Why is refreezing food bad? What exactly is freezer burn?
Dear Straight Dope:
Why is it a deadly sin to refreeze anything? What will happen if you do?
Dear Straight Dope:
What exactly happens to food when it gets freezer-burned?
For a long time humans have known that chilling food retards spoilage, and freezing preserves it even longer, provided you don't mind some deterioration in taste and texture. Cold helps fight two agents that ruin food: microorganisms and the food's own enzymes. The colder you keep the food, the more likely you are to prevent spoilage from molds and bacteria, and the slower the enzymes within the food work to chemically change it. However, even in a cold refrigerator (33-40 degrees F), food will spoil, as anyone who has had to clean a few science experiments out of the crisper drawer can attest. Meat can be especially susceptible to this, as bacteria will still slowly and steadily work to ruin your food.
Freezing food stops most enzyme activity and inhibits bacteria and mold growth. However--important point--it doesn't sterilize food. Though some microorganisms die, others resume activity as soon as the temperature increases. A more immediate problem is that the water in the food ruptures the cell membranes as it expands into ice crystals. This effect is more pronounced in meat than in vegetables and fruits according to Harold McGee:
The intercellular spaces [in meat], which are often large and which contain some water but very little else, freeze first, and then draw water out of the cells by osmosis. This in turn increases the concentration of dissolved substances within the cell, which further lowers the freezing point. Parts of the cell interior do not freeze at all.
When the cell membranes in meat are ruptured by freezing, the loss of muscle proteins and other cell fluids results in a dryer and tougher meat when thawed. In addition, reactions between the fluids liberated from the cells and the fluids that were in the intercellular spaces to start with almost always result in a poorer taste. Finally, freezing causes the fat in meat to oxidize and become rancid, especially in the case of fish, chicken, and pork, which contain unsaturated fats that are more prone to that sort of thing. Because beef contains mostly saturated fats, it can be kept frozen longer without going bad..
Although your freezer may be filled with ice (especially if you never defrost it), the air in it is extremely dry. As a result, if unwrapped or poorly wrapped food is left there for some time, it'll undergo freezer burn. In freezer burn the ice crystals in the food evaporate without first melting (a process called sublimation), altering the taste of the dried-out parts when they're thawed and eaten and in extreme cases making them inedible.
Refreezing is bad for three reasons. First, by refreezing food you multiply the damage to it--any cells that escaped rupture the first time the food was frozen are at risk of being ruptured the second time. Second, when food has been frozen and thawed out, it has larger pockets of liquid within it than the first time due to the ruptured cells. When the food is refrozen, the larger pockets of liquid can freeze into much larger ice crystals, which can tear through many more cell membranes and lead to more damage to the food. (The best way to avoid cell damage, incidentally, is flash-freezing, which produces smaller ice crystals, minimal cell damage, and maximum freshness.)
The third and most important reason not to refreeze is increased risk of spoilage due to microorganisms. Many people thaw food by letting it sit at room temperature for several hours, giving the microorganisms in it time to get busy and partially spoil the food before it's refrozen. The problem is particularly pronounced in large pieces of meat such as a turkey, some parts of which may be at or near room temperature for hours during thawing. That's why turkeys should be thawed in a sink filled with water--the water equalizes the temperature and makes for faster thawing. Alternatively, you can thaw in the refrigerator, which is slower but retards spoilage by keeping the meat cool. Even so you're likely to have some multiplication of microorganisms. If you refreeze and rethaw, you've subjected the food to double the microorganism growth and double the fun.
If you must refreeze food that has been thawed in a warm place (or which has remained thawed in a cold place for a long time), you should cook the food properly first, then refreeze it. Even under the best circumstances, however, multiple freezing cycles aren't recommended. If you can't finish the food yourself, you're better off giving it to your household garbage disposal--ideally the one that barks or meows. References
On Food and Cooking: The Science and Lore of the Kitchen, Harold McGee, 1984.
"The Big Thaw: Safe Defrosting Methods for Consumers," Food Safety and Inspection Service Fact Sheets, U.S. Department of Agriculture