What is the difference between unbleached and bleached flour?
Apparently flour needs to be aged, just like fine wine. The bleaching is part of the aging process. Since a food scientist can explain this better than I can, let me quote Harold McGee from his book On Food And Cooking (Charles Scribner’s Sons, 1984).
“After the flour has been ground and blended to the desired mix of particles, it is treated chemically to accomplish in a matter of minutes what otherwise takes weeks. Bleaching removes the light yellow color caused by xanthophylls, a variety of carotenoid pigment also found in potatoes and onions. The color has no practical or nutritional significance and is oxidized simply to obtain a uniform whiteness. Bleaching does, however, destroy the small amounts of vitamin E in flour, which probably accounts for its bad reputation in some circles.”
“Bleaching is often accomplished with the same gas, chlorine dioxide, that is used to age, or ‘improve,’ the flour. But even unbleached flour has been aged with potassium bromate or iodate. Aging has important practical results. It has long been known that flour allowed to sit for one to two months develops better baking qualities; hence the practice of letting flour ‘age’ before use (during this period, it is also naturally bleached by oxygen in the air). But, done in this way, aging is a time- and space-consuming, somewhat unpredictable procedure. Hence the use of chemicals both to accelerate and to control flour improvement. Aging affects the bonding characteristics of the gluten proteins in such a way that they form stronger, more elastic doughs.”
Since we are not making many white breads these days, unbleached flour is fine for baking and cooking purposes. When it comes to cake flour, bleached is better for sweeter, more tender cakes.