Bleaching gets a very bad rap in many baking circles (I’m thinking specifically of artisan bread bakers), though it can be a very important thing when it comes to pastry making. Why? Is it so important that our flour be perfectly, pristinely white? And isn’t that racist? The truth of the matter is that bleaching is only incidentally about color. It’s mostly about batter/dough characteristics and rise.
But first what exactly is “bleaching”? In general, bleaching means exposing flour to chlorine gas via some sort of aeration mechanism. The gas bubbles though the flour leaving no residues or residual flavors. Nor does it, contrary to popular myth, diminish the flour’s nutritional value. What it does do is partially denature the protein (gluten) in the flour. This has the effect of making the batter and/or dough the flour is made from somewhat less extensible and springy, and therefore more tender.
However the chief effect that chlorine has on flour is that it renders the starch more susceptible to gelation. Which is to say it increases the tendency of individual starch molecules to break off from starch granules (i.e. bits of ground wheat endosperm) when they’re exposed to moisture and heat. These individual molecules go on to bond with one another (loosely) to form the various structures that allow a baked good to rise.
All that translates into three things. First, it means a higher rise generally, which is great for things like cakes and éclair shells. Second, the increased gelation means a pastry maker can add more liquid and/or sugar to a given recipe without having the pastry fall (more liquid and/or sugar mean increased tenderness and/or sweetness, both good things for the pastry arts). Lastly, the increased/quicker gelation keeps doughs like, say, biscuit doughs, from spreading out on the pan, which means a taller product and prouder Southern grannies.
Bleached flour isn’t good for everything, of course, bread being a great example. here you generally want as much extensibility and elasticity in your dough as possible (it creates big holes). You also 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 pastry or cake: reach for the bleached variety.