The Building Blocks

• The texture of a vegetable, and how well it holds up under prolonged cooking, is largely determined by its structure. Plants don't have skeletons, as most animals do, so they get their structural strength from the sturdy cellulose that forms their cell walls. The cells are held together by two different types of carbohydrates, the pectins and the hemicelluloses. They act as a sort of glue or cement, holding the cell walls together.

Firm vs. Soft Vegetables

• In carrots or rutabagas they form strong bonds between the cells, keeping the vegetable firm even as it cooks. In sweet potatoes or russet potatoes those bonds are relatively weak and are easily severed, either by your fork or by the gentle force exerted by your simmering stew. The effect is especially noticeable if you stir the stew frequently, helping break up the sweet potatoes, or if you simmer it at too high a temperature. A gravy that bubbles vigorously, on the edge of a full boil, makes the sweet potatoes dissolve much more quickly.

Coping Strategies

• There are a number of ways to compensate for sweet potatoes' tendency to disintegrate in the pot. The simplest is to cut them larger than dense vegetables such as carrots, so they'll take longer to cook. That's only moderately effective, because the outer edges will still dissolve before the middle cooks. A better option is to cook the sweet potatoes separately, in a small quantity of the same broth or gravy you're using for the rest of the stew. Add the sweet potatoes back to the pot immediately before serving, stirring the pot just enough to ensure that they're evenly incorporated.

Science to the Rescue

• A pair of lesser-known alternatives take advantage of some cell-wall quirks, spelled out by food-science writer Harold McGee in his book "On Food and Cooking." Acidity strengthens the molecular bonds holding cell walls together, so par-cooking the sweet potatoes in water with added lemon juice will help them stay firm after they're added to the stew pot. Alternatively, pre-cook the sweet potatoes gently in a pot of water at 130 to 140 degrees Fahrenheit for 20 to 30 minutes. This gives the sweet potatoes a firmness that lingers after they're added to the stew, and does it without changing their flavor. You'll need a kitchen thermometer to monitor the temperature of the water properly.