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Beer-spoiling yeasts and flocculation

One of the major effects of beer-spoiling yeasts on the fermentation process is related to the impact of contamination on the flocculation potential of the culture strain. Flocculation is a form of non-sexual aggregation and can be described as a reversible, calcium-mediated process that is characterized by the adhesion of cells within a population to form aggregates known as flocs (see Chapter 1). These sediment rapidly from the medium in which they are suspended, facilitating beer clarification and providing a cost-effective means of collecting yeast for re-pitching into a successive fermentation. Within the brewing industry, it is desirable that this occurs in a regular and predictable fashion, and preferentially towards the end of fermentation once fermentable sugars have been utilized. Consequently, the flocculation properties of the culture yeast are of great importance, since they have an impact on the consistency of the process, and the quality of the yeast and the final product.

In Saccharomyces yeasts, flocculation is governed by a group of closely related genes, referred to as the FLO gene family. This family incorporates several groups of genes, the first of which facilitate cellcell adhesion (FLO1, FLO5, FLO9, and FLO10). FLO8 encodes a transcriptional activator of FLO1 which itself is responsible for the structural protein directly involved in the flocculation process. The remaining gene of interest, FLO11, induces cell- substrate adhesion associated with invasive growth or pseudohyphae formation as described above (see section, ‘Vegetative growth, cell structure and sexual division'). The flocculation process is largely driven by lectin-like proteins (flocculins) coded for by the FLO1 gene (Teunissen et al., 1993). These extend out of the cell wall and bind to mannan receptor sites on adjacent cells (Miki et al., 1982; Kobayashi et al., 1998). Less is known about the other genes within the family; although they are structurally similar, the main difference is the degree of flocculation induced by their expression. Genome analysis of non-Saccharomyces yeast species have indicated that they also possess genes that code for cell wall lectins, suggesting that flocculation may be broadly similar in nature across the yeasts. Functional analysis of beer-spoiling yeasts including Debaryomyces (Cubells Martinez et al., 1996), Candida (Bauer and Wendland, 2007), Hanseniaspora (Suzzi et al., 1996), Pichia (Mbawala et al., 1990), Kluyveromyces (El-Behhari et al., 2000), Brettanomyces (Steensels et al., 2015), Torulaspora (Canonico et al., 2016), and Zygosac- charomyces (Suzzi et al., 1992), have indicated that each species demonstrates flocculation phenotypes of varying degrees. However, analysis of the mechanism of flocculation in K. marxianus has indicated that the specific structure and spatial arrangement of the cell wall groups involved in flocculation may be species- or genus-specific (Sousa et al., 1992), indicating that the underpinning mechanisms may differ.

Irrespective of the precise mechanism of aggregation, at the very basic level brewing strains are highly flocculent compared to most other types of yeast. Hence, it would be expected that the presence of beer-spoiling species would lead to a reduction in flocculation potential, causing issues for postfermentation processing. However, this is simplistic since flocculation potential in beer-spoiling yeasts is dependent on the species, its physical properties, and the level of contamination. Furthermore, it is known that mixing yeast cultures can give rise to coflocculation, a phenomenon first described by Eddy (1958), based on observations that non-flocculent strains became flocculent when mixed together. This definition has been extended to describe variations in flocculation observed when two different strains or species are mixed together, irrespective of their individual intrinsic flocculation capacity (Nishihara et al., 2000). Intra- and inter-specific co-flocculation has been reported in Kluyveromyces species (El-Behhari et al., 2000; Sosa et al., 2008), as well as in D. hansenii (previously C. famata) and Sch. pombe (Martinez et al., 1996), and is likely to be a prevalent phenotype across the yeasts. In fact, co-flocculation has also been observed between yeast and beer-spoiling bacteria (Peng et al., 2001), suggesting that such interactions may be important in developing microbial ecosystems. This hypothesis is supported by specific analysis into the genetic regulation of co-flocculation in yeast. It has been reported that individual FLO genes may impact differently on cell-cell adhesion phenotypes, favouring adhesion between some species while excluding others in mixed flocs (Rossouw et al., 2015). This type of interaction was observed between Hanseniaspora strains and S. cerevisiae wine yeasts, as well as between other species, including P. kudriavzevii. In the same study, FLO gene-specific differences in co-flocculation behaviour were observed between non-Saccharomyces strains analysed, with FLO1 overexpression consistently leading to increased co-flocculation. Analysis of the remaining FLO genes revealed a complex pattern of results dependent on the combination of strains investigated, indicating that co-flocculation is a multifaceted process. However, allowing organisms in co-culture to respond differently to one another may help explain the evolutionary persistence of the FLO gene family, comprising a number of genes that exhibit apparently similar function.

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