The concern about refreezing food is really a concern about two things, bacterial contamination and deterioration of texture and flavour. Let鈥檚 deal with the safety issue first. Bacteria like to dine on the same kinds of things as we do and therefore are often found on our foods. They also need water to survive, so they find moist environments particularly hospitable. Dry foods like salt, sugar and rice are rarely contaminated by bacteria. The presence of bacteria in a food, however, is not enough to make us sick. That depends on which bacteria in particular are present, and most important, in what numbers they鈥檙e present.聽It usually takes a large number of bacteria to cause illness.
Freezing does not kill bacteria but it greatly slows down the rate at which they multiply. When the food is thawed, bacterial growth speeds up, especially if the food reaches room temperature. Now, if it is refrozen, there are more bacteria than before, and when it thaws out again, we start with a greater pool of bacteria that can multiply. Why should this matter? Aren鈥檛 the bacteria going to be killed anyway when the food is cooked? Indeed, the bacteria may be killed by heat, but some bacteria, staphylococcus for one, produce a heat stable toxin.
If the food has not been allowed to warm up past about 5C, before refreezing, there is little likelihood of dangerous bacterial contamination. So here is the appropriate advice. You can safely refreeze food that still has ice crystals, or that has been no warmer than 5C or 40F and has been out of the freezer for no more than 24-48 hours.
The "do not refreeze" instruction originally came from Clarence Birdseye, the father of the frozen vegetable industry.聽He wasn鈥檛 worried about safety, he didn't want people removing the package from the freezer and putting it back because it would cause textural damage and reduce sales. Freezing does diminish the quality of food. Generally, water is retained inside cells because of osmosis through the cell wall. Dissolved solids are more concentrated inside the cell so water flows in. But with freezing, this osmotic capability is lost and water seeps out from inside cells. This means that textural rigidity is lost because the pressure caused by the swelling of one cell against another is reduced. Also when ice crystals form, they take up more space and the expansion bursts cell walls and pushes cells apart giving the water released an easy escape route. The higher the water content of food, the more mushy it will be after freezing and thawing. Compare a frozen steak that is thawed and a refrigerated steak. The difference in texture will be obvious. When liquid flows out of ruptured cells it also carries with it some flavour and nutrients and there is therefore a loss of quality.聽Vegetables survive freezing better than meat, seafood and fruit.
This texture problem has to be addressed by cryonics researchers interested in preserving human bodies in a frozen state with possible 鈥渞eanimation鈥� in the future. There is no way presently to eliminate the cellular damage caused by freezing but researchers are looking at certain proteins present in the blood of fish that live in cold water to see if this may be of help in preventing damage due to freezing. If you were frozen for a few hundred years and they managed to find a way to bring you back to life, you wouldn鈥檛 want to wake up limp and oozing water.聽
And how long can you keep frozen food in the freezer?聽Hamburger about 3 months, steaks a year, chicken 9 months, vegetables 8 months. In general nothing should be kept for more than a year. Not even frozen people.
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