I am highly allergic to sulfa drugs and cannot eat any dried fruit because it is cured with sulfur. My severe allergy to the sulfites in wine causes hand-sized hives, but I would like to cook with wine. Do you know if sulfites "cook out" during the cooking process?

You have consulted the wrong kind of doctor. I am a PhD, not an M.D. -- not a real doctor, as a mean aunt of mine used to announce at every opportunity. So I would suggest that you consult a medical doctor, preferably a specialist in allergies, to find out exactly what you are having reactions to, because there is no connection between sulfa drugs and sulfites. If you are indeed sensitive to both, it's a coincidence.

Which brings our discussion into the realm of chemistry, a field in which I am a real doctor. (So there, Aunt Bea!) You are confusing sulfa drugs with sulfur and both of them with sulfites. Sulfur is a chemical element that is part of the molecules of all chemical compounds known as sulfa drugs, sulfides, sulfates and sulfites, among many others. They are all different kinds of chemical compounds with different properties and uses. Let's straighten them out before focusing on the sulfites, which are indeed troublesome to an estimated 1 percent of the population and 5 to 10 percent of people with asthma.

The sulfa drugs -- sulfanilamide, sulfathiazole, sulfadiazine and literally thousands of others -- are a class of organic (carbon-containing) chemical compounds called sulfonamides, which are the amides of sulfonic acids. (That won't be on the test.) They inhibit the growth and activity of bacteria but don't actually kill them.

Sulfides and sulfates are simple inorganic chemical compounds of sulfur. A common sulfide is hydrogen sulfide, an evil-smelling, toxic gas. Calcium sulfate, or gypsum, is made into wallboard.

And now for the sulfites, which in sensitive individuals can cause anything from breathing difficulties to life-threatening anaphylactic shock -- and yes, hives.

Food manufacturers and processors are required to list sulfites on their labels if they are present in concentrations of more than 10 parts per million. Sulfites are added to food products for many reasons. Search the list of ingredients for a chemical whose name ends in sulfite, bisulfite or metabisulfite. They can all release sulfur dioxide gas, which is the real culprit.

Small amounts of sulfites are generated by the yeasts used in making wine, but more sulfites are usually added as a preservative -- more in white wine than red, because the reds' tannins help to preserve them.

And to answer your question (finally), I'm aware of no measurements that have been made on the elimination of sulfites during cooking. But from what I know about their chemistry, I would expect them to decompose into sulfur dioxide gas, especially in acidic foods. Breathing the gas can be just as bad for a sulfite-sensitive person as ingesting the sulfites themselves.

Some recipes call for using kosher salt, often when rubbing directly onto meat. Can you explain the differences between this and "sea salt" and plain old salt? Do I really need to keep three kinds of salt in my pantry?

Well, here we go again!

Five years ago in this space, I wrote a series of columns on salt. At the time, chefs were discovering that there were intriguing alternatives to the "plain old salt" in our salt shakers. Exotic sea salts became the rage, and chefs were heard to extol their purportedly enchanting flavors in lyrical terms.

My columns, intended to put scientific brakes on what Alan Greenspan might call irrational exuberance, caused a bit of commotion, winning me several journalism awards and a number of enemies -- opinionated, reading- impaired food writers who misrepresented what I said as "all salts are the same."

What I said was that they differ in physical form -- in crystal size and shape -- rather than in taste.

Here, then, is a brief reprise of the facts I presented, which I'm pleased to say have now been largely recognized and accepted by the food community:

* Commercial edible salts from the mine and from the sea are all about 99 percent pure sodium chloride, although some sea salts might contain 1 to 2 percent of magnesium and calcium chlorides.

* Sea salts are not "rich in essential minerals." The crystallization process itself, whether accomplished by solar evaporation of seawater or vacuum evaporation of brine from a salt mine, is a purification technique that has been used by chemists for centuries.

* Salt crystallized from seawater by slow solar evaporation develops larger, more complexly shaped crystals than rapidly crystallized mined salt. It is that coarser texture that gives sea salts their appeal. .

* Cooking with sea salt is a foolish extravagance because the textural effect is gone as soon as the crystals dissolve. So when a recipe specifies "sea salt," ignore it. Chefs have come to realize that it is best sprinkled on a dish just before serving.

* Some of the more exotic sea salts, such as Hawaiian red salt and Indian black salt, do have unique flavors. They taste like salt plus clay, which is what they are.

* Kosher salt is a coarse-crystal salt used in the koshering process to coat meat to extract residual blood. Chefs use it because its crystals make it easy to judge amounts by pinching it from a dish.

So do you really need three kinds of salt in your kitchen? Yes: kosher salt for cooking and seasoning; a coarse sea salt for topping servings of meat, fish and vegetables; and regular table salt, because it is the standard of measurement, especially in baking recipes.

Does that leave you with an unused salt shaker? Fill it with a cinnamon-sugar mixture to sprinkle on buttered toast.

Robert L. Wolke (www.robertwolke.com) 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, 2005). He can be reached at wolke@pitt.edu.