Enzymatic Generation of Factors from Malt Responsible for Premature Yeast Flocculation

Optimization of a Small-scale Fermentation Test to Predict the Premature Yeast Flocculation Potential of Malts

“…Factors originating from malt are carried into wort and cause yeast to flocculate prematurely and/or heavily in the presence of relatively high sugar concentrations 5,[10][11]16,21,[30][31][32] . This causes significant financial and logistical problems to the brewer due to incomplete conversion of sugars to alcohol resulting in higher residual extract and lower alcohol than specification demands, flavour abnormalities, disruption of process cycle times and potential issues with the re-use of the yeast in subsequent fermentations 2,3,[16][17][18]21,[30][31][32] .…”

“…Cellulose is relatively resistant to enzymatic degradation 23 and as a consequence has been suggested not to be associated with PYF 24 . Therefore, the degradation of the barley husk, predominantly comprising arabinoxylans and cellulose 12 , will produce only a wide range of high molecular weight (HMWt) acidic arabinoxylan products 30 . In the proposed mechanism of PYF, the HMWt polysaccharides bind to the yeast cells via zymolectins, (adhesing glycoproteins which act as cell-surface receptors and are activated upon wort sugar depletion 3,16 ), leading to cross-bridging between adjacent cells.…”

“…Candidates for this antimicrobial agent have been proposed and include thionins, defensins and non-specific lipid transfer proteins 3 . Thionins, defensins and non-specific lipid transfer proteins are cationic antimicrobials which are relatively small (5-10 kDa) 30 , stable due to multiple disulphide bridges and capable of persisting through both malting and the brewing processes 2,3,[30][31][32] . It has been suggested that their action on membranes leads to cell lysis 31 and that they may disrupt yeast membrane integrity, leading to impairment of sugar uptake and resulting in leakage of cell constituents 4 .…”

“…Impairment of sugar uptake may result in an induction of the starvation response in yeast, which has been linked to the regulation of the onset of flocculation 31 . Despite the fact that the sporadic occurrence of PYF has been associated with the presence of fungi on the surfaces of grain [30][31][32] , the effect of the steeping process on PYF grains has also been proposed to exacerbate the occurrence of "tailing" fermentations 5,34 . In particular, Axcell et al 5 in 1986 suggested that the PYF factor may be released during malting by forcing water into the grain at a late stage of steeping.…”

“…As standard malt analysis is unable to predict PYF fermentation potential 4,20,31 , small scale fermentation tests have been developed to perform this key function [8][9][10][11]13,20,22,30 . Fermentation assays normally require several days to be completed and rely on recording suspended yeast cell counts and residual extract 4 .…”