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How does the food in the supermarket refrigerated showcase change in freezing and freezing?
author: supermarket refrigerated showcase category: commercial refrigeration Views: 575Time: 2016-08-22
1) Ice crystals during food freezing
supermarket refrigerated showcase Food freezing is the process of turning most of the water contained in the food into ice. Therefore, the crystal surface is the most basic essence of the freezing 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.
2) Freezing of solutions in food. The freezing of
solution is different from pure water, and its freezing point is lower than that of water. The freezing point of solution E, 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 the remaining juice in the food, the greater its concentration, the more the juice freezes and the more the pepper is. The continued freezing of the food needs to be carried out under the conditions of greatly reduced temperature. The freezing point of most foods is between -1 and -2°C. 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 is frozen 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℃.
(3) The temperature curve of food freezing and the maximum ice crystal formation zone.
The temperature curve of food freezing changes according to the freezing speed, but whether it is fast freezing or slow freezing, the temperature drop during freezing can be divided into three stages:
In the first stage, the temperature of the food drops rapidly. Until it is reduced to the crystallization temperature. The second stage is the ice formation stage, which is represented by a nearly horizontal line. At this stage, it is at 0~-5℃. At this time, more than 80% of the water inside the food has been frozen. This is the temperature range where a large amount of ice crystals are formed., 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 that the frozen food in the supermarket refrigerated showcase continues to freeze to the specified final temperature.
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