packet / 2¼ tsp yeast

Baker's yeast (Saccharomyces cerevisiae) is a single-celled fungus used as a leavening agent in bread and pastry production. The organism ferments sugars present in dough, producing carbon dioxide gas and ethanol, which causes dough to rise. Commercial baker's yeast is cultivated from selected strains and sold in three primary forms: fresh (compressed cake), active dry (dormant cells with dead cells removed), and instant (smaller particles with faster hydration). Each form contains living microorganisms that remain viable during storage, with potency gradually declining over time.

A standard packet of active dry yeast typically contains 2¼ teaspoons (approximately 7 grams), the customary amount required to leaven one pound of dough. The yeast's fermentation activity depends on temperature, sugar availability, salt concentration, and pH levels within the dough matrix.

Baker's yeast is essential in bread baking, from rustic loaves to sandwich breads, enriched doughs (brioche, babka), and sweet pastries. It is used in both direct methods (mixing yeast directly with flour) and indirect methods (preferments like poolish or biga that develop flavor complexity). The fermentation process not only leavens the dough but also develops gluten structure, creates flavor compounds, and improves digestibility through enzymatic activity. Yeast is also employed in beer and wine production, though those applications use different strains and fermentation conditions than baking yeast.