Starchy Vegetables

Potatoes, sweet potatoes, winter squashes, and other starchy vegetables owe their distinctive cooked texture to their starch granules. In the raw vegetables, starch granules are hard, closely packed, microscopic agglomerations of starch molecules, and give a chalky feeling when chewed out of the cells. They begin to soften at about the same temperature at which the membrane proteins denature, the “gelation range,” which in the potato is from 137–150°F/58–66°C (it varies from plant to plant). In this range the starch granules begin to absorb water molecules, which disrupt their compact structure, and the granules swell up to many times their original size, forming a soft gel, or spongelike network of long chains holding water in the pockets between chains. The overall result is a tender but somewhat dry texture, because the tissue moisture has been soaked up into the starch. (Think of the textural difference between cooked high-starch potatoes and low-starch carrots.) In starchy vegetables with relatively weak cell walls, the gel-filled cells may be cohesive enough to pull away from each other as separate little particles, giving a mealy impression. This water absorption and the large surface area of separate cells are the reasons that mashed potatoes and other cooked starchy purees benefit from and accommodate large amounts of lubricating fat.