What is yeast fermentation in coffee processing?

Yeast fermentation in coffee processing is a controlled approach where specific strains of yeast - most commonly Saccharomyces cerevisiae, the same species used in bread and beer - are introduced as a starter culture to guide the fermentation stage of processing.

Adding specific yeast strains allows producers to direct the microbial environment towards known flavour outcomes. S. cerevisiae metabolises sugars in the mucilage into ethanol, CO₂, and various esters and organic acids that interact with the bean. Depending on the strain, temperature, and oxygen levels, results can include tropical fruit complexity, increased sweetness, or distinctive aromatic notes that wouldn't emerge from spontaneous fermentation alone.

Yeast fermentation can happen in aerobic (open tanks) or anaerobic (sealed vessel) conditions, and the environment significantly affects the microbial balance and flavour result. Like any controlled fermentation, the outcome depends on the precision of management - temperature, fermentation time, inoculation rate, and clean equipment throughout.