Discussion on several problems in using modified a

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Discussion on several problems that should be paid attention to when using modified atmosphere packaging technology for food

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modified atmosphere packaging technology for food first appeared in the European food industry, then spread to the Americas and Japan, and was widely used. At present, the application of modified atmosphere packaging technology to food packaging is rare in China's food industry. How to carry out modified atmosphere packaging for food is an important issue that packaging workers urgently need to study and solve

in order to prolong the shelf life of food, people mainly used heating, salt soaking, smoking, adding preservatives, disinfectants, antioxidants and other means to pretreat food. These methods will have some impact on food more or less. With the gradual improvement of people's quality of life, people began to look for other food preservation methods, of which modified atmosphere packaging is one

modified atmosphere packaging is to use packaging materials with gas barrier performance to package food, and then fill a certain proportion of mixed gas into the packaging to prevent or slow down the quality decline of food in physical, chemical, biological and other aspects, so that the food can have a relatively long shelf life. The changes of food during transportation and storage are very complex. The variety and quality of food, microbial environment, storage temperature, etc. will affect the changes of food quality. Modified atmosphere packaging is a technology that integrates physics, chemistry, biology and other disciplines

due to the different physiological characteristics of food and the different conditions encountered in the transportation and marketing of food, the requirements for packaging vary greatly. There are many factors to be considered when using modified atmosphere packaging technology for food, mainly including the following four elements

I. gas mixture with appropriate proportion

the most commonly used modified atmosphere packaging is nitrogen, carbon dioxide and oxygen or their mixture

nitrogen is stable in nature. It is generally used to remove oxygen, so as to slow down the oxidation and respiration of food. Nitrogen also has a certain inhibitory effect on the growth of bacteria. In addition, nitrogen is basically insoluble in water and oil. Food has little adsorption on nitrogen, so it will not shrink gradually due to the absorption of gas during packaging

carbon dioxide is the most critical gas in modified atmosphere packaging. It can inhibit the growth of bacteria and fungi. When used in fruit and vegetable packaging, increasing carbon dioxide has the effect of strengthening oxygen reduction and reducing respiratory intensity; However, when using carbon dioxide, it must be noted that carbon dioxide has a high solubility in water and oil. The formation of carbonic acid after dissolution will change the pH value and taste of food. At the same time, after the dissolution of carbon dioxide, the amount of gas in the packaging will be reduced, which is likely to lead to shrinkage and incompleteness of food packaging and affect the appearance of food. The use of carbon dioxide in modified atmosphere packaging must consider many factors, such as storage temperature, food moisture, microbial species and quantity

modified atmosphere packaging generally only uses oxygen in the following three cases: 1) use oxygen to maintain the color of food, such as the red of meat

2) maintain the respiration of fruits and vegetables

3) inhibit the growth of anaerobic microorganisms

II. Gas barrier membrane

gas barrier membrane is the most important part of modified atmosphere packaging. It must have high gas barrier performance to ensure that the mixed gas in the packaging does not leak out. In addition, for fruits and vegetables, the proportion of mixed gas will be changed due to their respiration. In this case, the mixed gas must be dynamically balanced, that is, the permeability of barrier film is used to maintain the ideal proportion of mixed gas. Modified atmosphere packaging has very strict requirements on the air permeability of packaging materials. In addition, the thermoformability of the film and the reliability of sealing must be considered. At present, the commonly used films include polyester (PET), polypropylene (PP), polystyrene (PS), polyvinylidene chloride (PVDC), ethylene vinyl acetate (EVA), ethylene vinyl alcohol (EVOH), various composite films and metal plating films

III. storage temperature

storage temperature is an important link affecting the shelf life of food. Under low temperature conditions, the oxidation rate and respiration rate of food will be weakened, and the growth of microorganisms will be inhibited. Even below a certain temperature limit, the activities of microorganisms will stop completely. Most of the microorganisms causing food spoilage belong to thermophilic microorganisms. Taking E. coli as an example, the lowest growth temperature is 10 ℃, and the most suitable growth temperature is 37 ℃. Therefore, freezing and cold storage methods are generally used to store food. But on the other hand, low temperature also has different effects on some foods. The specific storage temperature should be determined according to the packaged food. Generally, fish meat is frozen, while fruits and vegetables are stored in cold storage

IV. food quality and microbial environment

moldiness and corruption of food are mainly caused by molds, bacteria and other microorganisms, so the type and quantity of microorganisms attached to the food inside and on the surface before packaging are also part of the friction and wear testing machine, and an important factor affecting the food quality assurance period, especially for foods with high water activity. Besides the influence of temperature, the growth of microorganisms is also related to the water activity of food. Bacteria cannot reproduce in food with water activity lower than 0.90. The minimum water activity value of yeast reproduction is 0.88 and that of mold is 0.80. The water activity value of fish, fruit and other foods is as high as 0.98, which is very suitable for the reproduction and growth of microorganisms. It is very important for the sterilization of such foods before packaging

in addition to the microbial environment, the quality of food before packaging is very important. For example, the ripeness of the picked fruit and whether there is mechanical damage directly affect the respiratory intensity and antibacterial performance of the food, thus affecting the concentration of oxygen and carbon dioxide in the package. When using modified atmosphere packaging technology, the food quality before packaging is a factor that must be considered

the four elements of modified atmosphere packaging of food are interrelated and indispensable. Their joint action determines the length of food shelf life. Of course, due to the different packaged foods, the requirements for each element are also different. According to the water activity of food, food can be divided into three categories, namely, dry food with little water content, slightly wet food with less water content and high moisture food with high water content

the following is a brief introduction to the modified atmosphere packaging methods for these three different types of food

