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Bilverstone, Terry W.; Boulton, Chris A. and White, Rod
(2013).
Abstract
Volatile esters, produced as a result of the metabolism of wort by yeast during fermentation, are essential contributors to the flavor and aroma of beer. Since many esters arise at concentrations close to their flavor thresholds comparatively small variations in the concentrations of individual members can have a large impact on beer organoleptic properties. An essential part of the control of the brewing process is to ensure that esters are produced in the concentration ranges characteristic for individual beer qualities. This is especially the case where a single beer is brewed at several different production sites. The metabolic pathways that are implicated in volatile ester accumulation as a result of yeast activity during fermentation and the underlying genes responsible for the synthesis of the individual enzymes and their regulation are reasonably well-characterized, albeit with several caveats. Similarly, the process factors that are known to have an impact on ester formation have been subject to intensive study over the years, although often with contradictory results. We present volatile ester data relating to a pilsner-type lager beer produced at several international breweries that demonstrate significant site-specific variability although all of the breweries ostensibly used the same yeast strain and identical wort composition and fermentation control parameters. It is assumed that the observed variability must be a consequence of differences in plant and process operation associated with each site—in particular, fermentation vessel design and management. A laboratory model fermentation system is described that has been used to study formation during fermentation of the various groups of volatile esters known to contribute to beer quality as well as the synthesis and fate of their precursors. Trials are described that have been performed with a view to elucidating the cause-and-effect relationships between process conditions and ester accumulation for this combination of yeast strain and wort composition. Preliminary results are presented that demonstrate that this model system can be used to identify production-scale regimes that when implemented will allow ester formation to be controlled in a predictable fashion and so result in improved site-to-site product matching.