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Food walk-in cooler preservation principle (5)
Food walk-in cooler preservation principle
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Ice crystal growth in walk-in cooler. The ice crystal size of the frozen food just produced is not completely uniform. During the freezing process, due to the longer freezing time, the fine ice crystals will gradually merge and grow into larger ice crystals. This phenomenon is called ice crystal growth. One of the reasons for the growth of ice crystals is that the water or water vapor around the ice crystals moves to the ice crystals, adheres and freezes on the ice crystals, so that the larger ice crystals grow larger, while the small ice crystals gradually decrease and disappear. The formation of large ice crystals increases the degree of mechanical damage to cells, resulting in protein denaturation, increased juice loss after thawing, and food flavor and nutritional value are affected. Another reason for ice crystal growth is the growth of ice crystals caused by recrystallization. Recrystallization refers to a phenomenon in which the volume of crystals increases due to repeated thawing and recrystallization due to fluctuations in the freezing temperature during the freezing storage process. The degree of recrystallization directly depends on the number and degree of temperature fluctuations per unit time. The greater the range of temperature fluctuations and the greater the number, the deeper the degree of recrystallization, which will promote the rapid increase of ice crystal particles and rapid decrease in number. As a result, the tissue structure of frozen food is severely damaged, and it loses its elasticity after thawing. To prevent the adverse effects of ice crystal growth on frozen food during the freezing process, it can be prevented from the following two aspects: ①The deep-temperature quick freezing method is adopted to make 90% of the water in the food too late to move during the freezing process. The position becomes extremely fine ice crystals, so that the size and distribution of the ice crystals formed are more uniform. At the same time, due to deep-temperature quick freezing, the final temperature of frozen food is relatively low, the freezing rate of the food is increased, and the residual liquid phase is less, which can also reduce the growth of such ice crystals in the frozen storage. ② Frozen storage temperature should be as low as possible with little change, especially avoiding temperature changes above -18°C.
Food changes during the thawing process in the walk-in cooler and freezes. Food also has problems such as pollution and consumption during the thawing process, but the biggest problem is the loss of body fluids. After the food is thawed, the internal ice crystals melt into water. If they cannot be absorbed by the tissues and cells and return to their original state, this part of the water will separate and become a drained liquid. Drain fluid is not only water, but also water-soluble components, such as protein, salt, vitamins, etc. The loss of body fluids reduces the quality of food, and also loses nutrients and flavors. Therefore, the production rate of lost fluid has become one of the indicators for evaluating the quality of frozen. Water cannot be absorbed by the tissues when thawing, because the water holding capacity of the protein, starch and other ingredients in the food is lost due to irreversible changes during freezing and freezing, and the water holding capacity becomes dehydration. The outflow of body fluids isIt is caused by mechanical damage caused by ice crystals in the freezing process of fleshy tissue. When the damage is severe, the gaps between the flesh are large, and the melted water from the internal ice crystals flows out through these gaps: when the mechanical damage is slight, the melted water from the internal ice crystals is retained in the flesh due to capillary action, and only drains outward when pressurized. The greater the physical changes in the food when it is frozen, the more body fluids will be lost when it is thawed. Generally speaking, if the food raw materials are fresh, the freezing speed is fast, the freezing temperature is low and the fluctuation is small, and the cold storage period is short, there will be less fluid loss during thawing. If the moisture content is high, there will be more fluid loss. Such as the ratio of fish to meat, the water content of fish is high, so more fluid is lost. Compared with legumes, leafy vegetables lose more fluid. After pre-freezing treatments such as adding salt, sugar and phosphate, there is less fluid loss. The finer the food ingredients are cut, the more liquid is lost.
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