Yeast (Saccharomyces spp.) is the key microorganism of beer fermentation.

• Main role: to consume the fermentable sugars of the wort and produce:

  • Ethanol → alcohol

  • CO₂ → natural carbonation

  • Aromatic compounds → esters (fruit), phenols (spices), diacetyl (butter, flaw)

• Types of yeast:

  • Ale / top fermentation: Saccharomyces cerevisiae → 15–25 °C, fruity/spicy notes, fast fermentation

  • Lager / bottom fermentation: Saccharomyces pastorianus → 8–15 °C, slow fermentation, neutral taste

  • Special / wild yeasts: Brettanomyces, Lactobacillus (sour), Saccharomyces diastaticus (high attenuation)

• Parameters influencing fermentation: temperature, original gravity, oxygenation, nutrients, yeast stress

Yeast is therefore responsible for the alcohol, the carbonation and a large part of the aromatic profile, and it largely defines the final style of the beer.