The Role of Mannose in Saccharomyces pastorianus Flocculation During Industrial Brewing Fermentations

Yeast flocculation is a phenomenon in which yeast zymolectins bind to mannose on adjacent cells resulting in aggregation within fermentation media. Flocculation has been extensively studied and is often assumed to occur only after media sugars are depleted as these are thought to competitively bind to zymolectins. However, yeast within the brewing industry have often been observed flocculating prematurely or staying suspended after media sugars were depleted. In this study, the mechanism of yeast (Saccharomyces pastorianus) flocculation was further investigated, specifically, how flocculation ability, zymolectins, and mannose associated with yeast cells changed throughout fermentation. It was observed that yeast flocculation behavior changed throughout fermentation despite no changes in zymolectin concentrations (as measured using bound fluoroprobes) independent of media sugar concentration. However, a strong positive correlation was observed between yeast flocculation ability and mannose concentration which increased as the fermentation progressed. This implies that the changes observed in flocculation behavior during fermentation may be more dependent on cell mannose rather than media sugars or lectins. These results have implications for yeast immobilization and flocculation control schemes. This research enhances the current understanding of the yeast flocculation mechanism with applications in brewing, bioethanol production, pharmaceutical production, precision fermentation, and cell biomass industries.