Food that has been frozen and thawed already has its tissues damaged to some extent. Therefore, thawed food should be processed and cooked as soon as possible and should not be refrozen.
The requirement for "rapid freezing" is to lower the food's core temperature to -1°C to 5°C (meaning most of the water has turned into ice) within 30 minutes (the shorter the time, the better), and to continue cooling it down quickly. The reason for this is that water has the smallest volume at 4°C, and its volume increases as the temperature drops further. In the -1°C to 5°C range, the volume of ice crystals in the food continues to grow, which squeezes and damages or ruptures normal cells and tissues. Therefore, the time spent in the -1°C to 5°C range should be minimized during freezing to reduce damage to normal cells and tissues. This is the principle behind "rapid freezing."
Thawing must be done slowly. This is because when thawing slowly, the food's volume changes gradually, and the melted water can be reabsorbed by the food cells, restoring the food to its original state. If thawed rapidly, the melted water cannot be reabsorbed by the tissues and becomes "free water," which flows out, carrying nutrients with it and reducing the food's quality.
The correct way to thaw food is to let it thaw naturally in air below 15°C, or to place it overnight in the bottom of the refrigerator (where the temperature is about 10°C), so that it is thawed by the time it is to be processed the next day. It should be noted that thawed food should be placed on a tray to prevent blood and juices from contaminating the refrigerator.
Food that has been frozen and thawed already has its tissues damaged to some extent. Therefore, thawed food should be processed and cooked as soon as possible and should not be refrozen. This is because during the recrystallization process, the water in the food will further damage the already compromised tissues, causing them to lose elasticity and to lose more water and nutrients.