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The microbiota and metabolites of ageing gueuze beers

Young and old lambic beers are blended by the brewer to make gueuze beers, which spontaneously re-ferment after bottling, a process that is referred to as ‘ageing. Gueuze beers bottled between 5 months and 17 years prior to the sampling have been examined to study the changes in microbiota and metabolites that can be used as a proxy of the processes that occur during gueuze maturation (Spitaels et al., 2015a). All gueuze beers showed the characteristic presence of B. bruxellensis and comprised acetic acid, lactic acid, ethyl acetate, and ethyl lactate as the most abundant metabolites. While pediococci were readily isolated from 1-, 2- and 3-year-old lambic beers, and from a gueuze beer bottled a few months before sampling, LAB or any other bacteria were no longer isolated from gueuze beers bottled more than 3 years prior to sampling (Spitaels et al., 2015a). Different yeast species including B. bruxellensis, B. anomalus, P. membrani- faciens, and S. cerevisiae were isolated from recently bottled gueuze, but this diversity decreased with age until only B. bruxellensis was isolated via enrichment culturing, even from the 17-year-old gueuze beer. The low nutritional demand of this yeast species probably enabled its long-term survival in this environment (Aguilar Uscanga et al., 2000; Renouf et al., 2007). The latter is also supported by the versatile metabolism of Brettanomyces yeasts, which can both produce and assimilate carbon sources such as acetic acid and ethanol (Renouf et al., 2007). The yeast cells in these old gueuze beers most likely occurred in a VBNC state that could be reversed by the enrichment procedure. The VBNC state allows yeast (and bacterial) cells to withstand several stress conditions (Millet and Lonvaud- Funel, 2000).

Further, the metabolite analyses revealed that the ageing of gueuze beer is probably limited in time by the depletion of the available malto-oligosaccha- rides (Spitaels et al., 2015a). Malto-oligosaccharide concentrations were very low in 9-year-old and 17-year-old gueuze beer samples and no further increase in the concentrations of lactic acid was found in these gueuze beer samples (Spitaels et al., 2015a). Furthermore, the acetic acid concentrations were the lowest in the 17-year-old gueuze beers, suggesting that acetic acid was further metabolized and that no new acetic acid was produced. Lactic acid concentrations increased steadily for beers aged up to 10 years, but no further increase was noticed in the beers that were aged for 17 years.

Additionally, the typical fruitiness was no longer perceived in the sensory analysis in the oldest gueuze beers examined (Spitaels et al., 2015a). This was probably caused by the degradation of the fatty acid ethyl esters, which are known to add fruitiness in beer. The increasing concentrations of ethyl lactate and decreasing concentrations of ethyl decanoate could be considered as positive and negative gueuze beer-ageing metabolite biomarkers, respectively.

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