Yeast flocculation

Yeast flocculation typically refers to the clumping together (flocculation) of brewing yeast once the sugar in a wort has been fermented into beer. In the case of "top-fermenting" ale yeast (Saccharomyces cerevisiae), the yeast creates a krausen, or barm on the top of the liquid, unlike "bottom-fermenting" lager yeast (Saccharomyces pastorianus) where the yeast falls to the bottom of the brewing vessel. Cell aggregation occurs throughout microbiology, in bacteria, filamentous algae, fungi and yeast. Yeast are capable of forming three aggregates; mating aggregates, for DNA exchange; chain formation; and flocs as a survival strategy in adverse conditions. Industrial brewing strains rarely mate. Therefore, only chain formation and flocculation are of relevance to the brewing industry. Yeast flocculation is distinct from agglomeration (‘grit’ formation), which is irreversible and occurs most commonly in baker's yeast when strains fail to separate when resuspended. Agglomeration only occurs following the pressing and rehydration of yeast cakes and both flocculent and non-flocculent yeast strains have been shown to demonstrate agglomeration. It is also distinct from the formation of biofilms, which occur on a solid substrate. Louis Pasteur is erroneously credited with first describing flocculation of brewer’s yeast. Brewer's yeast flocculation has been the subject of many reviews. Flocculation has been defined as the reversible, non-sexual aggregation of yeast cells that may be dispersed by specific sugars or EDTA. The addition of nutrients other than sugars has been demonstrated not to reverse flocculation. This is as opposed to mating aggregates formed as a prelude to sexual fusion between complementary yeast cells. For flocculation to occur the yeast must be flocculent and certain environmental conditions must be present. Several factors are important in cell-to-cell binding such as surface charge, hydrophobic effects and zymolectin interactions. The importance of these forces in brewing yeast flocculation was unrecognized in the past but work by Speers et al.