You've probably had the experience of thawing a packet of quick-frozen shrimp, and only a small portion of the stir-fry, so you put the rest in the freezer and freeze it again for the next time. This is not a very good practice.
Quick freeze: It's not too cold this winter
The so-called quick freezing, as the name suggests, is to freeze quickly. Inhibit various chemical reactions by reducing temperature and water activity, delaying the deterioration of food. The quick-freezing process involves the crystallization of water molecules, in short, the slower the rate of cooling, the larger 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 the food as much as possible. In fact, if the cooling rate is fast enough (for example, using liquid nitrogen to quickly cool below minus 135 degrees Celsius), the water will exist as a highly viscous liquid without ice crystals at all. However, this technique is usually only used for "essence" resources such as freezing sperm and eggs, and is unlikely to be applied to food - first, the volume of food is too large to quickly reduce the core temperature; Second, the cost of cooling down so quickly is too high.
Figure 丨 Picture worm creativity
So how quickly and how low is the "flash freeze" in food processing? The usual requirement is to reduce 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. Since the ice crystals frozen in this way are less than 100 microns in diameter, which is exactly the same size as most animal cells (10-100 microns), the ice crystals can damage the structure of cell membranes in food. This is one of the reasons why quick-frozen food does not taste as good as fresh food.
However, these ice crystals are difficult to harm smaller bacteria (about 0.5-5 microns in diameter), which means that such temperatures cannot freeze bacteria at all, they are only temporarily hibernated due to the decrease in temperature and water activity.
Thaw: Spring of the bacterial class
With the exception of a few cold drinks such as ice cream, most foods still need to be thawed, reheated, and even cooked at high temperatures before they can be eaten (what?). Do you have good teeth? Love to nibble on frozen fish? ）。 As a result, two important changes occur during the thawing process: one is the increase in temperature, and the other is the increase in water activity.
Figure 丨 Picture worm creativity
When a lump of fish with minus 18 degrees Celsius warms up to room temperature, as the temperature and water activity increase, various chemical reactions that can lead to food deterioration are gradually accelerated, and the bacteria that were originally in hibernation are also awakened as if they have ushered in spring. Guess what the bacteria see first when they wake up? Since the cell membrane is destroyed by ice crystals during the freezing process, the cell fluid flows out after thawing. For bacteria, it's like a hungry person suddenly seeing a tempting slice of boiled meat and oily roast chicken drumsticks. And the ancients said, full of lust, this is no exception to bacteria, and what is more frightening is that the reproduction of bacteria is increasing exponentially. Just a few hours at room temperature can double the number of bacteria in this fish several times.
At this time, if you quickly make it into braised strip fish and eat it, it will not cause health problems, but if you put it back in the refrigerator, because the general household refrigerator has a limited freezing effect, it usually takes several hours to completely freeze the food from the inside out, and the bacteria will still use this time to raise the "population base" of "class members". In addition, this slow freezing process can form larger ice crystals, causing further damage to the fish cells. When the next time it thaws, due to the larger bacterial "population base" and the more damaged fish cells, this fish that has been frozen twice and the sad fish is more likely to deteriorate in a short time, and the consequences of eating this sad fish rashly are likely to run to the bathroom with your stomach, or maybe N trips!
Such a delicious thing, don't you eat it all at once? 丨Figureworm Creative
The secondary freezing of thawed food is very detrimental to the preservation of food, and the risk of food deterioration will increase, so the thawed food should be eaten at one time. For the safety of consumers, the EU stipulates that all frozen food packaging must be clearly marked "Do not refreeze thawed food".
Author: less screws
Editor: Forget the rivers and lakes