No Magic in Pasta Water

The danger, then, isn't in the freezing or refreezing; it's in the intermediate thawing. If frozen food is properly thawed -- either slowly in the refrigerator or in some cases more quickly by immersion in cold tap water -- then a second freezing shouldn't matter as far as safety is concerned.

But if the food has been thawed or partially thawed so that parts of it exceeded refrigerator temperature for more than an hour or so (no definite period of time can be stated), then the microorganisms that had gained a foothold during that time can continue to multiply while the food is being refrozen.

The reason is that our home freezers aren't cold enough to "quick-freeze" the food. They are set to about zero degrees, whereas commercial frozen-food plants may have freezers set at 50 degrees below zero or colder to do the job in a relative flash. Because home freezers therefore cool the food more slowly, an improperly thawed food may languish in the 40-to-70-degree region long enough for its bacterial burden to multiply dangerously.

So about that chicken: If it had been kept continuously at or below refrigerator temperature -- and you must trust your purveyor for that -- you can certainly freeze it without concern. But when you're ready to use it, thaw it properly in the refrigerator. And give it lots of time -- several days. Hordes of Thanksgiving guests are known to have become restive while waiting for turkeys that weren't completely thawed at roasting time.

And those partially thawed vegetables? They are undoubtedly still at or below refrigerator temperature, so there's no danger in refreezing them, especially if the package hasn't been opened. But their texture may suffer from the slowness of the refreezing. The slower the freezing, the bigger the ice crystals will be that form in the food, and the more they can damage a vegetable's cell structure.

That's why vegetables frozen slowly at home may be mushy when thawed. And obviously, freezing them a second time will exacerbate the damage.

When buying frozen vegetables in plastic bags, look or feel for ice or "snow" inside. That means that the food has been allowed to partially thaw, whereupon water leaked out of the damaged cells and was later refrozen. The vegetables will be withered and dry.

Robert L. Wolke is professor emeritus of chemistry at the University of Pittsburgh. His latest book is "What Einstein Told His Cook 2, the Sequel: Further Adventures in Kitchen Science" (W.W. Norton). He can be reached atwolke@pitt.edu.