Making My Own Baking Powder

We got Knuckles' allergy test results back in the mail, and discovered that they neglected to tell us that he's also allergic to corn.

Corn is one of the most difficult allergies to accommodate, since corn is in even more foods than wheat. Just about every commercial candy is sweetened with corn syrup. Anything with maltodextrin in it is out, since maltodextrin still contains corn proteins. Even baking powder has cornstarch in it.

I spent the entire weekend searching on the 'net and found three baking powders made without cornstarch. Two are single-acting baking powder and one is a double-acting baking powder.

For those of you for whom baking powder is a mysterious concoction, let me explain. Baking powder makes bubbles by mixing an acid with a base, just like your grade-school volcano project. Most baking powders use sodium bicarbonate (baking soda) as the base and cream of tartar as an acid. Single acting baking powders use only one acid, usually cream of tartar, which reacts in the mixing bowl. Double acting baking powders use two acids, one which reacts in the mixing bowl and the other which reacts at a higher temperature in the oven, giving the baking powder more leavening oomph. Cornstarch serves as a filler and helps separate the acid and the base and keep them from reacting before they hit the mixing bowl.

It is insanely easy to make your own single acting baking powder and there are plenty of recipes for it out there on the internet. A typical recipe will be two parts cream of tartar, one part baking soda, and one part cornstarch. So it would be very straightforward to make my own single acting baking powder substituting, say, arrowroot for the cornstarch.

The problem comes when I want to make a double acting baking powder. Most double acting powders use sodium aluminum sulfate as the heat-activated acid. I don't know where to get sodium aluminum sulfate. Not only that, but I don't know what proportions to use. I can get somewhat of a guide from this page (which also explains why the proportion of cream of tartar to baking soda is 2:1) but I don't know if sodium aluminum sulfate is readily available. I was thinking of using dicalcium phosphate instead, especially since it appears to be cheap and available as a dietary supplement for pets. But again, what proportions?

Any chemists or chemical engineers out there who could help me figure it out?