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 "kreuzen" on the top of the liquid, unlike with "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 (Lewin, 1984; Stratford, 1992). Yeast are capable of forming three aggregates; mating aggregates, for DNA exchange; chain formation; and flocs as a survival strategy in adverse conditions (Calleja, 1987). 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 (Guinard and Lewis, 1993). 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 (Guinard and Lewis, 1993). 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 (Stewart et al., 1975; Stewart and Russell, 1986; Calleja, 1987; Speers et al., 1992; Jin and Speers, 1999). Flocculation has been defined as the reversible, non-sexual aggregation of yeast cells that may be dispersed by specific sugars (Burns, 1937; Lindquist, 1953, Eddy, 1955; Masy et al., 1992) or EDTA (Burns, 1937; Lindquist, 1953). The addition of nutrients other than sugars has been demonstrated not to reverse flocculation (Soares et al., 2004). This is as opposed to mating aggregates formed as a prelude to sexual fusion between complementary yeast cells (Calleja, 1987).

For flocculation to occur the yeast must be flocculent and certain environmental conditions (such as agitation, absence of sugars, a microamount of Ca²⁺, ethanol, etc.; Jin and Speers 1999) must be present. Several factors are important in cell-to-cell binding such as surface charge, hydrophobic effects and zymolectin interactions (see following). The importance of these forces in brewing yeast flocculation was unrecognized in the past but work by Speers et al. (2006)^[1] have indicated the importance of zymolectin and hydrophobic interactions. As the cells are too large to be moved by Brownian motion, for binding of two or more cells to occur the cells must be subjected to low level of agitation.

Zymolectin Interaction Theory

The accepted mechanism of flocculation involves a protein-carbohydrate model (Miki et al., 1982) (figure 1.3). Fully flocculent yeast cells exhibit carbohydrate α-mannan receptors and protein zymolectins (section 1.5.4). Zymolectins are so termed as they may not be true bivalent lectins (Speers, Smart, Stewart and Jin,1998)^[2] It has been suggested that zymolectin interactions between the protein and mannan moities results in the flocculation phenotype (section 4.1) with Ca²⁺ ions required for the correct conformation of the zymolectins. Coflocculation between Kluyveromyces and Schizosaccharomyces has been shown to be by a “lectinic” mechanism (El-Behhari et al., 2000). This theory explains the essential role of calcium and how deproteinisation affects flocculation.

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