also quite different), maturity (such as the ripe banana's breathing rate is 4 to 5 times that of green banana), and the degree of damage during preparation (such as the bar-shaped carrot's breathing intensity is 6 to 7 times that without cutting carrots) ). Temperature is also one of the most important factors, with a temperature increase of 10. c, the breathing rate is generally increased by 2 to 3 times. However, at room temperature storage is very common, because the cost of refrigeration is too high, and in fact CO2 is more effective than low temperature, generally breathing can be controlled 4 to 5 times lower than the low temperature
Fresh fruits and vegetables are also subjected to vigorous respiration and evaporation after picking. Oxygen is extracted from the air, decomposing its own nutrients, producing carbon dioxide, water and heat. The main nutrient during breathing is sugar, so the respiratory response is mainly the oxidation of sugar. When the oxygen supply is not sufficient, this breathing is called hypoxic breathing, which is intermolecular breathing. In the absence of oxygen breathing, incomplete oxides such as alcohol will be produced.