Yeast flocculation

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Yeast flocculation typically refers to the clumping together of brewing yeast once the sugar in a beer brew has been converted into alcohol. In the case of ale yeast Saccharomyces cerevisiae the yeast floats to the top of an open tank, whereas with lager yeast Saccharomyces pastorianus the yeast will sink to the bottom of the tank.

Cell aggregation occurs throughout microbiology, in bacteria, filamentous algae, fungi and yeast [1]. Yeast are capable of forming three aggregates; mating aggregates, for DNA exchange; chain formation, for development and differentiation; and flocs as a survival strategy in adverse conditions.[2] Brewing strains are polyploid so mating aggregates do not occur. Therefore, only chain formation and flocculation are of relevance to the brewing industry.

Flocculation is distinct from agglomeration (‘grit’ formation), which is irreversible and most commonly found in bakers yeast strains, which fail to separate when resuspended [3]. 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 [4]. It is also distinct from the formation of biofilms, which occur on a solid substrate.

Pasteur first described flocculation of brewer’s yeast in 1876[5] which has since been the subject of many reviews [6]. Flocculation has been defined as the reversible, non-sexual aggregation of yeast cells that may be dispersed by specific sugars [7] or EDTA [8]. The addition of nutrients other than sugars has been demonstrated not to reverse flocculation [9]. This is as opposed to mating aggregates formed as a prelude to sexual fusion between complimentary yeast cells [10].

Flocculation is a bimodal process in which a non-flocculent population develops into one comprising of flocculent and non-flocculent cells.[11] The efficiency of flocculation is determined by the timing of flocculation onset and the rate of flocculation in conjunction with the ratio of flocculent to non-flocculent cells [12]. The rate-limiting step is doublet formation, requiring the presence of active surface proteins [13]. The mechanism by which this occurs is thought to be the lectin interaction theory.

Lectin Interaction Theory

The accepted mechanism of flocculation involves a protein-carbohydrate model [14] (figure 1.3). Fully flocculent yeast cells exhibit carbohydrate α-mannan receptors and protein lectins (section 1.5.4). It has been suggested that lectin like interactions between the two results in the flocculation phenotype (section 4.1) with Ca2+ ions required for the correct conformation of the flocculation lectins. Coflocculation between Kluyveromyces and Schizosaccharomyces has been shown to be by a “lectinic” mechanism [15]. This theory explains the essential role of calcium and how deproteinisation affects flocculation.

Flocculation Lectins and Phenotypes

Three flocculation phenotypes have been elucidated based on the lectins they produce: Flo1 [16] NewFlo [17] and Mannose Insensitive (MI) [18]. These flocculation phenotypes differ in the time of the onset of flocculation and the sugar inhibition of flocculation. Flocculation has also been classified according to time of onset and floc morphology.

Gilliland Class Flocculation Characteristics I Completely Dispersed II Flocculating into small, loose lumps late in fermentation III Flocculating into dense masses late in fermentation IV Flocculating very early in fermentation owing to non-separation of daughter cells

The genetic control of yeast flocculation has not been extensively studied. Recent reports suggest genes encoding lectin-like proteins exhibit close sequence homology [19]. Furthermore, it seems that FLO genes have interchangeable functions that can compensate for one another [20].

Flocculation Phenotypes

The Flo1 phenotype is inhibited by mannose [21] occurs in both ale and lager strains [22] and is associated with the FLO1 gene.[23].

The NewFlo phenotype differs from that of FLO1 in several ways. Firstly NewFlo flocculation is inhibited by mannose, glucose and maltose [24]. Secondly the NewFlo lectin is putatively encoded by the FLO10 gene [25] and is not expressed until stationary phase onset [26]. Thirdly lectin maturation occurs some fourteen hours after the cessation of cell division [27] and is therefore not concomitant with entry into stationary phase, although this is strain dependent.[28]

The MI phenotype appears to occur in ale (Saccharomyces cerevisiae), but not lager (saccharomyces pastorianus) strains [29] and is considered to be a rare phenotype. The FLO11 gene has however been identified as being essential for flocculation in S. bayanus [30] and characteristics such as invasive growth and pseudohyphal formation in Saccharomyces cerevisiae.[31] Although this flocculation phenotype has not been fully characterised, it is differentiated from other flocculation phenotypes by a lack of inhibition of the lectin like reaction in the presence of mannose.[32]

References

  1. Lewin, 1984; Stratford, 1992
  2. Calleja, 1987
  3. Guinard and Lewis, 1993
  4. Guinard and Lewis, 1993
  5. Pasteur, 1876
  6. Stewart et al., 1975; Stewart and Russell, 1986; Calleja, 1987; Speers et al., 1992; Stratford, 1992; Jin and Speers, 1999, Smart, 2001
  7. Burns, 1937; Lindquist, 1953, Eddy, 1955; Masy et al., 1992
  8. Burns, 1937; Lindquist, 1953
  9. Soares et al., 2004
  10. Calleja, 1987; Stratford, 1992
  11. Miki et al., 1982
  12. Stratford and Keenan, 1987; 1988; van Hamersveld et al., 1996
  13. Stan and Despa, 2000
  14. Miki et al., 1982
  15. El-Behhari et al., 2000
  16. Stratford and Assinder, 1991
  17. Stratford and Assinder, 1991
  18. Masy et al., 1992; Dengis and Rouxhet, 1997
  19. Jin and Speers, 1991, 1999; Smart, 2001
  20. Guo et al., 2000
  21. Burns, 1937; Miki et al., 1982; Nishihara and Toraya, 1987; Kihn et al., 1988; Stratford, 1989; Stratford and Assinder, 1991
  22. Miki, 1982; Stratford and Assinder, 1991; Masy et al., 1992; Smit et al., 1982; Stratford, 1993; Stratford and Carter, 1993; Teunissen et al., 1993; Teunissen et al., 1995a, b; Bony et al., 1997; Braley and Chaffin, 1999; Fleming and Pennings, 2001; He et al., 2002; Verstrepen et al., 2003
  23. Watari, 1991 Masy et al., 1992; Stratford, 1993; Stratford and Carter, 1993; Teunissen et al., 1993; Teunissen et al., 1995a, b; Bony et al., 1997; Braley and Chaffin, 1999
  24. Stratford and Assinder, 1991; Masy, 1992; Rhymes, 1999
  25. Guo et al., 2000; Smart, 2001
  26. Stratford, 1989; Stratford and Assinder, 1991; D’Hautcourt and Smart, 1999
  27. Stratford, 1989; Stratford and Assinder, 1991; Masy 1992; D’Hautcourt and Smart, 1999
  28. D’Hautcourt and Smart, 1999; Verstrepen et al., 2003
  29. Masy et al., 1992; Dengis and Rouxhet, 1997; Jin and Speers, 1999
  30. Ishigami et al., 2004
  31. Lo and Dranginis, 1998; Gagiano et al., 1999; Gancedo, 2001; Gagiano et al., 2002; Gagiano et al., 2003; Verduzco-Luque et al., 2003; Vivier et al., 2003; Guldener et al., 2004
  32. Dengis and Rouxhet, 1997; Guo et al., 2000; Smart, 2001