Flocculation in Saccharomyces cerevisiae

Abstract

Yeast flocculation is a reversible, non-sexual and multivalent process of cell aggregation into multicellular masses, called flocs, with the subsequent rapid removal of flocs from the medium in which they are suspended. Traditionally associated with beer production, flocculation might also be useful in modern biotechnology as a low cost and easy method of cell separation. Flocculation characteristics, namely the degree and the time of onset of flocculation, are of exceptional interest to brewing industry because they can affect beer characteristics. Flocculent cells have a specifi c lectin-like protein, which sticks out of the yeast cell wall, recognizes and interacts with the carbohydrates residues of -mannan (receptors) of adjoining cell walls; calcium ions are required to activate the lectin. Taking into account the sugar sensitiveness and ethanol dependence, four flocculation phenotypes have been described: Flo1 phenotype, NewFlo phenotype, mannose insensitive (MI) phenotype and strains whose flocculation requires the presence of ethanol. Yeast flocculation is a complex process, influenced by multiple factors, namely: cell surface characteristics, chemical characteristics of the medium, fermentation conditions and the expression of several specific genes such as FLO1 , FLO5 , FLO8 and Lg-FLO1 . This work reviews, discusses and updates the current knowledge on yeast flocculation with particular attention to the aspects related with brewing yeasts. The loss and the onset of flocculation in brewing yeasts belonging to NewFlo phenotype are also examined and discussed. Finally, the possibility of flocculation to constitute a long-term survival mechanism or a means of protection against an unfavorable environment is discussed.