I've been cooking for 45 years and have recently had a problem with potatoes turning black after they're cooked. Boiled or roasted, they are impartial to the process. I do NOT leave them in water, I do NOT peel them way ahead, and I DO make sure they are covered with water if they are being boiled. Can you help?

Sorry to say, your 45-year lucky streak has come to an end. I'm surprised that you haven't noticed this fairly common occurrence before. But it has nothing to do with peeling the potatoes or covering them with water.

On exposure to air, cut surfaces of raw potatoes do turn brown by enzymatic action, and keeping them under water prevents this. But such enzymatic browning is a completely different phenomenon from what you are describing.

The food scientists' technical term for potatoes' darkening after cooking is "after-cooking darkening." It is caused, as are virtually all changes that take place in foods, by a chemical reaction.

Potatoes contain varying amounts of iron in what chemists call the ferrous form (I'll abbreviate it as Fe-2), along with varying amounts of a chemical called chlorogenic acid (CgA). In the heat of cooking, the Fe-2 reacts with the CgA to form a complex (ferrous-chlorogenic acid) that I will call Fe-2CgA. Then, on exposure to air, the Fe-2CgA is oxidized to Fe-3CgA (ferri-dichlorogenic acid), a black pigment. It is perfectly harmless and does not affect the potato's flavor or nutritional value.

The amounts of iron and CgA in a given potato, and hence the degree of after-cooking darkening that can take place, depend on the potato's variety, its growing conditions, and its length and temperature of storage. The longer and colder the storage, the more CgA is formed.

So store your potatoes at room temperature, not in the refrigerator, even though you have no way of knowing how cold and how long they had been stored before you bought them; it can be many months. (Freezing slows the production of CgA and is often used commercially to deter after-cooking darkening.)

In 2001, the world consumed 7.4 million tons of frozen potatoes in the form of french fries alone. In the same year, the average American consumed a total of 58 pounds of french fries. Is it any wonder, then, that . . .

Oh, well, never mind.

Rudolph Chelminski, in his book "The Perfectionist," recounts a recipe for fried eggs attributed to the great and ultra-finicky French chef Fernand Point (1897-1955): "Place a lump of fresh butter in a pan or egg dish and let it melt -- that is, just enough for it to spread, and never, of course, to crackle or spit; open a very fresh egg onto a small plate or saucer and slide it carefully into the pan; cook it on heat so low that the white barely turns creamy, and the yolk becomes hot but remains liquid; in a separate saucepan, melt another lump of fresh butter; remove the egg onto a lightly heated serving plate; salt it and pepper it, then very gently pour this fresh, warm butter over it."

I followed these instructions to the letter and the results, while a culinary triumph, were an aesthetic disaster. First, maybe my egg wasn't fresh enough, but it had not one but two whites! The first was thick and formed a circle around the yolk. The second was quite thin and spread out to form a larger circle around the first. This made even cooking difficult, as the first white was still liquid after the second had congealed and begun to have bubbles of butter and steam pop through.

I've tried this twice with the same result. Why are there two layers of egg white?

All hens' eggs have two kinds of whites: a thick albumen and a thin albumen. The thick albumen mostly surrounds the yolk, while the thin albumen is predominantly located farther out toward the shell. Your thin albumen spread out more in the pan, and as a thinner layer, it cooked faster.

I also tried Point's recipe, with both a fresh egg and a rather stale one. When cracked into a dark-colored plate, the fresh egg showed a thick, jellyfish-like oval of albumen surrounding the high-standing yolk, with a shallow pool of thinner albumen spreading outward from it. The stale egg formed a thinner, wider and shallower oval around a somewhat flattened yolk, with an even thinner, quite transparent pool of albumen spreading outward. In each case, the inner oval was the thick albumen and the wide pool was the thin albumen. As an egg ages, the yolk flattens out, while both albumens thin out and become more transparent.

If I may be so bold as to suggest an improvement to Chef Point's recipe, you can even out the cooking of the thin and thick albumens by basting the latter with the hot, melted butter.

At local farmers markets, many vendors recommend washing fruits only just before consumption. Their explanation is that washing fruits in advance of consumption will enhance spoilage. My wife concurs with the vendors. I don't buy this old wives' tale. Please help in resolving our family disagreement.

Sorry, old boy, but your wife is right for a couple of reasons.

First, many fruits have wax coatings that are either natural or applied by packers to prevent moisture loss and maintain firmness during handling and storage. It's best to leave these coatings on as long as possible. Some commercial fruit coatings actually inhibit bacterial growth. At local farmers markets, of course, only the natural coatings should be of concern.

Second, and even more important, is that washing produce before putting it in the refrigerator's vegetable bin can trap moisture that will encourage mold and bacterial growth.

When you're ready to use the fruit or vegetable, wash it in clean, cold, running water, and when appropriate (not on strawberries!), scrub the skin with a vegetable brush. Commercial vegetable-washing products are unnecessary. Save your money.

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.