The Man Who Ate Everything - Jeffrey Steingarten [24]
The potato plant is a perennial herb of the Solanaceae family with purple or yellow flowers and, occasionally, a fruit that looks like an amiable little green tomato but poisons you like the berry of the nightshade. The edible potato is not a root vegetable at all but a tuber—the swollen tip of the plant’s underground stem. The eyes of a potato are not arranged randomly but continue the pattern set on the stem by its branches and leaves: a spiral with thirteen leaves or eyes for every five turns of the helix. Sugar is manufactured in the leaves and transported to the underground tuber, where it is turned into starch.
Why your potato turns gummy. Like other living things, a potato is composed of millions of cells all cemented together. Lining the walls of each potato cell are hard, closely packed microscopic granules of starch, impervious to the water that fills the rest of the cell. But when you heat a potato to about 140 degrees Fahrenheit, the starch granules begin to absorb the water around them, and by 160 degrees they have swollen to many times their original size. The starch is now a gel, a viscous complex with water, and fills up most of the cell. Separate, swollen, and perfectly intact potato cells make for smooth mashed potatoes. But at 160 degrees the cells are still strongly bound to one another, and if you try to mash the potato now, the cells will split rather than separate, and the starch gel will ooze out of them.
This is called free or extracellular starch, and it is the enemy. Free starch turns mashed potatoes gummy.
As the cooking time lengthens and the internal temperature of the potato increases to 180 degrees, the cement between the potato cells—pectic material similar to the pectin that thickens jams and preserves—begins to degrade, and the cells can now separate from one another. This is a good time to mash your potatoes. With further cooking, the cells begin to weaken and rupture, and some of the gelled starch leaks out. That’s why overcooked potatoes become sticky and gluey even though they are easy to mash. If 15 or 20 percent of the cells in your potato are ruptured, you will be very sorry.
Buying your potato. One common way of categorizing potatoes is by their starch content. A mealy or floury potato like the Russet Burbank is dense, high in starch, low in water, and, despite the distasteful sound of the word “mealy,” generally preferred in this country for mashing. A waxy potato like the White Rose is low in starch, high in water, and often specified in French recipes. (Joël Robuchon’s potato is the BF 15; translated from the French this means a small yellow-skinned potato with dark yellow waxy flesh.) The adjectives “mealy” and “waxy” refer to the texture of the potato after you cook it. Mealy potatoes become fluffy and almost grainy when you mash them; waxy potatoes become creamy and smooth. But waxy potatoes generally require longer cooking and greater mashing force to separate the cells. Some researchers feel that more free starch is thus released, increasing the risk of gumminess.
Once you have decided which type of potato to cook, just try to ask your greengrocer or the guy stamping prices on laundry soap at your supermarket to point out the waxy potatoes and the mealy ones. Your reward will be an uncomprehending stare. But you can test the potatoes yourself. Buy one of each and, when you get home, mix a bowl of brine with nine and a half cups of water and one cup of salt. If a potato sinks into the brine, it is high in starch and will cook up mealy. If it floats, it belongs in the waxy category—unless it suffers from a potato ailment called hollow heart, in which case it will probably float in anything.