By Harold McGee
There are two general cooking categories of potato, called the “mealy” and the “waxy” for their textures when cooked. Mealy types (russets, blue and purple varieties, Russian and banana fingerlings) concentrate more dry starch in their cells, so they’re denser than waxy types. When cooked, the cells tend to swell and separate from each other, producing a fine, dry, fluffy texture that works well in fried potatoes and in baked and mashed potatoes, which are moistened with butter or cream. In waxy types (true new potatoes and common U.S. red- and white-skinned varieties), neighboring cells cohere even when cooked, which gives them a solid, dense, moist texture, and holds them together in intact pieces for gratins, potato cakes, and salads. Both types can be made firmer and more coherent, less prone to the “sloughing” of outer layers when boiled, by treating them to the low-temperature precooking that strengthens cell walls.