Bleaching gets a very bad rap these days. It’s frequently portrayed as a trivial cosmetic process that comes at the steep cost of adding chemicals — chemicals! — to our food. More than that it could be racist. But in fact bleaching is not primarily about a flour’s whiteness, it’s about a flour’s performance.
But first what exactly is “bleaching”? In general, bleaching means exposing flour to a compound like chlorine gas, ascorbic acid (vitamin C) or perhaps an enzyme like lipoxygenase (derived from fava or soy beans). These agents leave no residues or residual flavors, nor do they, contrary to popular myth, diminish the flour’s nutritional value.
So what do bleaching agents do? Different agents perform differently, but in general they do three things. First they react with the bonds at the ends of the string-like gluten molecules, making them more inclined to bond with one another and form stretchy gluten networks. Second, they make the starch in the flour more susceptible to gelation. Which is to say, they increase the tendency of individual starch molecules to break off from starch granules (bits of ground wheat endosperm) when they’re exposed to moisture and heat.
All that translates into three things. First, a higher rise generally, and that’s handy for a something like cake batter, which is heavily weighed down with fat and sugar. Second, the increased gelation means a pastry maker can add more liquid and/or sugar to a cake batter without the risk of a fall. Lastly, the increased/quicker gelation keeps doughs and batters from spreading out too much in the pan.
Bleached flour isn’t good for everything of course, bread being a great example. There you don’t necessarily need the extra structure that bleaching delivers. Also you want as much flavor as you can get, and bleaching, truth be told, does tend to mute the wheaty taste of many kinds of flour.
So, different flours for different uses. I keep a supply of both bleached and unbleached flour around and so should you. But as a general rule of thumb whenever you’re making pie dough, American biscuits, pancakes and especially layer cake: reach for the bleached.