Wheat flour is naturally slightly yellowish because it contains carotenoid pigments, natural yellow and orange compounds found in many fruits, vegetables, and grains.
Carrots’ famous orange color, carotene, is the mother of them all. But most people are less color tolerant than you and don’t like their flour to be yellow. The major exception is the semolina flour used in making pasta.
Although it contains even more carotenoid pigment than other wheat flours, it isn’t usually bleached. Pasta that is slightly yellow, with its implication of eggy richness, is more appealing than if it were dead white.
If given half a chance, though, flour bleaches itself. That is, as it ages in contact with air, the pigments are oxidized and transformed into colorless compounds. But aging requires storage time, and time is money. That’s why “unbleached”, meaning naturally self-bleached during storage, flour costs more.
Flour millers can simulate the effects of aging by adding an oxidizing agent such as potassium bromate (in which case the flour is said to be bromated), or chlorine dioxide, or benzoyl (BEN-zo -eel) peroxide.
The bleaching of flour isn’t mere cosmetics.
Flour that has been matured, either by natural aging or by being treated with oxidizing agents, makes doughs that produce finer-grained, higher-volume bread and a dough that bakers report as being more elastic during kneading. That’s because oxidation not only removes the yellow color of flour but removes certain sulfur-containing chemicals (thiols) that interfere with the formation of gluten, the sticky, elastic protein in dough that traps gas bubbles and gives bread its light texture.
Some people are concerned about the intimidating names and properties of the bleaching chemicals. But after doing their jobs they are gone, having been chemically transformed into harmless substances. Chlorine dioxide is a gas that dissipates, so there is none of it left in the flour. And any excess of benzoyl peroxide would decompose in the heat of an oven.
After reacting with the carotenoids and thiols in the flour, the 50 or 75 parts per million of added potassium bromate turn into potassium bromide, a perfectly harmless salt. For most of the eighty-plus years of bakers’ use of potassium bromate, no one was able to detect any residual excess bromate in baked goods.
However, chemists today have such sensitive analytical methods, down to billionths of a gram in many cases, that extremely low levels of residual bromate can be detected in baked goods made with bromated flour. Analytical detection instruments are so sensitive these days that they can find traces of almost any given chemical in almost anything, a fact that much of the public doesn’t understand. A finding that a certain food “contains toxic XYZ” often generates unfounded fears. But everything is toxic in large enough amounts and harmless in small enough amounts.
Nevertheless, because high levels of bromate have been found to cause cancers in rats, many consumers fear it, and the American baking industry, in consultation with the FDA, has voluntarily stopped using it. Canada and the United Kingdom have banned the chemical altogether.
Incidentally, the claim that the bleaching of flour destroys its vitamin E is true but empty, because wheat flour contains negligible amounts of vitamin E to begin with.
About self-rising flour:
Self-rising flour is flour that has some baking powder and salt already mixed into it. Most of the bigger supermarkets carry it. But to make it yourself, just add about teaspoons of baking powder and 1/2 teaspoon of salt to each cup of all-purpose flour or cake flour and mix well. A gently wielded whisk does the best mixing job.