Effects of Fermentation Parameters and Cell Wall Properties on Yeast Flocculation

摘要

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 floes 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 floccu-lence and cell surface hydrophobicity values rapidly increased to high numbers from 24 h onward. Changes in the orthokinetic capture coefficient (α_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.