Provocation: all yeast cells are born equal, but some grow to be more equal than others

Abstract

Yeast is central to the brewing process and as such deserves great care and attention to ensure it is fit-for-purpose. It is widely accepted that assessment based on simple viability testing is useful but far too blunt an instrument to give much useful information regarding the physiological state of the live yeast cells within the population. The need to address this issue has spawned a plethora of procedures collectively termed ‘vitality tests’. The concept of yeast vitality is nebulous, and this goes some way to explain why no single test has been universally adopted. There is, however, another major flaw in the underlying assumptions of the majority of vitality tests. This is, most are based on bulk samples which ignore heterogeneities within the population of cells being examined.

The asymmetric mode of proliferation of Saccharomyces yeasts means that mother and daughter cells are always unequal. Individual cells have finite lifespans, old cells die and must be replaced with new fitter progeny. We are fortunate in that budding yeasts are widely used as a model cell for studies into ageing in higher eukaryotes and as a result there is a huge and ever-expanding body of literature devoted to this subject. This has shown that yeast populations comprise several distinct sub-populations which respond differently to external messengers. These messengers may be generated by one sub-set of the population and received and acted upon by another indicating a degree of cooperation designed to ensure survival of the whole. Death of individuals within populations can be as a result of the application of lethal external stresses but more usually is a programmed process in which targeted cells commit suicide in an expression of altruism for the greater good of the whole. On reflection, perhaps this is unexpected but perfectly logical behaviour for single celled organisms to adopt.

Most yeast population studies have been performed using haploid laboratory strains cultivated under relatively defined conditions. The industrial scale model is, of course, far removed from this. Brewing yeast strains are very different, and the complexities of serial fermentations and associated yeast handling processes introduce added layers of complexity. Nevertheless, there is every reason to believe that similar population heterogeneities and cooperative behaviour exist within the context of brewing. The thrust of the arguments presented in this article is that we should embrace this and consider individual cells within yeast populations within a fermenter, for example, as being individual free-living components of a multicellular organism. Further, intercellular cooperation is managed by a network of messenger molecules many of which are likely to trigger the global shifts associated in metabolic activities which accompany the reversible passage from growth to quiescence and which coincidentally are likely to be of importance in beer flavour.

The purpose of this article is to argue that the majority of current yeast vitality tests are not fit for their intended purpose. They do not provide outputs that are predictive of fermentation performance or an indication of the metabolic status of the population as a whole. Simple viability tests based on differential responses to biological stains, including methylene blue, are of equal value in commercial brewing. This situation can be improved by the introduction of tests which give information of how sub-populations develop and interact with each other. Probably dual staining techniques which probe membrane competence and detect cells in the early stages of programmed cell death, as discussed in this article, are promising candidates. © 2021 The Institute of Brewing & Distilling