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.
VIEW PRESENTATION 9
