Leavening Agent - Biological Leaveners

Biological Leaveners

Microorganisms that release carbon dioxide as part of their life cycle can be used to leaven products. Varieties of yeast are most often used, particularly Saccharomyces species (such as baker's yeast, Saccharomyces cerevisiae), though some recipes also rely on certain bacteria. Yeast leaves behind waste byproducts (particularly ethanol and some autolysis products) that contribute to the distinctive flavor of yeast breads. In the naturally leavened sourdough breads, the flavor is further enhanced by various lactic acid bacteria (Lactobacilli) or acetic acid bacteria (Acetobacter).

Leavening with yeast is a process based on fermentation, biologically changing the chemistry of the dough or batter as the yeast works. Unlike chemical leavening, which usually activates as soon as the water combines the acid and base chemicals, yeast leavening requires proofing, which allows the yeast time to reproduce and consume carbohydrates in the flour. Yeast can unhydrate itself and then rehydrate itself later.

Yeast can also be used to make alcoholic beverages like beer or wine. The resulting cast-off yeast, known as barm, can be used as a leavener and was probably ancestral to the use of modern pure-cultured yeast. Non-European cultures have used other by-products of making alcoholic beverages as leaveners, as in Ecuador: "In olden times when the sediment of chicha called concho was used as a ferment, we had good bread; and now with better mills good quality bread has disappeared entirely."

While not as widely known, bacterial fermentation is sometimes used, occasionally providing a drastically changed flavor profile from a yeast fermentation; a well-known example is salt rising bread, which uses a culture of the Clostridium perfringens bacterium.

Some typical biological leaveners are:

• beer (unpasteurised—live yeast)
• buttermilk
• ginger beer
• kefir
• sourdough starter
• yeast
• Whey protein concentrate
• yogurt