Frozen food secondary freezing, bacteria like best!

You've probably had the following experience, thawing a packet of frozen shrimp, only a fraction of the stir-fry, so put the rest in the fridge freezer and freeze it again for the next time you eat it. In fact, this is not a good practice.- it's not too cold this winter so-called fast-frozen, as the name suggests, is to freeze quickly. By reducing temperature and water activity, various chemical reactions are inhibited, which slows the food's desalination. The frozen process involves the crystallization of water molecules, in short, the slower the cooling rate, the greater the ice crystals formed by water molecules. So the purpose of rapid cooling is to make the ice crystals formed by water as small as possible, so as not to destroy the integrity of the microstructure of food. In fact, if the cooling rate is fast enough (e.g. using liquid nitrogen to cool rapidly below minus 135 degrees Celsius), the water will exist in a highly viscous liquid without ice crystals at all. But the technique, which is typically used only on "serminate" resources such as frozen sperm eggs, is unlikely to be applied to food - first, because food is too large to slow down the central temperature;how fast and how low is the "frozen" in food processing? The usual requirement is to lower the temperature of the food center to about minus 5 degrees Celsius within 30 minutes and then store it in an environment of minus 18 degrees Celsius. Because the frozen ice crystals are less than 100 microns in diameter, which is exactly the same as the diameter of most animal cells (10-100 microns), the ice crystals destroy the structure of the cell membranes in food. This is one of the reasons why frozen foods don't taste as good as fresh food.However, these ice crystals are difficult to hurt smaller bacteria (about 0.5-5 microns in diameter), which means that such temperatures simply do not freeze to death bacteria, they are only due to the temperature and water activity of the reduction, temporarily into hibernation.- Spring of the Bacterial Class In addition to a few cold drinks such as ice cream, most foods we still need to thaw, reheat, and even cook at high temperatures to eat (what?). You have good teeth? Just love to gnaw frozen fish? ）。 Two important changes occur during the thawing process: an increase in temperature and an increase in water activity.When a fish with minus 18 degrees Celsius warms up to room temperature, as temperature and water activity rise, chemical reactions that can cause food to go bad are accelerating, and bacteria that are already hibernating wake up like spring. Guess what bacteria wake up to see the first time? Because the cell membrane is destroyed by ice crystals during freezing, the cell fluid flows out after thawing. For bacteria, it's like a hungry person suddenly seeing scented, tempting slices of boiled meat and oily roast chicken legs. And the ancients said, full of warm thinking ah, this is no exception to bacteria, more terrible is that the reproduction of bacteria is exponential growth. In just a few hours at room temperature, the number of bacteria in this fish can be doubled several times.This time, if you quickly make it into red-burned fish to eat, not to cause health problems, but if put back in the refrigerator, because the general household refrigerator freezing effect is limited, usually it takes several hours to let food from the inside out completely frozen, bacteria will still use this time to improve the "class members" of the "population base." In addition, this slow freezing process creates larger ice crystals, causing further damage to the fish cells. Wait until the next time to thaw, due to the larger bacteria "population base" and more destructive with fish cells, this is frozen twice, sad with fish, it is easier to spoil in a short period of time, and the consequences of eating this sad fish, it is likely to cover the stomach to run the bathroom, may also be N trip Oh!the second freezing of thawed food is very detrimental to food preservation, the risk of food quality will increase, so thawed food should be eaten as much as possible at once. For the safety of consumers, the EU requires that all frozen food packaging must be clearly marked "Do not freeze the unfrozen food again."