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Future perspective

It will be apparent from the material covered in this chapter that yeast cells, in particular S. cerevisiae, have been subjected to intense study. Much of the elucidation of the breathtakingly elegant ways in which eukaryotic cells regulate all aspects of cellular function, from birth to self-programmed death, have been performed using this model cell. However, the results of very few of these studies can be used with any degree of certainty to interpret the behaviour of brewing yeast strains growing under brewing conditions. Thus, the combination of growth on an uncharacterized and variable but comparatively unbalanced medium, transient aerobioisis, and yeast recycling coupled to serial fermentation make for a fascinating but perhaps bewildering level of complexity. Nevertheless there are some important lessons to be learnt, which might be used profitably in attempts to make brewing yeast behave in a predictable and more productive manner.

In yeast, the signalling pathways that respond to external nutritional cues do so in a way that multiple nutrients can elicit a common set of responses. The pathways respond to the presence or absence of comparatively simple molecules. Since brewers have yeast ready to pitch in storage vessels, it may be possible to use this as an opportunity to initiate key pathways before pitching has occurred and thereby shorten lag times, improve the performance of newly propagated cultures, remodel the phenotype to a state that is more amenable for very-high-grav- ity brewing, or simply provide methods for more precise control of the formation of yeast-derived beer flavour metabolites.

An aspect of yeast activity that is not usually considered in normal brewing practice is that of population heterogeneity. For obvious reasons, it is usual to deal with yeast in bulk and by inference there is a tacit assumption that all cells present within that population will behave in a similar fashion. Clearly this is incorrect. The understanding of the relationships between cell age and size and the risks of selection of an inappropriate portion of a bottom crop from fermenter are well-known. It is certain that yeast populations are inherently heterogeneous and there is some evidence that there is some degree of cooperation. The phenomenon of flocculation is considered to be a stress response in which cells within the inner parts of the cell mass are more likely to survive than those on the outside. This may be a purely random process, although it is possible that there is some element of choice in terms of which cells are selected for sacrifice or survival based on their relative positions in the floc. As alluded to already, the phenomenon of apoptosis carries the assumption that members of a population are selected for death. Whether this requires input from all members of the population or it is simply a manifestation of the end of the lifespan of an individual unrelated to potential cooperative behaviour is not known.

It is a sobering thought that the contents of a large fermenter represent an enormous number of individuals. A terminal count of 60 million cells/ml equates to a total of around 1 x 1016 cells at the end of growth in a 2000 hl vessel. This compares with the total human population of the earth for 2015 of approximately 7 x 109. Assuming a pitching rate of 15 million cells/ml and initial and final viabilities of 95%, an additional 4.5 x 1014 dead cells would be present in the crop. Of course, this does not include any cells that would have disappeared via lysis or those that might be approaching senescence. Quorum sensing, the ability of microbial populations to monitor cell densities, is a well-established phenomenon in bacteria. The evidence suggests that a similar system occurs in populations of S. cerevisiae and that the process uses aromatic alcohols such as phenylethanol as signalling molecules (Zupan et al., 2013; Wuster and Babu, 2010). In another example, important beer esters such as ethyl acetate and ethyl hexanoate have been shown to function as fruit fly pheromones and by inference may be involved in dispersing natural yeast populations (Siderhurst and Jang, 2006). A greater understanding of the biological basis of, for example, the formation of beer flavour compounds, is likely to lead to the development of more directed control procedures.

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