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The principle of preservation of food supermarket refrigerated showcase (3)
author: supermarket refrigerated showcase category: commercial refrigeration Views: 553Time: 2016-08-22
The principle of preservation of food supermarket refrigerated showcase (3)
The distribution of freezing speed and frozen crystals in supermarket refrigerated showcase.
The distribution of water in food can be roughly divided into two parts: water in cells and water in intercellular spaces. In the intercellular space of food, the water vapor tension is smaller than that of
, the salt concentration is also lower, and the freezing point is higher. When food is frozen, the water in the intercellular spaces first forms ice crystals. Since the saturated vapor pressure of ice is lower than that of water
, in the initial stage of food freezing, when the water outside the cell has frozen into ice, and the water inside the cell is still in a liquid state due to the low freezing point, because the two are saturated with steam
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, and larger ice crystals are formed. When the water turns into ice, the volume increases by 9%-10%, resulting in the cells being deformed due to compression, and even causing the membrane of the cells
to rupture. So when the food is thawed, the ice crystals melt into water, and the food juice is lost. If fast freezing is used, because the ice crystal formation speed is greater than the diffusion speed of water vapor
, ice crystals can be evenly distributed in the food cells and intercellular spaces, and form small crystals, so that the cells will not be deformed or broken
. When the food freezes fast and slowly, the ice crystals in the tissue will change accordingly.
2) The changes in food freezing and freezing in supermarket refrigerated showcase.
(1) Volume expansion produces internal pressure. Water has the smallest volume at 4°C, and therefore the highest density. When it is 1000kg/m3aO°C, the water freezes and the volume increases by about 9%. In food
, the volume increases by about 6%. When the temperature of ice drops by 1℃, its volume shrinks by 0.005%~0.01%0=Compared to those, the expansion is much larger than the shrinkage. Therefore,
food with more water content will expand in volume when it freezes. When food is frozen, first the surface moisture freezes, and then the ice layer gradually extends to the inside. When the internal water is frozen and the volume of
expands, it will be hindered by the external freezing layer to generate internal pressure, which is called freezing expansion pressure, and its value can be as high as 8.7MPa by pure theoretical calculation.
(2) Dry consumption. The loss of water in food, resulting in reduced quality and reduced quality is called dry consumption . Dry consumption exists in the freezing and freezing process
. The reason for dry consumption in the freezing process is that the air in the freezing room has not reached the saturation state of water vapor, and its vapor pressure is less than the saturated water vapor pressure, while fish, meat, etc. contain
. The water content is high, and the surface layer is close to the saturated water vapor pressure. Under the action of the vapor pressure difference, the moisture on the food surface evaporates into the air. After the moisture on the surface layer evaporates, the inner water
moves to the surface layer under the action of diffusion. Because the air in the freezing room continuously passes through the evaporator, the water vapor in the air condenses on the surface of the evaporator, and after dehumidification
, it is often in an unsaturated state, so the cost of the freezing process is constantly going on. The reason for dry loss during the freezing process is similar to the freezing process, the difference is that the freezing
during the freezing process is lost in the form of ice crystal sublimation. At the beginning of dry consumption, only ice crystal sublimation occurs on the surface layer of the frozen food, that is, the so-called dehydration multiply
L layer. After a long time, it gradually pushes inward to reach the deep internal ice crystal sublimation, and the dehydrated porous layer diffuses inward, so that the internal The dehydrated porous layer is constantly deepened.
this not only makesFrozen food is dehydrated to reduce weight, resulting in quality loss, and because the ice crystals sublimate into small cavities, the contact area of quick-frozen food and air
is greatly increased. The de-porous layer can easily absorb inwardly diffused air and frozen storage. The various peculiar smells in the warehouse are easy to cause strong oxidation reaction. Under the action of oxygen,
, animal food fats are oxidized and rancid, the surface becomes yellow and brown, the appearance of the food is damaged, and the taste and nutritional value are deteriorated. This phenomenon is called
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n in the supermarket
refrigerated showcase. ). The moisture content of the frozen part of the food is very low, 2%~3%. At this time, the protein is also dehydrated and denatured, and the quality of the food is seriously reduced.
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