Impacts of Premature Yeast Flocculation Factors on Yeast Physiological Characteristics and Metabolite Profiles during Stirred and Unstirred High-Gravity Fermentations

Abstract

Premature yeast flocculation (PYF) is a sporadic condition of brewery fermentations in which yeast settles out from the fermenting wort earlier or more heavily than usual and before full attenuation has been reached. Factors originating from malt (i.e., high molecular weight polysaccharides and antimicrobial peptides) are believed to cause this accelerated flocculation. The impacts of PYF factors on yeast physiological characteristics and metabolite profiles were investigated during high-gravity (15°P) stirred and unstirred laboratory-scale fermentations. The PYF positive (PYF+) malt was known to have caused PYF in brewery fermentations and was confirmed as PYF+ using our in-house PYF test. However, comparing a range of yeast health, growth, and metabolic aspects of fermentation (gravity drop, ethanol yield, and CO2 evolution), only minor differences were observed between PYF+ and PYF negative (PYF–) fermentations. In particular, sugar uptake was not significantly affected by the PYF status of the wort. Hence, it is unlikely that significant amounts of membrane-disruptive antimicrobial peptides were present in this PYF+ sample; rather, it is more likely that it was inducing PYF primarily through the “bridging” polysaccharide mechanism. The main differences observed in these experiments were between the stirred and unstirred fermentations. Keeping yeast in suspension by mechanical agitation enabled more rapid yeast growth and fermentation progression.