Description of the temporal dynamics in microbial community composition and beer chemistry in sour beer production via barrel ageing of finished beers.

Currently, there is a strong interest in barrel ageing of finished, conventionally fermented beers, as a novel way to produce sour beers with a rich and complex flavour profile. The production process, however, remains largely a process of trial and error, often resulting in profit losses and inconsistency in quality. To improve product quality and consistency, a better understanding of the interactions between microorganisms, wood and maturing beer is needed. The aim of this study was to describe the temporal dynamics in microbial community composition, beer chemistry and sensory characteristics during barrel ageing of three conventionally fermented beers that differed in parameters like alcohol content and bitterness. Beers were matured for 38 weeks in new (two types of wood) and used (one type of wood) oak barrels. Beer samples were taken at the start of the maturation and after 2, 12 and 38 weeks. Microbial community composition, determined using amplicon sequencing of the V4 region of the bacterial 16S rRNA gene and the fungal ITS1 region, beer chemistry and sensory characteristics substantially changed throughout the maturation process. Likewise, total bacterial and fungal population densities generally increased during maturation. PerMANOVA revealed significant differences in the bacterial and fungal community composition of the three beers and across time points, but not between the different wood types. By contrast, significant differences in beer chemistry were found across the different beers, wood types and sampling points. Results also indicated that the outcome of the maturation process likely depends on the initial beer properties. Specifically, results suggested that beer bitterness may restrain the bacterial community composition, thereby having an impact on beer souring. While the bacterial community composition of moderately-hopped beers shifted to a dominance of lactic acid bacteria, the bacterial community of the high-bitterness beer remained fairly constant, with low population densities. Bacterial community composition of the moderate-bitterness beers also resembled those of traditional sours like lambic beers, hosting typical lambic brewing species like Pediococcus damnosus, Lactobacillus brevis and Acetobacter sp. Furthermore, results suggested that alcohol level may have affected the fungal community composition and extraction of wood compounds. More specifically, the concentration of wood compounds like cis-3-methyl-4-octanolide, trans-3-methyl-4-octanolide, eugenol and total polyphenols was higher in beers with a high alcohol content. Altogether, our results provide novel insights into the barrel ageing process of beer, and may pave the way for a new generation of sour beers.