MBAA TQ https://doi.org/10.1094/TQ-58-2-0609-01 | 
VIEW ARTICLE
Graham G. Stewart and Anne M. Anstruther. The International Centre for Brewing and Distilling, (ICBD), Heriot-Watt University, Riccarton, Edinburgh, EH14 4AS, Scotland
Abstract
The importance of flocculation in order to crop a brewer’s yeast culture at the end of primary wort fermentation, so that the yeast culture can be reused during a subsequent fermentation, cannot be overstated. Modification to either flocculent or nonflocculent characteristics can affect (in a different manner) overall wort fermentation performance. Flocculation is a yeast cell surface phenomenon and cell wall structure is critical, and appropriate findings on this aspect are discussed. There are many definitions of yeast flocculation, but the one that has been employed for many years, almost universally, is that “flocculation is the phenomenon wherein yeast cells adhere in clumps and either sediment from the medium (usually wort) in which they are suspended or rise to the medium’s surface”. This definition excludes forms of cell chain formation, which will be discussed separately in this review. Microamounts of calcium are necessary for the flocculation of brewer’s yeast strains. The sedimentation performance of a brewer’s yeast strain is often modified during repeated repitching of a brewing fermentation. In principle, this modification could be due to irreversible or reversible genetic changes. Alternatively, it could be due to long-lasting physiological responses to modifications in yeast handling and the fermentation environment employed (for example, high-gravity wort). Flocculation is reversible with the addition of sugars such as mannose and also chelating agents such as EDTA that complexes with Ca++ ions. Cells will reflocculate following the addition of Ca++ ions. Genetic studies on yeast flocculation have concluded that there are at least nine FLO genes that code for flocculation that have been identified in both Saccharomyces cerevisiae and S. pastorianus strains. These genes encode for lectin-like proteins that are known as adhesants, zymolectins or flocculins. FLO1 is the most studied flocculation gene, which has been mapped, cloned, and sequenced by a number of research groups. The flocculin encoded by FLO11 strains differs from the others in that it is involved in filamentous (chain) growth and also adhesion to solid surfaces. It is important to enhance our understanding of both the environmental and genetic factors (nature-nurture) that affect the flocculation characteristics of yeast cells. Knowledge of yeast cell flocculation should be increased, because it is the primary option to control yeast sedimentation and culture collection in a brewery. There are other routes to bring about yeast separation and/or cropping (for example, centrifugation and filtration), and these are also discussed in this review.
Keywords: ale yeast, biofilms, cell wall structure, centrifugation, chain formation, co-flocculation, flocculation, FLO genes, lager yeast, sedimentation, yeast harvesting
