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The brewing yeast cycle

An obvious characteristic of yeast belonging to the genus Saccharomyces is an ability to survive in a wide variety of conditions; perhaps more specifically, an ability to adapt its phenotype rapidly in response to changes in the environment in order that it may grow and proliferate where conditions permit this or simply survive where they do not. Modern brewing practices test these abilities to the full as will be described.

The majority of modern commercial brewers practise a semi-conservative process in which yeast derived from fermentation is recovered from the immature beer, retained in a storage vessel and a proportion used to inoculate a subsequent fermentation. This process of serial fermentation is continued typically for 5-15 successive fermentations (referred to by brewers as ‘generations'), after which the yeast is disposed of and replaced by a new culture derived from laboratory stocks and introduced into the brewery via a process of propagation (Fig. 1.1).

The brewing yeast cycle has several consequences. The sequence of events represented by inoculation (pitching), growth, cropping, and storage requires cells to undergo a series of transitions in which they are exposed to rapid and dramatic changes in environmental conditions. In the storage phase, cells are starved of nutrients but usually exposed to low temperature (2-4°C), anaerobic conditions, and relatively high ethanol conditions. In many breweries, before pitching, yeast cells are treated with an acidulant such as phosphoric acid with the aim of killing less acid- tolerant bacteria. In this ‘acid washing' step, the pH is reduced from around pH 4.0 to pH 2.1-2.3 and, at least in a properly controlled process, the

The brewing yeast cycle

Figure 1.1 The brewing yeast cycle.

cells are held at a temperature of around 3°C for a period of 1-2 hours (Simpson and Hammond, 1989). During pitching, the cells undergo a shift to higher temperature (although one much lower than that optimal for growth) and exposure to oxygenated wort. Thus, cells undergo a transition to aerobic conditions in a complete growth medium, albeit one that is relatively unbalanced, having a very high sugar concentration and comparatively smaller concentrations of other essential nutrients. Transfer to wort is accompanied by a concomitant osmotic shock and in the large capacity and tall vessels most commonly used in modern brewing the cells have considerable hydrostatic pressure exerted on them. As growth proceeds, the yeast exhausts the available oxygen and conditions revert to anaerobic; nutrients are assimilated; and the principal products, ethanol and CO2, accumulate together with a multitude of other metabolic products, some of which influence beer flavour. Ethanol and CO2 both exert toxic effects on yeast cells. In modern high-gravity brewing practice, it is common to use highly concentrated worts as a means of increasing fermentation productivity. The concentrated beers produced in this process are then diluted to ‘sales gravity' prior to packaging. The osmotic pressures generated as a consequence of high-sugar concentrations in these worts and the consequent high yields of ethanol increase the stress levels experienced by yeast (Udeh and Kgatia, 2013). Commonly, wort strength is increased by the addition of relatively pure sugar syrups since these are usually less expensive compared to malted barley. The effect of this is to dilute the non-sugar components and thereby render the wort an even more unbalanced growth medium.

As fermentation proceeds, the majority of brewing strains settle to the bottom ofthe vessel to form a sediment in which the cells are subject to the potentially damaging combination of high hydrostatic pressure and high levels of carbonation and ethanol. In those strains capable of doing so, sedimentation is aided by the process of flocculation (Vidgren and Londesborough, 2011). At some stage in the fermentation, yeast, with some entrained beer, is cropped from fermenter, chilled and transferred back into a storage vessel where it may be held for up to 7 days (typically 2 - 5 days). After this time, another cycle of growth is initiated by pitching stored yeast into fresh wort. Cylindroconical fermenters (the type most commonly used in modern brewing practice) are designed to allow removal of yeast crops with a minimum of back- mixing. Thus, there is a spatial relationship between the location of yeast in the crop in the cone of the vessel and the timing of its removal. By inference, the brewer has the opportunity to make a choice over which fraction to retain for re-pitching. This is significant in that Powell et al. (2002) noted that there was considerable heterogeneity in terms of location of cells in the cone crop and factors such as cell size, replicative age, flocculation characteristics and levels of storage carbohydrates. Predictably, there was concomitant variability in subsequent fermentation performance depending on which cells were re-pitched.

The modern brewing process is potentially harmful to yeast and prolonged serial fermentation increases the likelihood of contamination and the potential for the emergence of genetic variants. It is for this reason that after a given number of serial fermentations, the precise number chosen by individual brewers on a more or less empirical basis, the yeast culture is disposed of and replaced with a new one of guaranteed identity and purity. The propagation process used by the majority of large- scale commercial brewers involves carrying out a series of batch cultures of ever-increasing size until sufficient biomass is generated to pitch a full- scale production fermentation. In the propagation phase, wort is used as the growth medium and yeast growth, in excess of that observed in fermentation, is encouraged by the provision of continuous oxygenation. Fed-batch approaches, in which biomass yields far in excess of those achievable by conventional brewery propagations on wort are used for the production of active dried yeast, a product used by many of the new generation of craft brewers (Jenkins et al., 2011).

Compared with many other industrial processes and certainly the majority of laboratory studies, brewing fermentation pitching rates are relatively high, typically 15-20 million cells per ml (ca 0.81.2 g/l dry weight) in a high-gravity wort of 15-20° Plato. During fermentation, growth is modest, usually a 3- to 4-fold increase in cell biomass. A result of this is that the pitched cells not only play an active part in fermentation but may also survive through to cropping and re-pitching. Yeast cells undergo an ageing process that, unless cut short by other causes, culminates in senescence and death (Steink- raus et al., 2008). Depending on the fraction of crop retained for re-pitching, there is the possibility of a gradual increase in the average cell replicative age with each generation.

 
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