Q. My kitchen is populated by a variety of flours. An unbleached bread flour is one. Can it be substituted in recipes calling for all-purpose? Wlhat are their culinary and nutritional differences? Would a bleached bread flour behave differently from an unbleached bread flour?

A. "Bread" and "all-purpose" describe flours intended for sale in supermarkets. There are no federal regulations mandating how these names relate to flour behavior.

In order to protect their product's market position, however, most millers hold to fairly consistant standards. The most important measurement of flours is protein content, as protein determines flour's behavior when mixed with water. Bread flour, the highest in protein, contains 12 to 13 percent and all-purpose 10 to 11 percent. Wheat protein, when mixed with water, forms the elastic matrix called gluten. It is crucial to the texture and volume of breads, pastries and cakes.

Substitution of one flour for another will never result in exact duplication in a recipe. But then, there are so many other sources of variability that, if one at least accounts for differences in protein content, one should obtain similar results.

To obtain the desired results after substituting bread for all-purpose flour, keep the following in mind:

* When bread flour is substituted for all-purpose in bread doughs, use 1 tablespoon less per cup. Once the dough is mixed, knead it 3 to 5 minutes longer, as bread flour protein takes longer to develop gluten.

* Never use all bread flour in a pie dough. Instead, substitute 1/2 cup of bread flour and 1/2 cup of cake flour for every cup of all-purpose. Cake flour is about 6 percent protein so the half-half mix produces the tender crust associated with less protein but a cohesive, smooth dough associated with more protein.

* Never use all bread flour in biscuits. It produces a desirably smooth, light texture but undesirable rubberiness. Again, a half-bread, half-cake flour mix is quite satisfactory.

* Never use any bread flour in cookies. Pastry flour is best; some supermarkets now carry it, as do most health-food stores.

* Never use any bread flour (or all-purpose) in egg-foam cakes (angel food, genoise, sponge). Wheat protein withdraws moisture from the delicate egg foams, causing collapse of bubbles and formation of gluey strands. Pastry or cake flours are preferable.

* Bread flour can be substituted for all-purpose in shortened cakes (yellow, white, poundcake or chocolate) provided you mix an equal amount with cake flour.

* Bread flour does a fine job in popovers and cream puff batters. For every cup of all-purpose flour, substitute 1/3 cup bread and 2/3 cup cake flours.

Because of the variability among flours' protein contents, one can only make generalizations regarding their protein value. Wheat protein is not nearly as complete in its amino acid content and balance as dairy and egg proteins. But bread flour's higher protein content makes it slightly more valuable nutritionally than all-purpose. Assuming both the unbleached bread and the all-purpose flours were enriched (addition of riboflavin, niacin, thiamin and iron to levels higher than those of whole wheat), all-purpose and bread flours are equal in B-vitamin and iron content. The established federal standards are identical to all enriched flours, whatever the name or use.

And, to your final question: assuming that the unbleached flour has been properly aged, a bleached flour would offer no advantages. If you find that a bleached flour produces bread of more flavor or volume, it's not because of the bleaching. It's because of the quality of the wheat from which it was made. And conversely, one cannot generalize that unbleached bread flour is superior in baking quality to its bleached peers.