New insights into the mechanisms underpinning diacetyl formation and reduction in large-capacity cylindroconical fermentations

Technical Session 03: Yeast I Session
Christopher A Boulton, University of Nottingham
Co-author(s): Joseph Sebastian, University of Nottingham, UK

ABSTRACT: In previous studies we have demonstrated that when using large capacity cylindroconical vessels, where filling times may be prolonged and require several individual batches of wort, the timing of pitching and wort oxygenation can have a profound influence on subsequent fermentation performance and beer analysis. It is known that the appearance in fermenting wort of free diacetyl and its immediate precursor, alpha-acetolactate is related to the assimilation of amino nitrogen. In this regard, the extra- and intra-cellular concentrations of valine, a group B amino acid not assimilated until mid-fermentation, are significant. It would be predicted that the ordered sequence of amino acid assimilation might be perturbed where there is a long interval between pitching and the completion of wort addition. Here these possibilities are discussed, and the results of relevant trials are presented. The situation is made more complex since it has also been shown that for much, if not all, of primary and secondary fermentation, conditions within these large vessels are heterogeneous. In particular, even with relatively non-flocculent yeast strains, a large proportion of the yeast population begins to form a crop in the cone before primary fermentation has reached completion. It is accepted brewing wisdom that where it is practice to eliminate diacetyl via a warm rest period at the end of primary fermentation it is essential to ensure that sufficient suspended yeast cells are present to ensure efficient assimilation and reduction of free diacetyl to less flavor-active metabolites. Since a large proportion of the yeast has already formed a sediment in the cone during the warm diacetyl rest this brings into question how the whole of the population contributes to the removal of diacetyl in the later stages of fermentation. Here the results of trials are presented in which these aspects of fermentation performance are explored. These support the contention that the underlying mechanism that produces the visible changes in total VDK concentration throughout large-scale production fermentations is more complex than the literature would sometimes suggest. The ways in which these new insights can be applied to produce more consistent and predictable overall fermentation performance are discussed, and supporting evidence is provided.

Chris Boulton gained his first and doctorate degrees at the University of Hull. The latter for an elucidation of the biochemistry of lipid accumulation in oleaginous microorganisms. He joined the research Department of Bass Brewers in 1984, where he worked as a fermentation scientist. Over the next 25 years, working with the same company and later with Molson Coors in a number of roles, he has continued to carry out research into how the physiology of yeast is influenced by the conditions it encounters during production-scale brewing; in particular, the ways in which the genome responds to modern intensive fermentation practices, and how it can be manipulated to ameliorate the effects of applied stresses and provide consistency in performance and outcome. In 2007 he joined the Department of Brewing Science at the University of Nottingham as a teaching fellow and special professor, where he teaches and continues to pursue his interests in fermentation science.