Effects of Fermentation Parameters and Cell Wall Properties on Yeast Flocculation¹

Abstract

Industrial wort was fermented with a NewFlo phenotype ale yeast in lab-scale cylindrical fermenters. The effects of various fermentation parameters and yeast cell wall properties on yeast flocculation were studied during 120 h fermentation. The evaluation of the cell volume during the fermentation revealed a non-normal distribution (p < 0.05) at most fermentation times. Overall yeast cell size initially decreased during the first 24 h of fermentation then increased during the 24-60 h fermentation period. Cell size subsequently declined until the end of fermentation presumably due to floc settling. While yeast flocculation began after 24 h fermentation, most flocs remained in suspension until 60 h when the average turbulent shear rate caused by CO 2 evolution declined to below 8 s -1 . Both the Helm's flocculence and cell surface hydrophobicity values rapidly increased to high numbers from 24 h onward. Changes in the orthokinetic capture coefficient (␣ 0 ) value with fermentation time, measured in fermenting worts, indicated a significant increase (p < 0.001) after 24 h of fermentation. Presumably, this change was due to increases in ethanol and the decline in sugar concentration with time. Although a significant positive correlation (p < 0.05) was observed between zymolectin densities and cell surface areas, the total zymolectin level on yeast cell walls did not change significantly with fermentation time (p > 0.05). Interestingly, no significant difference existed in Helm's flocculation values of suspended and settled yeast cells (p > 0.05). The flocculation rate of LCC125 was readily inhibited by addition of glucose or maltose. Results suggest that fermentable sugar levels and shear force exert major influences on yeast flocculation during beer fermentations.

125th Anniversary Review: Yeast Flocculation and Sedimentation in Brewing

“…an increase of CSH and decrease of cell surface charge occur at the onset of flocculation 2,33,71,78,91 . Cell surface hydrophobicity is reported to increase rapidly as cells pass through the exponential phase and to reach high, stable levels in stationary phase 78 . The low CSH in exponential phase can be explained by the presence of many young daughter cells, which are significantly less hydrophobic than older cells 58 .…”

“…and used to calculate what proportion of the total cells has sedimented. Less attention has been paid to assaying how complete sedimentation will eventually be attained or to how strongly the cell flocs are held together (which can be estimated by thermal deflocculation 93 ), though the reader is directed to work modelling the kinetics of cell collision and capture (the orthokinetic capture coefficient) 27,78 . The sedimentation rate is apparently simple to measure and of immediate practical importance to brewers.…”

“…However, in NewFlo phenotype strains flocculation is inhibited by mannose, glucose, sucrose, maltose and maltotriose (also by galactose in some strains) suggesting that flocculin proteins in these cells are able to bind this wider range of sugars 67,86 . Most brewer's yeast strains (both ale and lager) have been reported to possess the NewFlo phenotype 9,78,86 . This makes them particularly suitable for brewing since flocculation is delayed until the fermentable wort sugars (glucose, maltose and maltotriose) that inhibit this kind of flocculation have been largely consumed as the fermentation approaches completion.…”

“…In flocculating strains, cells that are not entrapped in flocs have not necessarily lost the ability to flocculate 78,102 . Instead, there is a dynamic equilibrium between flocs and free cells resulting in a continuous exchange between cells entrapped in flocs and free cells.…”

“…Turbulent shear force caused by CO 2 evolution during fermentation is an important factor restricting sedimentation 78 . Speers et al 78 showed that an ale strain with the NewFlo phenotype started to flocculate after 24 h of fermentation, but most flocs remained in suspension until 60 h, when the average turbulent sheer rate caused by CO 2 evolution declined to below 8 s -1 .…”