Bromated flour contains potassium bromate, an oxidizing agent widely used in commercial baking to strengthen and improve dough and promote rising. Some brands of flour sold in supermarkets for home use contain potassium bromate. Concerns about bromated flour stem from studies dating back to 1982 that found that the chemical causes several types of cancer in lab rats. For that reason, potassium bromate is considered "possibly carcinogenic to humans" by the International Agency for Research on Cancer, and bromated flour is banned by a number of governments including the European Union, Canada, Brazil, Peru and China.
In the U.S., the FDA has encouraged bakers to voluntarily stop using bromated flour, but most of them haven't. The agency continues to permit the use of the flour as long as potassium bromate doesn't exceed .0075 parts for each 100 parts of the weight of the flour, or 750 parts per million. The only other regulation concerning the use of bromated flour is a California law that requires a store warning on any bakery goods made with it if the products contain more than a specified level of the chemical. As a result, most California bakers have switched to bromate-free flours.
Potassium bromate is supposed to cook out of bread as the dough bakes, thus removing any potential health threat. Some of it may not, however, and any amount that you ingest could prove harmful. For this reason, some manufacturers have substituted ascorbic acid, which provides the baking benefits of potassium bromate without the potential risks.
Many national bakeries including Pepperidge Farm, Pillsbury and Best Foods, Inc. (brands include Arnold and Entenmann's) switched to non-bromated flour more than a decade ago, as did some large supermarket chains that make their own baked goods. When you buy commercial bread or baked goods, look on the ingredient lists for "potassium bromate" or "bromated flour." Avoid products that contain them.
Some major brands of flour, such as Gold Medal, include both bromated and unbromated products. (These are listed on the General Mills website.) On its website, Whole Foods lists bromated flour and potassium bromate as "unacceptable ingredients for food" and sells no foods containing them. I've read that some fast food rolls and buns are likely to be made with bromated flour, and reportedly, it is also widely used in pizza dough. (Some national chains such as Domino's and Pizza Hut don't use it.)
Andrew Weil, M.D.