Van Opstaele, F. | bioRxiv Science

bioRxiv2022

Development of a tractable model system to mimic wood-ageing of beer on a lab scale

Wood-ageing of conventionally fermented beers is gaining increased attention in the production of sour beers with a noteworthy balance between sourness, wood aroma and flavour complexity. Besides the extraction of wood-derived compounds into the beer, wood-aged sours owe their layered flavour profile to the activity of a variety of wild microorganisms that reside in the barrels or that emerge from the brewing or maturation environment. However, until now wood-ageing of craft beers largely remains a process of trial and error that often generates unexpected or undesirable results. Therefore, to better understand the process and develop control strategies to improve the consistency, predictability and overall quality of the resulting beer, more insight is needed into the interactions between the wood, the microorganisms and the maturing beer. Nevertheless, as studying these interactions on an industrial scale is highly challenging, the objective of this study was to develop a reproducible and easy-to-manipulate experimentally tractable system that can be used to study wood-ageing of beer on a lab scale. Barrel-ageing was mimicked in a 0.5 liter glass jar filled with beer and closed off by a wooden disk. Furthermore, the system was equipped with a synthetic community composed of four bacterial species (Acetobacter malorum, Gluconobacter oxydans, Lactobacillus brevis and Pediococcus damnosus) and four fungal species (Brettanomyces bruxellensis, Candida friedrichii, Pichia membranifaciens and Saccharomyces cerevisiae) that represented key microbes previously identified in wood-ageing experiments with 225-liter barrels. In order to test the hypothesis that the barrel-ageing process of beer can be replicated in the simplified in-vitro system, the system was subjected to 60 days of ageing and microbial community dynamics and beer chemistry were compared with a 38-week industrial barrel-ageing experiment using the same beer. Beer samples were collected at regular time points and subjected to both qPCR assays targeting the eight selected species and chemical analysis. Results revealed that in vitro ageing showed similar trends in the temporal dynamics of the microbial populations and beer chemistry as those observed during 38-weeks of barrel-ageing in 225-liter barrels. Furthermore, results were found to be highly reproducible. Altogether, the in-vitro system was found to be a robust and reproducible system that has great potential to perform more in-depth research about the intricate interactions between microbes, wood and maturing beer and to develop control strategies to improve the consistency, predictability and overall quality of the resulting beer.

microbiology↗

bioRxiv2022

Beer ethanol and iso-α-acid level affect microbial community establishment and beer chemistry throughout wood maturation of beer

Sour beers produced by barrel-aging of conventionally fermented beers are becoming increasingly popular. However, as the intricate interactions between the wood, the microbes and the beer are still unclear, wood maturation often leads to inconsistent end products with undesired sensory properties. Previous research on industrial barrel-aging of beer suggests that beer parameters like the ethanol content and bitterness play an important role in the microbial community composition and beer chemistry, but their exact impact still remains to be investigated. In this study, an experimentally tractable lab-scale system based on an in-vitro community of four key bacteria (Acetobacter malorum, Gluconobacter oxydans, Lactobacillus brevis and Pediococcus damnosus) and four key yeasts (Brettanomyces bruxellensis, Candida friedrichii, Pichia membranifaciens and Saccharomyces cerevisiae) that are consistently associated with barrel-aging of beer, was used to test the hypotheses that beer ethanol and bitterness impact microbial community composition and beer chemistry. Experiments were performed using different levels of ethanol (5.2 v/v%, 8 v/v% and 11 v/v%) and bitterness (13 ppm, 35 ppm and 170 ppm iso--acids), and beers were matured for 60 days. Samples were taken after 0, 10, 20, 30 and 60 days to monitor population densities and beer chemistry. Results revealed that all treatments and the maturation time significantly affected the microbial community composition and beer chemistry. More specifically, the ethanol treatments obstructed growth of L. brevis and G. oxydans and delayed fungal growth. The iso--acid treatments hindered growth of L. brevis and stimulated growth of P. membranifaciens, while the other strains remained unaffected. Beer chemistry was found to be affected by higher ethanol levels, which led to an increased extraction of wood-derived compounds. Furthermore, the distinct microbial communities also induced changes in the chemical composition of the beer samples, leading to concentration differences in beer- and wood-derived compounds like 4-ethyl guaiacol, 4-ethyl phenol, cis-oak lactone, vanillin, furfural and 5-methyl furfural. Altogether, our results indicate that wood-aging of beer is affected by biotic and abiotic parameters, influencing the quality of the final product. Additionally, this work provides a new, cost-effective approach to study the production of barrel-aged beers based on a simplified microbial community model.

microbiology↗