The Science of Cooking Potatoes

By David Joachim and Andrew Schloss

from Fine Cooking #125, pp. 26-27

Potatoes are among the world’s most important food crops, along with corn, wheat, and rice, and Americans consume more of the humble spud than they do any other vegetable. That’s because potatoes are easy to grow and versatile in the kitchen. But to cook them just right-and make them taste their best-it helps to understand their inner workings.

What exactly is a potato?

Most people believe that it’s a root, since it’s often referred to as a “root vegetable.” But believe it or not, the potato is actually a stem. The leaves of the potato plant (Solanum tu-berosum) manufacture starch, which is transferred through the plant to its underground stems, known as stolons. Starch collects at the end of the stolons, forming swollen sections called tubers. These are the potatoes.

Each potato has several buds, or eyes, that can grow new plants. When your potatoes sprout during storage, they’re using available light and stored starch-the same starch that becomes deliciously creamy when cooked- to grow a new plant.

Rich in nutrients, the average 8-ounce potato provides about 35 grams of carbohydrates, 4 grams of fiber, 3 grams of protein, 11 milligrams of vitamin C, and 350 milligrams of potassium.

Where is the best place to store potatoes?

Store potatoes in a cool (45°F to 50°F), dark, well-ventilated place, such as an unheated basement, closet, or root cellar. Under these conditions, potatoes can last for months and actually improve in flavor, as enzymes slowly break down lipids in the potato cell membranes, making them tastier.

Cool conditions are important because potatoes can begin to sprout within a week if stored at room temperature. But when stored below 40°F in a refrigerator, their metabolism changes, causing them to convert some of their starch to sugar. A little extra sugar improves browning and crispness on roasted and fried potatoes (for more on this, see the sidebar below), but too much can ruin a potato’s flavor. The starch-to-sugar conversion can be reversed by taking the potatoes out of the refrigerator and storing them in a cool, dark cabinet for a week or two.

Keep potatoes out of the light, too. When exposed to light, potatoes manufacture increasing amounts of chlorophyll as well as two bitter-tasting alkaloid compounds, solanine and chaconine. In high concentrations, these can not only make potatoes taste bitter but also can cause headaches and stomachaches. A 4-ounce (114 gram) potato naturally contains 5 to 6 milligrams of solanine and chaconine, which is considered harmless. But when a potato looks green from an increase in chlorophyll, it indicates that it will also have higher levels of these alkaloids-  as much as 20 times in potatoes that have turned completely green. Therefore, it’s best to avoid green potatoes, or deeply cut away any green parts. Potato sprouts, which develop from storing potatoes at room temperature for too long, are also high in solanine and should be cut away.

Never store potatoes in an airtight container, such as a tightly closed bin or a sealed plastic bag. Without ample ventilation, potatoes develop damp spots that are prone to bacteria and mold infestation.

For a cook, why does a potato’s starch content matter?

High-starch potatoes, such as russets (baking potatoes), have densely packed starch cells that swell and separate from one another when cooked, resulting in a dry, fluffy texture. High-starch potatoes also make creamy mashed potatoes and french fries with a flaky interior. On the other hand, low-starch potatoes, such as round red- or white-skinned potatoes, have moister, loosely packed starch cells that don’t separate from one another, so these potatoes retain their shape better, even when boiled. That’s why they work so well in salads and stews. New potatoes, or creamers, are also low-starch and slightly sweeter than other low-starch potatoes because they are harvested young, before all of their sugars have converted into starch. Finally, mediumstarch potatoes, such as Yukon Golds and Yellow Finns, occupy the middle ground. They’re known as all-purpose potatoes because they have a moderate starch content, which makes them suitable for any cooking technique.

If you’re not sure about the starch content of a potato, there’s a quick way to check: Cut the potato in half. If it sticks to the knife or leaves a creamy white residue on the blade, it’s a high-starch potato. If it doesn’t, it’s a low-starch potato.

How to make spuds that aren’t duds

For the crispest french fries, use high-starch potatoes and fry twice. Highstarch potatoes are lower in moisture so they brown and crisp better than lowstarch potatoes. First fry the potatoes at 325°F to evaporate moisture and help prevent a soggy interior; next, fry them at 375°F to crisp and brown their surfaces. You can also dip the potatoes in a light batter or dust them with starch (potato starch or cornstarch) after the first frying to help them develop a crisp crust.

For better browning on roasted potatoes, chill the whole potatoes for a day or two before cutting and cooking them. Chilling converts some of the potato starch to sugar and helps promote browning. This trick works for extra-brown fried potatoes, too, but chill the potatoes for only one to two hours because heat transfer occurs faster during frying and can cause burning.

To retain a potato’s shape in salads or long-cooked stews, parcook in 130°F to 140°F water for 20 to 30 minutes. This activates an enzyme in the potato cell walls that prevents the cells from weakening, thereby helping keep the potato intact during long cooking. Then continue to cook as necessary for the dish you’re using them in (boil in the water until tender for salads, or transfer to a stew pot for stews).

To prevent gluey mashed potatoes, don’t use an electric mixer or food processor for mashing because too much beating causes potatoes to release excess starch and become gluey. Instead, mash quickly with a potato ricer, masher, or food mill.

For more nutrients and flavor, don’t peel your potatoes (but do scrub them). The skin contains about half of the potato’s fiber and helps retain vitamin C and other nutrients in the flesh during cooking. But if you’re using the peels, it’s wise to buy organic. Conventionally grown potatoes usually show up on the Environmental Working Group’s Dirty Dozen list of produce with high levels of pesticide residues.

David Joachim and Andrew Schloss are the authors of the award-winning reference book The Science of Good Food.