Bacterial and Fungal Communities of Barley Malts Associated with Premature Yeast Flocculation During Beer Fermentation

Malt-induced premature yeast flocculation (PYF) is an intermittent malt-quality problem, which represents a major risk to the brewing process. Although the mechanism of PYF is not fully understood, it is accepted that PYF is closely associated with fungal infection of barley or malt. However, it remains unclear how microbial ecology of malt exactly impacts the PYF phenomenon. In this work, microbial communities of five malt samples (including four PYF-positive malts with varying PYF potential) were assessed at species level through a sensitive metagenomic sequencing method named 2bRAD-M. The results indicated that PYF cannot be simply associated with the diversity of malt microbial community. Specifically, putative biomarkers for PYF-positive malts were discovered from both fungal and bacterial species. Results indicate a hitherto unappreciated role of bacteria in PYF formation or prevention. PYF potential was not simply due to microbial load, but rather the result of microbial interactions. Improved understanding of the PYF-associated microbiome provides insight for further studies into the PYF phenomenon with a view to eventually introducing control strategies for process management, particularly in the malting process.