Yeast

What yeast does

Baker’s yeast is Saccharomyces cerevisiae — a microscopic fungus that consumes sugar and produces:

• Carbon dioxide — the gas that lifts bread.
• Ethanol — driven off in baking, retained in beer and wine.
• Flavor compounds — esters, acids, alcohols that develop during fermentation.

The same species is used for bread, beer, and wine, with strain differences fine-tuned for each application.

Three commercial bread yeast forms

• Active dry yeast — granules. Must be rehydrated in warm liquid before use. Long shelf life.
• Instant (rapid-rise) yeast — finer particles, bred to skip the rehydration step. Mix directly into dry ingredients.
• Fresh yeast (cake yeast) — moist blocks. Most active, traditional in European bakeries. Short shelf life.

The three forms give roughly the same end result; instant yeast is the most convenient for home baking.

Wild and sourdough

Sourdough isn’t made with packaged baker’s yeast. It’s leavened by wild yeasts and bacteria that occur naturally on grain and in the air, captured in a flour-and-water “starter.” The mixed-microbe fermentation produces more complex flavors and the characteristic sour tang.

Wild sourdough cultures are surprisingly stable — passed down across generations of bakers, with documented starters dating back over a century.

Nutritional yeast

A separate product: deactivated yeast cells, sold as flakes with a cheesy-nutty flavor. Used as a vegan parmesan substitute, sprinkled on popcorn, or stirred into sauces. Naturally rich in B-vitamins, fortified with B12 in many commercial brands — a vegan-diet staple.