The Dragon Boat Festival holiday is not long, but it is still a very pleasant choice to cook at home and digest the food that was hoarded before. There is just one thing to pay attention to: the food frozen in the refrigerator is not suitable for repeated thawing and then freezing.
You may have had the experience of defrosting a package of frozen shrimp, only to use a small portion for stir-frying, so you put the rest in the freezer of the refrigerator to freeze again for the next serving. Actually this is not a good practice.
Frozen: Not too cold this winter
The so-called quick freezing, as the name suggests, is to freeze quickly. By reducing temperature and water activity, it inhibits various chemical reactions and delays the spoilage of food. The quick freezing process involves the crystallization of water molecules. In short, the slower the cooling rate, the larger the ice crystals formed by the water molecules. Therefore, the purpose of rapid cooling is to make the ice crystals formed by water as small as possible, so as not to damage the integrity of the food microstructure as much as possible. In fact, if the cooling rate is fast enough (such as using liquid nitrogen to quickly cool down to below minus 135 degrees Celsius), the water will exist as a highly viscous liquid without ice crystals at all. However, this technology is usually only used for “essential” resources such as frozen sperm and eggs, and it is unlikely to be applied to food. First, the food volume is too large to quickly reduce the core temperature; second, the cost of such a rapid cooling is too high.
Source | pixabay
So how fast is the “quick freezing” in food processing, and how low is the temperature? The usual requirement is to reduce the temperature of the center of the food to about minus 5 degrees Celsius within 30 minutes, and then store it in an environment of minus 18 degrees Celsius. Because the ice crystals so quick-frozen 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 foods are not as tasty as fresh foods.
However, these ice crystals are difficult to damage smaller bacteria (about 0.5-5 microns in diameter), which means that such a temperature cannot freeze the bacteria at all. They are only temporarily hibernated due to the reduction of temperature and water activity. status only.
Thawing: Spring in the Bacteria Class
Except for a few cold drinks such as ice cream, most of the food still needs to be thawed, reheated, or even cooked at high temperature before eating (what? You have a good mouth? Just love to eat frozen hairtail?). Therefore, two important changes will occur during the thawing process: one is an increase in temperature, and the other is an increase in water activity.
Source | Zhanku Hailuo
When a lump of hairtail at minus 18 degrees Celsius warms to room temperature, with the increase in temperature and water activity, various chemical reactions that can lead to food spoilage are gradually accelerated, and the bacteria that were originally in a hibernation state are also like ushering in spring. They all woke up. Guess what the first thing the bacteria see when they wake up? Since the cell membrane was destroyed by ice crystals during the freezing process, the cell fluid flowed out after thawing. For bacteria, it’s like a starving person who suddenly sees deliciously fragrant boiled meat slices and oily grilled chicken thighs. Moreover, the ancients said, “Satiated warm and lustful, this is no exception to bacteria. What is even more frightening is that the reproduction of bacteria increases exponentially.” In just a few hours at room temperature, the number of bacteria in this octopus may double several times.
At this time, if you quickly make it into braised hairtail and eat it, it will not cause health problems, but if you put it back in the refrigerator, due to the limited freezing effect of ordinary household refrigerators, it usually takes several hours to completely freeze the food from the inside to the outside. , the bacteria will still use this time to increase the “population base” of the “class members”. In addition, this slow freezing process forms larger ice crystals, causing further damage to hairtail cells. When it is thawed again next time, due to the larger bacterial “population base” and the more severely damaged hairtail cells, the tragic hairtail that has been frozen twice is more likely to deteriorate in a short period of time, and The consequences of rushing to eat this tragic oarfish are likely to be running to the bathroom with your stomach covered, or maybe N times!
Such delicious food, haven’t eaten it all at once? ｜Source: Zhanku Hailuo
The secondary freezing of thawed food is not conducive to the preservation of food, and the risk of food spoilage will increase, so thawed food should be eaten at one time as much as possible. For the safety of consumers, the EU stipulates that all quick-frozen foods must be clearly marked “Do not freeze and thawed food again” on the outer packaging.
Author: few screws
This article is from Nutshell and may not be reproduced without authorization.
If necessary, please contact [email protected]
This article is reproduced from: http://www.guokr.com/article/461581/
This site is for inclusion only, and the copyright belongs to the original author.