If aluminum in baking powder sometimes makes baked goods taste "tinny", why do they put it in, since they can make it without (Runford, Red mill)? Am I going to be missing something when I use Rumford? (as David Lebovitz recommends)
The lack of aluminum shouldn't make a big difference in your product
It's not aluminum per se that can taste bitter but the compound sodium aluminum sulfate that may have a slight aftertaste. It's included in some brands of baking powder to add extra lift during the baking stage. I haven't done any side by side tests myself (yet) but Rose Beranbaum says she substitutes the two types without significant difference in the finished product.
Both are "double acting" baking powders, which have that 2nd stage, "extra lift" -- I've always used Rumfords or comparable.
Baking powders with aluminum compounds exist because (1) they work and because (2) there are plenty of people who don't notice or complain about the negative effects on taste. Aluminum-free products also work, so no, you won't be missing anything by opting for them.
Maybe I should have written "extra extra" lift during the baking phase.
I've used Rumford's exclusively for more than 20 years, and have no problem with everything rising as it should.