Pottasche, also written as potash or pearlash, potassium carbonate, salts of tartar, and carbonate of potash, sometimes appears in the ingredients lists of old-fashioned German baking recipes, especially classic gingerbread (lebkuchen) recipes. You may see it in conjunction with hartshorn or baker's ammonia (ammonium carbonate) in other cookie recipes such as springerle that benefit from a light but notably crisp texture.
Early Uses of Potash and Pearlash
Potash starts with lye, produced by passing water through hardwood ashes. Evaporating the lye water leaves behind the solid potash. Commonly in use in America during the 17th and 18th centuries, potash added a distinct ash flavor to baked goods. Pearlash, the purified version of potash, eliminated some of that undesirable smokiness.
Early bakers thought pearlash might replace yeast as a leavener, but because of its bitter aftertaste, it not only did not replace yeast but was eventually replaced by baking soda. Additionally, pearlash did not work well in batters containing a high ratio of fat, producing a soapy flavor not entirely unexpected since potash and fat form the basis for homemade soap.
Functions of Potash and Pearlash
An alkaline salt, pearlash or potassium carbonate (K2CO3) reacts with water or an acid such as sour milk, fruit juice, or molasses to create carbon dioxide, which gives baked goods lift.
Baking soda (sodium bicarbonate) all but replaces it in modern recipes for traditional German baked goods. Use 1/2 teaspoon of baking soda for every teaspoon of pearlash called for in a recipe. Note, however, that the taste of the final product may differ from the original recipe.
Historian David Walbert explains that pearlash decomposes to potassium and carbonate ions, which become carbonic acid and bubble out as carbon dioxide.
The potassium from the pearlash binds with the hydroxide ions from the liquid ingredient, producing potassium hydroxide. To prevent a soapy or bitter taste from the potassium hydroxide, you must add an acid to neutralize it, though you can harness the leavening power of the pearlash without acid.
Modern double-acting baking powder combines baking soda with acid for use in quick breads and other baking recipes that do not include an acidic ingredient such as buttermilk, honey, or yogurt. German baking powder, a single-acting leavener, does not work interchangeably with the double-acting variety more commonly found in the United States.