Once I thought: not much. And even until very recently I made my biscuits by blending canned baking powder with White Lily all purpose flour, which is the same stuff just without the pre-blended pop. My assumption has always been that it’s safer to use canned leavening as you never know how long your bag of self-rising flour has been on the shelf (after several months the baking powder starts to lose its potency).
That turned out to be Yankee thinking. The few times I bought White Lily in Chicago I was disappointed. But when you’re that far north of Mason-Dixon line what can you expect? Here in Louisville you don’t have that problem. The stuff moves off the shelves at a very brisk clip so you’re practically guaranteed a fresh bag. The one exception to the rule is the little 2-pound bags, which I made the mistake of buying just after I arrived here. No real Southerner would ever one of those. I deserved what I got.
Does it make a difference? Yes it does. Or at least it does in my biscuits. The reason is the blending. A flour mill can blend baking powder into flour more evenly than a home baker every could. And as I’ve written before back when I was talking about high-ratio cake, a strong, even rise depends on even distribution of ingredients. So if your recipe calls for self-rising flour, I recommend using it provided you can find it fresh. It’s not just hype, it really does work.