Some old cookie recipes call for something called baker's ammonia. What is it?
Baker’s ammonia, also known as ammonium bicarbonate (and often sold as ammonium carbonate), was the primary leavening agent used by bakers before the advent of baking soda and baking powder in the 19th century. In fact, certain recipes for European and Middle Eastern cookies and crackers still call for it today. When we purchased the powder from a mail-order source (it can also be found at some Greek and Middle Eastern markets), we quickly discovered its biggest drawback: an extremely potent smell. (In fact, it turns out baker’s ammonia is the stuff that was passed under Victorian ladies’ noses to revive them when they swooned.) Because of its noxious scent, it is used to leaven only low-moisture baked goods like crisp cookies and crackers that thoroughly dry out during baking, lest the ammonia linger.
When we tried trading baker’s ammonia for baking powder in a recipe for crisp sugar cookies, we found that not only can the two products be used interchangeably, but the baker’s ammonia produced a lighter, crunchier crumb. This is because when its tiny crystals decompose in the heat of the oven, they leave minuscule air cells in their wake from which moisture easily escapes. Furthermore, this leavener leaves none of the soapy-tasting residue of baking powder or baking soda. It works so well, we’d be tempted to use it for crisp baked goods all the time if it were more readily available.