Yeast flocculation
Reversible clumping of yeast after sugar fermentation.
Yeast flocculation refers to the reversible clumping together of brewing yeast once the sugar in a wort has been fermented into beer. In top-fermenting ale yeast (Saccharomyces cerevisiae), the yeast creates a krausen on the top of the liquid, whereas bottom-fermenting lager yeast (Saccharomyces pastorianus) falls to the bottom of the brewing vessel. This process is distinct from irreversible agglomeration and biofilm formation, and is of relevance to the brewing industry.
- Flocculation phenotypes
- Flo1, NewFlo, Mannose Insensitive (MI)
- Flo1 inhibition
- mannose
- Newflo inhibition
- mannose, glucose, maltose
- Newflo putative gene
- FLO10
- Key researchers
- Stratford and Assinder (1991), Speers et al. (2006)
Lore & Background
Cell aggregation occurs throughout microbiology, and yeast can form three aggregates: mating aggregates for DNA exchange, chain formation, and flocs as a survival strategy in adverse conditions. Industrial brewing strains rarely mate, so only chain formation and flocculation are relevant to brewing. Yeast flocculation is distinct from agglomeration, which is irreversible and occurs in baker's yeast upon rehydration, and from biofilm formation on solid substrates. Louis Pasteur is erroneously credited with first describing flocculation of brewer's yeast. Flocculation has been defined as the reversible, non-sexual aggregation of yeast cells that may be dispersed by specific sugars or EDTA; addition of other nutrients does not reverse it. For flocculation to occur, the yeast must be flocculent and certain environmental conditions must be present, including low agitation, as cells are too large for Brownian motion to bring them together.
Reader's Guide
The accepted mechanism of flocculation involves a protein-carbohydrate model, where fully flocculent yeast cells exhibit carbohydrate α-mannan receptors and protein zymolectins. Zymolectin interactions between protein and mannan moieties result in the flocculation phenotype, with Ca2+ ions required for correct zymolectin conformation. Three flocculation phenotypes have been elucidated: Flo1, NewFlo, and Mannose Insensitive (MI), which differ in the time of onset and sugar inhibition. The Flo1 phenotype is inhibited by mannose and occurs in both ale and lager strains. The NewFlo phenotype is inhibited by mannose, glucose, and maltose, and its lectin is putatively encoded by the FLO10 gene. Studies indicate little change in zymolectin levels through fermentation, and lectin maturation is argued to occur some fourteen hours after cessation of cell division, though molecular proof is near non-existent. The picture is complicated by changes in cell surface hydrophobicity and CO2-driven shear, which confound flocculation measurements often erroneously attributed solely to zymolectin interactions.
Did You Know?
- Yeast flocculation is reversible and can be dispersed by specific sugars or EDTA.
- Three flocculation phenotypes exist: Flo1, NewFlo, and Mannose Insensitive (MI).
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