leaven

Leaven is a substance used to make dough or batter rise by producing gas, usually carbon dioxide, during fermentation or chemical reaction. The most common leavening methods in baking are biological leavening with yeast or wild yeasts in a sourdough culture, and chemical leavening with agents such as baking soda or baking powder. In traditional breadmaking, yeast metabolizes sugars to release carbon dioxide and alcohol. The gas inflates the dough and, with gluten networks, creates a light, porous crumb. Sourdough leavening relies on a mixed culture of yeast and lactic acid bacteria, which adds acidity and a distinctive flavor.

Chemical leavening provides a faster rise: baking soda requires an acid to activate, and baking powder contains both acid and base. When heated, these reactions release carbon dioxide quickly, enabling quick breads, muffins, and cakes. Steam and air can also contribute to lift, especially in high-moisture doughs and laminated pastries.

Process: Yeast doughs are mixed, allowed to ferment (proof) to expand volume, then shaped and allowed to rise again before baking. Sourdough requires longer fermentation and ongoing feeding of the starter. Chemical-leavened batters are mixed and baked promptly to capture the gas before it dissipates.

Storage: Dried yeast keeps for months; fresh yeast has shorter shelf life. A sourdough starter is refreshed regularly with flour and water. Leavened doughs should be baked after suitable proofing to avoid over-proofing.