Flocculation
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At the end of fermentation, when all fermentable sugars are converted into alcohol and carbon dioxide, yeast cells in suspension clump together and sediment at the bottom of the fermenter, a process called flocculation.
Some strains of yeast are known to generally flocculate very well, and other strains flocculate poorly. Often the yeast lab providing a beer yeast culture will describe the flocculation tendency. Low flocculation strains will stay in suspension and add a haze, such as Weizen yeast for example, whereas high flocculation strains will settle quickly and leave little to no haze.
Compared to other brewing technical properties of yeasts, flocculation is one of the most variable,[1] meaning even the same exact strain of yeast may flocculate to a different degree or at a different time, depending on the conditions. Unfortunately the mechanisms are not currently well understood, and so manipulating this characteristic is difficult.
Regardless of its influence on fermentation performance or growth, Ca has been recognised for some time to play a critical role in cell flocculation64 and the so called ‘calcium bridging’ hypothesis was initially proposed as an explanation of the mechanism of yeast cell flocculation81,144. It was suggested that the divalent Ca cation would form a link between negatively charged sites on the surface of neighbouring cells. Flocculation can however occur with concentrations of Ca as low as around 40 ppb208, suggesting the ionic interactions alone may not be sufficient to support this phenomenon200 and it is now generally accepted that flocculation is governed primarily by the interaction of cell surface zymolectins with mannose residues on the surface of adjacent cells142,143, with Ca ions having a role in stabilising these bonds200. There is some evidence that flocculation of bottom (lager) and top (ale) fermenting strains is different with respect to Ca induction. Dengis et al.51,52 observed that flocculation could be induced by 200 ppm Ca in lager strains, but that Ca had no effect on the flocculation of ale strains51,52. This is, however, not a universal phenomenon and Stratford and Assinder200 reported that all 28 ale strains in their study were dependent on Ca for flocculation. Ale strains, which are not dependent on Ca for flocculation, are usually (though not always) part of the relatively rare ‘maltose insensitive’ category of brewing yeast51,52,155. The relationship between the two phenomena, i.e., Ca-independent flocculation and maltose-insensitive flocculation, to date is still unclear. It is also known that laboratory culture conditions can influence flocculation. The strains included in the Dengis et al.51,52 study, in which ale strains flocculated in the absence of Ca, were cultivated in YE (yeast extract) broth rather than wort and the results may not represent flocculation during a brewery fermentation. Ca concentrations of laboratory media (50–500 ppm) may also not be representative of the concentrations in wort, which are commonly below 50 ppm94. It remains to be seen if Ca concentrations in wort can limit flocculation performance. Ca ions at micromolar levels may promote flocculation208 and it is unlikely that total wort Ca concentrations drop to these levels. Strauss and co-workers201 described a yeast strain with an inverse flocculation pattern, i.e., it remained flocculent until stationary phase. This property was dependent on the growth media and suggested that one possible reason was the pH-dependent availability of Ca ions. Stratford199 showed that some ale strains flocculate only within a narrow pH range and flocculate poorly in many commonly used yeast growth media because of non-optimal pH conditions. The antagonistic effects of other ions such as Li+ , K+ , Na+ , Ba2+, Mn2+, Sr2+, Al3+, Fe3+, La3+ and Pb2+ 76,113,155,198,208 must also be taken into account and while these elements may, individually, be at low concentrations, their combined effect may have a significant impact on flocculation in wort containing low Ca. Machado et al.131 have proposed the use of flocculent brewer’s yeast as a bioremediation agent for metalliferous industrial effluent. Yeast cells were capable of removing cations of Ni, Cu and Cr from waste water via biosorption of the metals to cells and removal of cells from the system through flocculation in the presence of 160 ppm Ca131. That brewing yeast cells retained their Ca-induced flocculation potential in the presence of excessively high concentrations of metal ions suggests that the trace metal composition of wort is unlikely to deter normal flocculation.[2]
Mg-induced flocculation is more sensitive to the presence of metal-chelating agents than Ca-induced flocculation and it is likely that the greater specificity of Ca in yeast flocculation relates to a greater affinity for cell wall-binding sites51,198. Smit et al.190 reported that onset of flocculation was triggered in a lager strain by the depletion of any of a number of different nutrients from the growth media. The exception was Mg, the loss of which did not result in flocculation, suggesting that nutritional Mg has a role in the process. This role is not fully understood but may relate to cell surface hydrophobicity, which is reduced in cells grown in Mg-limited media (0.7 ppm). Inclusion of Mg in the assay medium did not increase the hydrophobicity (or flocculation potential) of the cells in that study190.[2]
- Virve Vidgren, John Londesborough 125th Anniversary Review: Yeast Flocculation and Sedimentation in Brewing
See also[edit]
References[edit]
- ↑ Van Mulders, S.E., et al. "Flocculation gene variability in industrial brewer’s yeast strains." Appl Microbiol Biotechnol, vol. 88, 2010, pp. 1321–1331.
- ↑ a b Gibson BR. 125th anniversary review: improvement of higher gravity brewery fermentation via wort enrichment and supplementation. J Inst Brew. 2011;117(3):268–284.