The label on the bag of Pillsbu\ry's bleached all-purpose flour claims that "bleaching improves baking performance"; at the same time, the label also says that all-purpose bleached and unbleached flour can be used interchangeably.

Interchangeably? Shirley Corriher, an Atlanta cooking teacher and biochemist, begs to differ. She maintains that since Pillsbury's unbleached is slightly higher in protein, it therefore is not as good at making tender cookies and pie crusts. (The company acknowledges this protein difference, but the nutrition label does not reflect this; see Page E5 for an explanation.)

In fact, she says an experiment she devised proves that even a small difference in protein can make a big difference in your dough: Place 2 cups of unbleached flour in a food processor and add 1 cup of water. Process for 10 seconds. Then do it again with the bleached flour. The flour higher in protein (and, thus, with stronger gluten strands) should form a sturdier, stickier mass than the smoother, low\er-protein flour, which will be more delicate and relaxed.

We tried it. And we agree. And we who didn't think there was any difference, and who bought unbleached flour because it seemed the thing to do, are about to change our ways. When it comes to delicate baked goods, bleached is the way to go.

Flour, the workhorse of baked goods, gets taken for granted. After all, with Valrhona chocolate and real vanilla in the world, who can get excited over an ingredient so dull and common?

Or is it? For as Shirley Corriher's disagreement with the advice on the Pillsbury package shows, it is also supremely misunderstood. That's because flour is far more complex than that sack of white fluff lets on. And knowing what's what this holiday baking season could make the difference between people saying "That's good" and "Wow! Gimme the recipe!" (And -- wouldn't you know it? -- the Pillsbury test kitchens wouldn't dream of using that "interchangeable" unbleached flour for pie crusts and the like.)

For starters, the biggest misconception about flour -- that it's simply a commodity, that flour is flour. As Brinna Sands, one of the owners of King Arthur Flour in Norwich, Vt., said, "It's kind of like wine. It has a lot of characteristics that change from year to year."

Those characteristics are largely influenced by weather and growing conditions, but there are many other factors that make flour not as monolithic as it seems. It Starts With the Wheat

There are six different classes of wheat, distinguished between winter and spring (depending on the growing season) and hard and soft (depending on the protein content). The largest class is Hard Red Winter, grown primarily in the Great Plains states of Kansas, Nebraska and Oklahoma, according to Jim Bair, vice president of the Millers' National Federation, a trade association of the nation's millers. There's the relatively new Hard White, also grown in the Great Plains, which makes a milder flour, and Hard Red Spring, grow in the Northern Plains. Moving onto the soft wheats, there's Soft White, grown in the Pacific Northwest and Michigan, and Soft Red Winter, grown east of the Mississippi. And finally, there's durum wheat, grown almost exclusively in North Dakota.

So why should you care?

Because it's a big deal. The hardness of the wheat determines the baking qualities of the flour produced from it -- the harder the wheat, the more protein the flour will contain.

And why should you care about protein?

Protein is the largest single distinguishing feature of flour. When you mix flour with liquid -- as in Corriher's experiment or in everyday use -- it's the proteins that form rubber band-like strands of gluten, the stuff that makes dough elastic and traps gas from yeast, allowing the dough to rise. The more protein in a flour, the stronger the gluten and the sturdier the dough.

This means that hard wheats, with more protein, produce flour that is best suited for baking breads. Bread flour, for example, made from hard, high-protein wheats, has a protein content of between 12 and 14 percent, according to the Wheat Foods Council. Soft, low-protein wheats, which produce flour with weaker gluten strands, are better bets for cakes, cookies and pastries, where lightness and tenderness are key. Cake flour, for example, milled from soft, low-protein wheat, has a protein content of between 7 and 9 percent. All-purpose flour is usually a blend of hard and soft wheats, and thus its protein content is somewhere in between, varying from 8 to 11 percent. More Than Just Plain Wheat

What's malted barley flour, anyway? Malted barley flour oftens appears as the second ingredient on flour labels -- primarily those used for baking bread and other yeast doughs. But even though it's second in weight, the ingredient is actually only a tiny percentage of the flour. The Food and Drug Administration says that the amount of malted barley flour may not exceed 0.75 percent of the weight of the flour.

And why is it there in the first place? As "food for the yeast," says Judi Adams, president of the Wheat Foods Council put it. That's because in order for yeast to grow, it needs a starch that has been turned into a simple sugar, adds Sands of King Arthur Flour. So when yeast is added to flour, it gets a jump start from the malted barley flour.

Although it is not a requirement, many flours are also "enriched" with three B vitamins (niacin, riboflavin and thiamine) and iron. Enrichment of flour began during World War II, according to the FDA. The purpose? To deal with pellagra, a niacin deficiency disease that flourished among low-income people, particularly those in the South. Pellagra is no longer a problem, but enrichment has continued.

In fact, enrichment does replace some of the nutrients lost during the milling of all-purpose flour. It is at that point that the endosperm, or center of the kernel, is separated from the wheat germ and wheat bran -- two sources of some of those nutrients, according to General Mills, which makes Gold Medal flour.

You may also have heard of bromated flour, although this practice is largely being phased out among millers in the United States. Bromates were traditionally added to bread flours to strengthen the dough, but questions about possible carcinogenicity spurred the industry to back off, says Bair of the millers' group. Ascorbic acid and natural enzymes have replaced them. As for bromates, they're still legal, although the FDA is currently reviewing their safety. Bleached Versus Unbleached

This is one of the enduring questions in the modern world of flour, and the one that elicits the most differences of opinion.

