Is it a good idea to add a little pasta water to cooked pasta, or should I just add the sauce? Why would it make any difference as long as there is enough moisture?
Your reasoning is impeccable. Why add water if there is already enough water -- especially since you've just finished draining it?
However, if there should be a substantial delay between draining the pasta and saucing it, it may dry out and stick together. In that case, you'd want to both reheat and re-moisturize so it will mix well with the hot sauce. If you retain all the cooking water by fishing out the pasta rather than by dumping everything into a colander, all you have to do is put the pasta back in the water for a few seconds and it can come out as hot and moist as new. Many restaurants precook and drain their pasta, separate it into individual portions, and then dunk the portions back in boiling water as needed before saucing and serving.
The more usual reason for retaining at least a cup or so of the pasta water is for thinning the sauce in case it's too thick or dry. There's nothing special about this hot water; it's just a handy source of, well, hot water. It's also useful for warming the serving bowls. Of course, if the pasta had been cooked in salted water, adding some of that water to the final dish will certainly kick up its flavor.
But believe it or not, some people claim that you can use a bit of the pasta water to thicken a sauce by adding the water and then simmering the sauce down. After all, the theory goes, the water contains starch dissolved out of the pasta, and starch will thicken a sauce.
"Huh?" I asked myself. "Can there be enough starch in the water to do any significant amount of thickening?" I decided to find out.
I cooked eight ounces of spaghetti in three quarts of water for the six minutes recommended on the package, fished out the spaghetti and boiled the cooking water down all the way to dryness to see how much starch was left. (The final stages of evaporation were done in a dish in a 220-degree oven.) Then I weighed the dried starch.
I found nine grams of starch in the entire three quarts of cooking water. A little calculation showed that, if you were to add a quarter-cup of this water to a pint of sauce in an attempt to thicken it, you would be adding about one-fifteenth of a teaspoon of starch.
That's supposed to thicken it? No way!
We're told not to refreeze food we've thawed because it is unsafe. But how about this "fresh" -- presumably not previously frozen -- chicken I bought? Can I freeze it at home, or must I use it promptly? And if I have a package of frozen vegetables that has partially thawed, can I put it back in the freezer?
There is nothing inherently wrong with refreezing foods. It depends on the food's history before you refreeze it.
The blanket dictum "do not refreeze" is overly cautious. It is based on the worst-case scenario that the food had been thawed by leaving it out on the kitchen counter -- a definite no-no, because bacteria and other microorganisms can thrive at temperatures between refrigerator (40 degrees) and room (70 degrees). And refreezing won't kill them.
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 at firstname.lastname@example.org.