Malt-induced premature yeast flocculation: current perspectives

Abstract

Premature yeast flocculation (PYF) is a sporadic problem for the malting and brewing industries which can have significant financial and logistical implications. The condition is characterised by abnormally heavy (and sometimes early) flocculation of yeast during brewery fermentations. The resulting low suspended yeast cell counts towards the end of the fermentation can result in flavour defects and incomplete attenuation (fermentation of sugars to alcohol). Despite several decades of research into the phenomenon, its precise nature and mechanisms have not been fully elucidated. In part this is because the term PYF has become a 'catch-all' syndrome which can have multiple origins. Furthermore, there are complex interactions in the malting and brewing processes which together mean that the PYF status of a malt sample is hard to predict at a generic level. Whether or not PYF is observed depends not only on barley quality, but on process factors in the maltings and to a substantial extent on the brewing yeast strain concerned. This article highlights the significance of PYF, and reviews current knowledge relating to the origins of this complex phenomenon.

The Microbiology of Malting and Brewing

“…This normally occurs at the end of fermentation, as sugars present during early fermentation associate with the lectin surface, preventing interaction (180). In PYF, yeast aggregation and settling occur prior to full attenuation of sugar, resulting in incomplete fermentation, off-flavors, and significantly decreased beer quality (181)(182)(183). PYF can be initiated by a range of polysaccharides naturally occurring in the barley husk (183)(184)(185), released either in response to microbial infection or by degradation of the husk by microbial enzymatic activity (186).…”

Saccharomyces and Non-Saccharomyces Starter Yeasts

“…These polysaccharides can act as a bridge between cells by interacting with yeast lectin-like proteins. It has been suggested that the binding between these polysaccharides and yeast cells is mediated by cationic antimicrobial peptides [22]. These compounds are produced by barley to protect against microbial atack, or in response to fungal contamination during the malting process.…”

125thAnniversary Review: Developments in brewing and distilling yeast strains

“…Recent studies have found that nitrogen starvation induces flocculation in bottom‐fermenting yeast. Both fungi and malt have also been implicated in premature yeast flocculation .…”