My e-mail in-box is long overdue for a housecleaning. Here are some accumulated readers' questions.

Q: Why does a recipe tell me to use unsalted butter, and then later to add salt?

A: It sounds silly, but there's a reason.

A typical stick of salted butter contains about two grams or half a teaspoon of salt, but different brands and regional products may contain very different amounts. A carefully formulated recipe, especially one that uses a lot of butter, can't afford to play Russian roulette with something as important as salt. That's why serious, high-quality recipes will specify unsalted, or "sweet" butter and leave the salt for a separate seasoning step.

Many chefs also prefer sweet butter because it is often of higher quality than salted butter. Salt is added partially for its preservative effect, and butter intended for use while very fresh doesn't need it. In unsalted butter any off-flavors are more readily detected.

Q: I like to make garlic oil by infusing olive oil with raw garlic. But I've heard that it's dangerous.

A: Yes, it is. Anaerobic (non-oxygen-using) Clostridium botulinum bacteria can thrive in that strictly airless environment.

Their growth is stopped by air, acids, extreme dryness or 10 minutes of boiling. But even if the bacteria are killed, any toxin they have produced will still be present. And keeping garlic in oil in the refrigerator may not help, because the bacteria can continue producing toxin down to 37 degrees Fahrenheit, which is colder than many refrigerators. Garlic in oil has been implicated in several deaths from botulism. So considering the seriousness of the hazard, just don't do it. Why not just press some garlic into the olive oil just before using it?

Q: My wife and I recently purchased a set of copper cookware and it looks great. How can we keep it looking new?

A: Whatever you do, don't cook in it.

Really, now. Shiny copper is beautiful, but are you a cook or a decorator? The great virtue of copper or copper-clad cookware is that it conducts heat superbly and evenly. For that it deserves to be cherished, not polished. If you try to keep your copper cookware in its virginal state you will have taken on a full-time job. I let my copper pans oxidize naturally, and over time they develop a handsome, reddish brown color like an old penny.

But to avoid having them look too blotchy, there are a few simple things you can do. Don't ever put them in the dishwasher; the highly alkaline detergent can discolor the copper. Dry them completely after washing with dishwashing liquid. Make sure to get all the grease off with a mildly abrasive cleaner such as Soft Scrub, because it will burn into a black, rather than a brown, color. Finally, don't heat the pans too hot, either with oil in them or especially when empty. Dark copper oxide forms most readily on the hottest spots, and you may wind up with the pattern of the burner imprinted on the pan's bottom.

Q: How does one make parboiled rice?

A: Parboil it.

Parboiling is boiling a food just enough to cook it partially, but not completely. When you're cooking a casserole that contains rice along with faster-cooking ingredients, the recipe may tell you to parboil the rice first to give it a head start so that everything will be done at the same time.

At the mill, rice is often treated with high-pressure steam before the hulls are removed, and they call this treatment parboiling also. Beneath the hull is a thin layer of bran, which is left on for brown rice or removed by abrasion to make white rice. The steam treatment forces nutrients from the bran into the starchy white grains or endosperm, but it doesn't qualify as parboiling in the time-saving kitchen sense; the rice is still "raw."

In case you've wondered what "converted" rice is, it's Uncle Ben's trademark for factory-parboiled rice. "Quick" or "instant" rice is rice that has been fully cooked and then dried, for people who are really in a hurry.

Q: Can raw eggs be frozen in the shell in a home freezer?

A: Yes. But (1) the shells will crack from expansion of the whites and (2) the yolks will be gummy and lumpy when they're thawed out. The latter effect is called gelation.

Large quantities of eggs used in food manufacturing are preserved by freezing after shelling. Gelation is prevented by adding salt, sugar or gelatin before freezing, but it would be rather difficult for you to get these substances into your whole eggs. Nevertheless, if you don't mind lumpy egg yolks, you can freeze them and they'll be perfectly fine.

Q: I use sponges to wipe up the kitchen sink, and I know they should be boiled from time to time to sterilize them. Is it possible to sterilize them in the microwave oven?

A: Sure. Put them in wet and zap them on high until they're steaming hot for a few minutes. Heat is heat, whether made by high- or low-tech methods. Some people put their sponges in the dishwasher but not all dishwashers get up to sterilizing temperatures.

Robert L. Wolke is professor emeritus of chemistry at the University of Pittsburgh and the author of "What Einstein Didn't Know--Scientific Answers to Everyday Questions." Send your food or cooking questions to wolke@pop.pitt.edu.