Turkey Day Chemistry in the Kitchen

Mashed-Potato Primer

Whether you like your potatoes fluffy or creamy or something in between, you can use science to get them just the way you want. There are three basic variables-potato type, cooking method and mashing technique. Here's a primer:

Potato type:
There are more than 200 species of potatoes, but for cooking purposes they can be split into two general categories. The more delicate "floury" or "mealy" potatoes, such as Idaho or russet, have more starch granules in their cells. When heated, they combine with water molecules, and the cells expand and separate, making for fluffier spuds. "Waxy" varieties, such as new potatoes, have cells that stick together and stay firm and dense when cooked. They make a creamier mash.

Cooking:
Potatoes contain an enzyme in their cell walls, activated at 120, that helps its cells resist breakdown. So science's best answer for mashed potatoes is to precook chunks of the mealy type at 130 to 140 for 20 minutes to activate the enzyme and keep the cell walls strong. Drain them and then re-cook. When they are tender, drain them again and return them to the pan to dry off any extra moisture.

Mashing:
If you've ever had the not-so-great idea of making mashed potatoes in a food processor, then you know what happens when cells get damaged and too much starch is released. You make glue. Even precooking won't prevent that. The fluffy but fragile granules of the floury type of potato are best separated using a food mill or gentle mashing, and their vast surface area can absorb a good amount of cream, butter or milk. The waxy type can take more of a beating, and less add-ins-they're best for rough-mashed, country-style potatoes.