1. Modified atmosphere packaging of dry food

there is almost no water in milk powder, coffee, potato chips, cheese, pinecones, peanuts and other foods, so ordinary microorganisms cannot reproduce and grow, and their shelf life only depends on the oxidation of food. For example, peanut, potato chips and other foods will change taste due to oxidation, laver will change color due to oxidation, meanwhile, the flavor and vitamins will disappear, and the flavor of tea will also change due to oxidation. The modified atmosphere packaging of this kind of food mainly considers how to inhibit the oxidation of food. Generally, the vacuum nitrogen filling method can be used to reduce the oxygen content to less than 1%, so as to inhibit the oxidation of food. At the same time, the moisture absorption of food must be prevented. The packaging material should adopt the film with good barrier to oxygen and water vapor. Generally, this kind of food can not be stored at low temperature

2. Modified atmosphere packaging of slightly wet food

the storage of slightly wet food is more complex than that of dry food. At this time, it is necessary not only to inhibit the oxidation of food, but also to prevent food mildew. This kind of food includes roast meat, snacks, flour products, etc. its water activity is lower than 0.90. Mold and yeast can grow and reproduce in this environment. However, due to the low water content, their growth speed will not be too fast, especially at low temperatures. When modified atmosphere packaging is used for this kind of food, oxygen is generally controlled at a low level, and a certain amount of carbon dioxide is added to inhibit the oxidation of food and the growth of microorganisms. Due to the low water content, high concentration of carbon dioxide can be used, and even pure carbon dioxide can be used in some cases, which will not lead to packaging shrinkage and food acidification. Packaging materials can be films with good barrier to oxygen, carbon dioxide and water vapor, and they are mostly stored at low temperature. If properly designed, modified atmosphere packaging can increase the shelf life of this kind of food by more than four times

3. Modified atmosphere packaging of high humidity foods

fish meat, fruits and vegetables are foods with high water content, and the water activity value is more than 0.98. This environment is very suitable for the growth and reproduction of microorganisms, including anaerobic and aerobic microorganisms. When designing the modified atmosphere packaging of this kind of food, the key is to pay attention to the oxygen concentration of the mixed gas and the oxygen transmittance of the barrier film, and ensure the appropriate storage temperature to achieve the purpose of inhibiting the growth of microorganisms

for modified atmosphere packaging of fresh meat, the main consideration is how to inhibit the growth of microorganisms and how to maintain the color of fresh meat. Carbon dioxide is generally used to inhibit the growth of microorganisms, and oxygen is used to maintain the color of fresh meat and inhibit the growth of anaerobic microorganisms. The color of fresh meat will darken after long-time contact with air, and it will remain bright red after contact with a certain amount of oxygen. However, it must be noted that the oxygen content must not be too high, otherwise the meat will turn brown. The mixed gas of carbon dioxide, oxygen and nitrogen is generally used for modified atmosphere packaging, in which the concentration of carbon dioxide is greater than 20% and the concentration of oxygen is 45 ~ 80%. The packaging material adopts gas high resistance diaphragm, but the requirements for water vapor transmission are not strict. The storage method is mostly frozen to inhibit the growth of microorganisms

seafood contains a large number of anaerobic microorganisms, and oxygen is required to inhibit the growth of such microorganisms. Therefore, in order to keep seafood fresh, it is very important to design the mixing ratio of carbon dioxide and oxygen. At present, materials with good oxygen barrier performance are generally not used for modified atmosphere packaging of seafood, so that external oxygen can penetrate into the packaging, so as to ensure the proportion of oxygen in the mixed gas. This is a relatively safe packaging method. In addition, there is a very complex situation for fish. The changes after the death of the fish are closely related to the situation of the fish before the death. The most representative is that the former Soviet Union classified the fish captured by the friction and wear testing machine according to the damage situation of the simulated friction surface and suffocated and the fish immediately slaughtered. The changes after the death are very different. Different packaging methods should be used for different situations

the modified atmosphere packaging of fruits and vegetables is the most complicated. After harvest, fruits and vegetables are still a living organism. They also carry out complex life activities such as dormancy, germination, water evaporation and respiration. These life activities all affect and restrict the packaging and storage time of fruits and vegetables, and the most important one is respiration. Under normal circumstances, the stronger the respiration of fruits and vegetables, the shorter the storage time, and the more prone to decay and deterioration; But it can not completely inhibit the respiration of fruits and vegetables, otherwise fruits and vegetables will wilt, lose luster, and be vulnerable to microbial infection. Generally, the respiratory intensity of fruits and vegetables can be controlled by controlling the proportion of oxygen and carbon dioxide in the mixed gas and the storage temperature. For example, for the modified atmosphere packaging of citrus, controlling the oxygen concentration in the mixed gas to be 12.5 ~ 17.5% and the carbon dioxide concentration to be 1.35 ~ 3.65% can better inhibit the respiratory intensity of citrus and reduce the rotten fruit rate from 7.63% to 2.32%. Of course, the design of fruit and vegetable modified atmosphere bag

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