Dawn Miller, age 13, of Barre., Vt., for her question:

HOW IS WHITE FLOUR MILLED?

Flour is powdery food made by grinding wheat and other grain. White flour made from wheat accounts for more than 90 percent of the flour produced in the United States.

Wheat kernels form the raw material used for flour. They consist of a tough covering called the brain, a mellow inner part called the endosperm and a tiny new wheat plant called the germ.

To make white flour, millers separate the endosperm from the bran and germ and grind the endosperm into flour.

Various cleaning machines first remove dirt, straw and other impurities from the grain. Next, the wheat is tempered or moistened. The moisture makes the endosperm more mellow and the bran tougher.

The tempered wheat passes between a series of rough steel rollers that crush the endosperm into chunks. Pieces of bran and germ cling to the chunks of endosperm or form separate flakes. Then the crushed grain is sifted. The tiniest bits of endosperm, which have become flour, pass through the sifter into a bin. Larger particles collect in the sifter.

Next, these larger particles are put into a machine called a purifier. There, currents of air blow flakes of bran away from the endosperm particles. The endosperm particles are then repeatedly ground between smooth rollers, sifted and purified until they form flour.

In most mills, about 72 percent of the wheat eventually becomes flour. The rest is sold chiefly as livestock feed.

Newly milled flour is cream colored, but some mills bleach it to make it white. They also may add chemicals that strengthen the gluten. Most millers also enrich their product by adding iron and vitamine to white flour made for home use. Most U.S. bakers use enriched flour, or they add vitamins and minerals to dough made with unenriched white flour.

Wheat is rich in starch, protein, B vitamins and such minerals as iron and phosphorus. But the vitamins and some of the minerals are chiefly in the bran and germ, which milling removes from white flour. For this reason, often millers enrich their product by adding iron and vitamins to white flour.

The enriching of white flour has probably helped millions of people avoid malnutrition. Diseases caused by a lack of B vitamins were common in the United States before 1941. That year, the nation's bakers and millers began enriching white flour products. Today, few Americans suffer those diseases.

There are three main types of white wheat flour: bread flour, cake flour and ail purpose flour. Bread flour is milled chiefly for commercial bakeries. Cake flour is made for both commercial and home baking. All purpose flour is used mainly at home.

Bread flour contains at least 11 percent protein and cake flour contains less than 8 1/2 percent protein. Ail purpose flour, which is a blend of bread flour and cake flour, has a protein content of about 10.5 percent.