Potatoes' Pigment Only Skin Deep

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By Robert L. Wolke
Wednesday, July 6, 2005

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?


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