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The principle of food commercial refrigeration (two)
food commercial refrigeration principle of preservation (two)
Changes in food during cold processing are a series of changes in the process of cold processing, that is, frozen food during storage. Some of the common changes are related to the low temperature environment of food storage. Knowing what changes are caused by low temperature in food will help us determine the correct cold processing technology and improve the preservation of food frozen processing and preservation.
1. Changes of food in commercial refrigeration cooling and cooling storage 1) Water evaporation When food is cooled, not only the temperature of the food drops, but the concentration of juice in the food also increases, and the moisture on the surface of the food evaporates, causing drying. When the moisture in the food is reduced, it will not only cause quality loss (commonly known as dry consumption), but also make plant foods lose their fresh and plump appearance. When the weight loss reaches 5%, fruits and vegetables will wilt. During the cooling and storage, the meat will also be consumed due to the evaporation of water. At the same time, the surface of the meat shrinks and hardens to form a dry film, and the color of the meat also changes. In the cold storage of eggs, the air chamber enlarges due to the evaporation of water, which squeezes the inner tissues of the eggs and causes quality degradation. 2) Cold contraction
After slaughtered beef is rapidly cooled in a short time, the muscle will contract significantly. Even after the maturation process, the meat quality will not be very soft. This phenomenon is called cold contraction. Generally speaking, within 10 hours after slaughter, the meat temperature drops below 8°C, which is prone to cold shrinkage. However, the temperature and time are not fixed, and the meat of adult cattle and calves, or different parts of the same cattle are different. For example, if the temperature of mature beef is lower than 8°C, while the temperature of calf is lower than 4°C, cold shrinkage occurs. According to the past idea, meat should be cooled quickly after being slaughtered. However, in recent years, as the sales volume of chilled meat has continued to expand, in order to avoid cold shrinkage, the international community is studying cooling methods that do not cause cold shrinkage.
3) chill
In the process of commercial refrigeration cooling and storage, when the storage temperature is lower than a certain limit temperature, the normal physiological functions of some fruits and vegetables will encounter obstacles and will lose their balance. This is called cold injury. The symptoms of chilling injury vary from species to species. The most obvious symptoms are softening spots on the epidermis and discoloration of the flesh around the nucleus, such as pits on the surface of watermelon, black heart disease of pears, and sweetness of potatoes.
There are also some fruits and vegetables that do not show symptoms of chilling injury in appearance, but after refrigerating them at room temperature, they lose their normal ability to promote maturity, which is also a kind of chilling injury. For example, if bananas are put in a cold storage room below 11.7°C for a period of time, the skin will turn black and rotten after being taken out of the cold room, commonly known as see wind black , and the ripening ability of raw bananas has been completely lost. Generally speaking, fruits and vegetables produced in tropical and subtropical regions are prone to chilling injury. It should be emphasized that it takes a period of cold damage to appear in an environment below the limit temperature. The earliest symptoms are bananas, like cucumbers and eggplants, which generally take 10^-14d.
4) Propagation of microorganisms
According to their adaptation temperature in commercial refrigeration, microorganisms in food can be divided into low temperature bacteria, medium temperature bacteria and high temperature bacteria. In the state of cooling and refrigeration, the reproduction and decomposition of microorganisms, especially low-temperature microorganisms, are not fully inhibited, but the speed becomes slow. The amount of fly increases. If the time is longer, food will occur. corruption. The reproduction of low-temperature bacteria becomes slow below 0°C, but if they are to stop reproducing ' Generally speaking, the temperature should drop below -10°C. For individual low-temperature bacteria, they still reproduce at a low temperature of -40°C.
5) Lipids changes
Lipids in the commercial refrigeration storage process, the fats contained in the food will undergo hydrolysis, fatty acid oxidation, polymerization and other complex changes, the reaction of low-grade aldehydes, ketones will make food The taste deteriorates, causing the food to appear discolored, rancid, and sticky. When this change is very serious, it is called oil burning .
2. Food changes during freezing and freezing
During the process of cooling the food in commercial refrigeration, there is no substantial change in structure, but most of the water freezes during the freezing process. This change will have a great impact on the structure and even the quality of food.
1) Ice crystals during food freezing
The freezing of food is the process of converting most of the water contained in the food into ice. Therefore, the most basic essence of the crystallization process. When the liquid water in the food forms solid ice crystals, a large amount of heat is transferred from the food, and the temperature of the food also decreases.
(l) Freezing of solution in food. The freezing of a solution is different from that of pure water. Its freezing point is lower than the freezing point of water. The freezing point of the solution, the concentration of the solution, and the degree of dissociation of the solute are related to the nature of the solvent. When food is frozen, the change process of solution concentration is much more complicated than that of ordinary solution, because the water contained in food contains many kinds of minerals and organic substances. Therefore, during the freezing process, as the water in the juice is analyzed, ice crystals are formed, which increases the concentration of the unfrozen juice and reduces the freezing point. The less juice remaining in the food, the greater its concentration, and the lower the freezing point of the juice. In this way, the continued freezing of food needs to be carried out under conditions where the temperature is greatly reduced.
The freezing point of most food is -1~-2℃. Foods containing a large amount of solutes (sugar, salt, acid) have a low freezing point of -3.5 to -5°C. Generally, when the temperature of food is -20℃, about 90% of the water freezes into ice. The lower the final freezing temperature of the food, the more water is frozen, which is conducive to the long-term preservation of the food. Generally, the final temperature (center temperature) of food freezing is -12~-5℃. (2) The temperature curve of food freezing and the maximum ice crystal formation zone. The temperature curve when the food is frozen changes according to the freezing speed, but whether it is fast freezing or slow freezing, the temperature drop during the freezing process can be divided into three stages:
In the first stage, the temperature of the food drops rapidly until it decreases Until the crystallization temperature. The second stage is the ice crystal formation stage, which is represented by a nearly horizontal line. At this stage, it is between 0 and -5°C. At this time, more than 80% of the water inside the food has been frozen. This temperature range where a large amount of ice crystals are formed is called the maximum formation zone of ice crystals. The latent heat released when ice crystals are formed is quite large. Therefore, the heat load is the largest when passing through the maximum ice crystal formation zone, and it takes a relatively long time. When freezing slowly, ice crystals in the food move from the surface to the center at a slower speed, and the temperature of the food center stays in a stagnant stage for a long time, and the horizontal line segment is longer. When freezing quickly, due to the strong heat conduction, the formation of ice crystals quickly moves from the surface of the food to the center of the food, so the horizontal line segment is very short. Finally, enter the third stage. This line segment indicates the cooling process of frozen food in commercial refrigeration that continues to freeze to the specified final temperature.
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