By Harold McGee
A new method of leavening made its first appearance in the first American cookbook, Amelia Simmons’s 1796 American Cookery. Four recipes, two for cookies and two for gingerbread, call for the use of “pearlash,” a refined version of potash, which was made by soaking the ash produced when plant materials are burned, draining off the liquid, and drying it down to concentrate the substances dissolved in it. Pearlash is mostly alkaline potassium carbonate, which reacts with acid ingredients in doughs to generate carbon dioxide gas. It was the precursor to baking soda and baking powders, which arrived between 1830 and 1850. These chemical ingredients made it possible to leaven instantly mixtures that living, slow-growing yeasts couldn’t very well: such things as fluid cake batters and sweet cookie doughs. Purified commercial yeast cultures for loaf breads, more predictable and less acidic than brewer’s yeast, became available from specialist manufacturers around the turn of the 20th century.