It is time to gird your insides with potato against the coming cold. I seem to forget the humble potato through the summer and other warmer months, it holds little sway in my sunlit kitchen. But come daylight-savings time I’ve only got eyes for roots. There is, of course, historical precedent for this: the hearty vegetables we tend toward throughout the winter (squash, potatoes, parsnips, etc.) are those which can be stored for many months. In the pre-refrigeration kitchens of our many ancestors, this was how they survived until spring.
And as with most vegetables, we are presented with a wide variety of potatoes at the market. Understanding the difference between them (from a chemical standpoint there are wild variances) can make and/or break your cooking.
Waxy versus Starchy Potatoes
Surely, we’ve all seen these descriptors used at various times to label potatoes in bins. Or perhaps you’ve been baffled by the use of those words in recipes as you prepare for Thanksgiving or Christmas. But the distinction is significant, and it is worth understanding how a potato is built in order to affect your cooking.
Potatoes, like all vegetables, are a collection of specialized plant cells. Those cells are held together with pectin (the magical plant glue). In the walls of potato cells we find lots of starch granules. Those starch molecules are tightly packed, they’re infinitely small when uncooked. But a miraculous thing happens when the potato is subjected to warm water. Those starch molecules inflate, they suck up the water like dry sponges and greatly increase their size. They become engorged and soft (that is how we understand the difference between a cooked and uncooked potato).
But the amount and kind of starch to be found in the cell walls is what determines the final texture of each potato. Starchy potatoes (Russets, Idahos, etc.) tend to contain more starch (obviously), about 22% by weight. Their starch is a blend of both amylose and amylopectin. Waxy potatoes (your red skinned, your blue potatoes, your fingerlings), on the other hand, contain about 16% starch by weight and that starch is nearly entirely amylopectin. Amylopectin sound familiar? That starch shares qualities with the magical plant glue (pectin) and consequently also shares some of its name.
Starchy potatoes cook up dry, fluffy, and crumbly. They’re best used for baking, and frying. Waxy potatoes, because of the lower starch percentage and higher quantity of amylopectin, are able to hold their shape longer. Consequently, we use those potatoes for potato salads and other boiled applications in which we prize shape.
Of course, all of this is useless information unless we put it in action. And, because Thanksgiving is around the corner, we will investigate how the waxy vs. starchy choice affects your mashed potatoes.
To start, you should favor a starchy potato over a waxy potato for mashing. When it comes to mashed potatoes, you want the least amount of agitation to achieve the final dish. If you choose a waxy potato, the sort of potato that holds its shape quite well in boiling water, you’ll have to mash harder, and longer in order to produce the final dish.
Ever been stuck with a pot of gluey mashed potatoes? That’s precisely because you stirred or mashed too long! Remember the little botany lesson earlier: cooked potatoes are a collection of swollen starch cells. When we start mashing them, the cell walls are severely damaged and starch begins to rush out. The more you stir, the more the starch molecules begin to link up. Are you using an electric mixer to make mashed potatoes? Stop!
The best way to achieve ethereal mashed potatoes is to use a potato ricer. This tool will mash the potatoes with minimal structural damage to the cell walls, leaving you with a lovely base of potato in which to stir your cream, butter, and other flavorings. Remember, less is more when it comes to manual labor here: don’t over stir!
Another reason to use starchy potatoes over way potatoes? Because they have less amylopectin and readily lose their shape, starchy potatoes are also more welcoming to liquid, i.e. they are terribly absorbent. Think about a baked potato. When you split it open the pillowy insides are all but begging for a blanket of butter and sour cream. They drink it up greedily. The same goes for mashed starchy potatoes. They will happily accept butter and cream, while mashed waxy potatoes will be slightly more standoffish when new ingredients are introduced.