First, there's disagreement as to why flour is bleached at all. Bair says that its primary purpose is not to make flour whiter, but to age it and improve its baking qualities. Flip to Marlene Johnson, spokesperson for Pillsbury, who says that flour is bleached a bright white because of consumer preferences established years ago. "It came out of the premise that only the wealthy could eat white bread," she said.

Whatever the reason, bleaching does make flour whiter; in doing so, it's simply fast-forwarding nature. That's because right after flour is milled, it's a yellowish color. But while oxidation turns a pear or apple brown, air turns the carotenoid pigments in flour white, explains Nick Malgieri, author of "How to Bake" (HarperCollins, $35). And like a stew that improves with age, flour functions better with some maturity under its belt.

Of course, 100 years ago, by the time flour was packed into barrels, shipped by railroad and sat around in a general store for a while, the flour was plenty grown up. Even nowadays, by the time unbleached flour gets from a miller to a warehouse to a supermarket to your house, sufficient aging has taken place.

If so, why resort to chemicals to bleach flour? Sands of King Arthur, which sells only unbleached flour, just doesn't like the idea. At the same time, Shirley Corriher, the Atlanta biochemist, believes that the notion that unbleached flour is somehow better for you is "a batch of hooey."

In fact, the nutritional value of flour is not changed during bleaching, according to the Wheat Foods Council. Corriher agrees, adding that trace amounts of Vitamin E may be lost, but flour was never a very good source of it anyway.

And then there's the question of performance -- the bleached flour versus the unbleached. Corriher is not the only food professional who differs with the Pillsbury-bag claim. Many baking experts agree that there are substantial differences in performance, as do we (see box at right for results of a Food section baking test).

According to Jim Vetter, vice president of technical research at the American Institute of Baking, bleaching agents help the flour hold onto the shortening and sugar in cakes, resulting in a better crumb. They also modify the protein in flour, resulting in more elastic gluten. The net result is flour with a silkier, finer grain, says Vetter. The chemical and physical reasons for all this are not well understood, he added.

This is why professional bakers such as Malgieri recommend bleached flour for soft cookies and cakes. But Malgieri says he sticks to unbleached flour for "any kind of bread or yeast product." And of course, like other bakers, Malgieri also does a lot of mixing and matching of flours in a single recipe; he'll use a combination of bleached all-purpose flour and bleached cake flour for biscuits, for example. Who's Still Milling Around

It used to be that practically every town had a mill. Even as recently as 1960, there were 2,000 mills in the U.S. Now there are about 200, according to Mike Everett, vice president and chief operating officer of Wilkins-Rogers, the Ellicott City, Md., mill that started in Georgetown in 1913. The company, which makes Washington brand flour, Indian Head cornmeal and muffin and cake mixes, is one of that dwindling number of regional mills.

Like many other industries, the milling business has become extremely concentrated. Only two companies, Pillsbury and General Mills, sell supermarket flour nationally, and together, they claim more than 50 percent of the retail market, according to Bair. (Another quarter of the market goes to regional millers; the rest is private-label supermarket brands, Bair says.)

In fact, General Mills is the only national retail miller left in the country. Pillsbury, which got its start milling flour in 1869, bailed out in 1992. It sold its mills to Archer Daniel Midlands, which joins giants Cargill and ConAgra as the nation's largest milling companies. But you won't see their names on food packages; they sell to commercial bakers and also produce store brands under private labels.

Is there any difference among the flours these companies produce? Yes, and it depends on what and who they're milling it for.

Commercial bakeries, for one, have widely varying specifications. Even when it comes to crackers, the flour used to make "a Triscuit is different from a Wheat Thin is different from a Saltine," says Bair.

As for supermarket flour, regional and national brands may vary significantly. The biggest difference: Again it's those protein levels. And even a little variation can make a big difference, say baking experts.

Which is why, at the margins of the market, away from the great "interchangeable" mass, there is King Arthur's unbleached all-purpose flour, made from Hard Red Winter wheat and high in protein, best used in sturdy yeast doughs. And, in striking contrast, there is White Lily all-purpose bleached flour, made from soft, low-protein wheat, and applauded by fans for its flaky, tender biscuits. Label It Frustrating

If different flours have different protein levels, why do most flour-bag nutrition labels say the same thing, 3 grams of protein per 1/4 cup? Shirley Corriher, an Atlanta cooking teacher and biochemist, knows why, and she's frustrated by it. She can't tell whether the flour she's buying is high in protein, and thus better for baking bread, or low in protein, better for making pie crusts and quick breads.

The nutrition label used to assess 1 cup of flour. If the protein content on the label said 9 grams, it was a low in protein, according to Corriher. If the protein was 14 grams per cup, it was high-protein.

But when the Food and Drug Administration's labeling law went into effect last year, the official serving size of flour dropped from 1 cup to 1/4 cup. And now it's impossible to tell how high in protein a particular flour is. Here's why: The 3 grams in a 1/4 cup of flour could range from about 2.5 to about 4 grams (a combination of rounding up or down plus FDA permission for food manufacturers to under-declare protein content), which means that a whole cup could have anywhere from 10 to 16 grams of protein. In other words, the flour could be high or low in protein and the label will say the same thing.

Good, possibly, if the goal is realistic portion sizes. Not very helpful if you want to know how to use a particular flour.