Q: I have a few questions about substitutions: for 1/2 cup of chopped nuts, can I substitute an equivalent amount of wheat germ, bran or oats? If there are no nuts in a recipe, should I reduce the flour content before adding nuts? And what is the best substitute for buttermilk?

A: With the exception of Viennese and German nut tortes that rely on nut flours for part of their structure, there is no recipe in which nuts play more than a secondary role. Being at least 30 percent fat and up to 30 percent protein, they are poor sources of starch and rarely contribute to the structure of a bread, pastry or cake. Instead, nuts are flavoring and texture agents. They provide textural relief from the sameness of the baked dough or batter. And their oils contain flavors that permeate and give the baked product its special essence. There is therefore no need to alter a recipe in any way should you choose to add or delete nuts from the formula.

If, however, you add oats, which are terrific water-absorbers, it would be wise to reduce the volume of flour by half the volume of added oats (roasted or raw). Therefore, if you add a half-cup of rolled oats, you should reduce the recipe's flour by a quarter cup. This will keep the dough or batter at about the same consistency.

The best substitute for buttermilk is plain yogurt. Both have virtually identical pHs (acidity levels), and their flavors are also quite similar although different bacteria are used to produce each. Sour cream is not a suitable substitute, though its pH is also 4.5, as it contains at least 20 percent fat. Yogurt and buttermilk are between 1 and 4 percent fat. Milk is also a suitable substitute for buttermilk, provided you add 1 tablespoon of distilled vinegar to the recipe for every cup of milk.

Q: Please discuss varieties of potatoes sold in the store and the purposes for which they are best suited.

A: Although there are more than 30 varieties of commercially produced potatoes, they can be classified into four categories based on shape, size and color. These categories are:

Russet: This is a dry, mealy potato with rough skin and shallow eyes. The russet has large starch granules and rounded cell walls, which cause it to form granular but fluffy mashed potatoes, mealy french fries and fluffy, dry baked potatoes. The russet is also called "Idaho potato," although New York, Maine and other states produce large quantities of this variety. Half of all potatoes grown in the United States are russets, and virtually all dehydrated potato buds and flakes come from russets too irregular in shape or too small to market fresh.

Long White: This is a potato of firm, smooth texture, tan skin and shallow eyes that is used for frying and boiling. Most fast-food restaurants use long whites, as their flesh isn't granular like the russet but is creamy and moist.

Round Red: A potato of firm texture, smooth, thin, reddish skin. The round red is ideal for boiling peeled or unpeeled. One should use the round red for potato salads, boiled and scalloped potatoes. Because of its small starch granule size, sticky cell walls and high moisture content, the round red makes a soggy baked potato and a floppy, squishy fry.

Round White: This is a potato of firm texture and smooth skin. In the restaurant industry, it's called a "chef potato." Its skin color is very light buff and its flesh is cream-colored. The round white is also a boiling potato, as its cell walls are sticky, it contains a lot of moisture and its starch granules are small and fewer in number than the russet. The round white makes the ideal mashed potatoes, not as gummy as those made with the round red and not as granular as when made with the russet or the long white.

When any potato, regardless of variety, is picked immature and sent to market immediately, it is called a "new potato." Generally, the new potato has a higher sucrose content and lower starch content than mature potatoes. The higher sucrose (also known as cane, beet or table sugar) content slows browning during frying; hence, a new potato makes the fries of lightest color. Whatever the variety, a new potato also has a higher moisture content and smaller starch granules. It therefore produces a moister french fry and baked potato.