Q. I substitute yogurt for sour cream in my stuffed potato recipe. Can it also replace sour cream in cakes, pies, cookies and other recipes?

A. Baking is an exact, measured science. Cooking is the art of combining handfuls and pinches. Since yogurt and sour cream are two very different beasts, any substitutions belong in the realm of art, not science. To substitute yogurt for sour cream in a pastry, cake or cookie recipe, you would have to accommodate for a higher water and lower fat content. In cooking, though, you can substitute yogurt cup-for-cup for milk, buttermilk, cream or sour cream, changing only the flavor and texture, not the essence of the product.

Yogurt is 87.9 percent water and sour cream is 70.95 percent water. Yogurt is only 3.25 percent fat while sour cream is 20.96 percent fat. These amounts vary somewhat depending on brand and batch. The point is, though, that adding an ingredient with more water and less fat really changes a pastry, cookie or cake.

Yogurt is an acidic protein gel composed of billions of protein (casein) molecules that change shape and precipitate during heating due to the lactic acid produced by bacteria. Sour cream also ferments, but with more fat and less protein. The product is more an emulsion -- fat particles surrounded by protein and water -- than a gel. This means that yogurt is more sensitive to heat than is sour cream, thus it is more likely to separate into curds and whey while sour cream's emulsion holds the curds and whey together during mild heating.

Instead of only thinking substitution, seek out recipes designed for yogurt. Here is a bread recipe I have found to be simple and very adaptable to the home kitchen. YOGURT BREAD (1 or 2 loaves) 4 cups bread flour (or 2 cups bread flour, 2 cups whole-wheat flour) 1 cup plain yogurt 1 1/2 teaspoons salt 1 package dried yeast dissolved and foaming in 1/4 cup lukewarm water with a pinch of sugar 1 tablespoon yogurt mixed with 1 tablespoon water Flour for dusting

Stir 4 cups flour, 1 cup yogurt, salt and yeast mixture together, adding enough extra water to form a medium-stiff dough. Knead 10 minutes or until firm and not sticky. Lightly grease the surface of the dough, place in a greased bowl, cover and let rise, in a warm, humid spot until it is 50 percent larger (about 30 minutes). Punch down and form into freeform loaves or loaf. Let rise again, this time until doubled in bulk. Pour the yogurt-water mixture over the loaf, spreading it around with your hands. Dust liberally with flour and spread it around, too. Make any slits desired and bake in a 400-degree oven for approximately 35 minutes -- until the bottom, when sharply rapped, responds with a dull thud.