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Brewing fermentations

Modern brewing practices used by global, regional, and craft brewers are essentially derivatives of those used in traditional, historical brewing. The late nineteenth-century innovations concerning the concept of single-strain brewing and the use of propagation systems eclipsed the practice of continual serial repitching. Although some breweries do practice this even today, they face the unintended consequences of hygiene issues and genetic drift of primary strains. Most global and regional brewers use propagation in specially designed vessels to ensure sufficient inoculating biomass (pitch) of the desired brewing strain. Craft brewers tend to use dried yeast or yeast slurries acquired from Regional or Global neighbouring breweries. Yeast is propagated in wort under aerobic conditions, achieved through the addition of sterile air or oxygen (Boulton et al., 2000). For further discussion of yeast handling, see Chapter 3.

Following propagation, the fresh yeast slurry is transferred (pitched) into aerated or oxygenated wort in a cylindroconical (CC) fermentation vessel. For the most part, these vessels are vertically placed to alleviate footprint constraints but on occasion companies adopt a horizontal position for these vessels presumably to reduce hydrostatic pressure impacts on some strains with the additional consequence of a modified ester (fruity) profile. CC fermenters may, or may not, be mixed using pumped loop systems or internal agitators where appropriate, though this practice is relatively new to the sector. At the beginning of fermentation a brief lag phase occurs. This is partly caused by the requirement for the yeast to exit from G0 of the yeast cell cycle and commence replication, but is also a consequence of the adjustment of the cells to changes in the nutrient, gaseous, and physical environment upon pitching. Following the lag phase, yeast grows exponentially, rapidly depleting the available oxygen and key nutrients before entering an anaerobic environment. The fermentable wort sugars and assimilable nutrients are rapidly utilized, resulting in carbon and nutrient limitation, typically prompting the cell to enter a quiescent state (see Chapter 1). These carbon- and nutrient-limited conditions coincide with an increase in ethanol concentration (Casey et al., 1984). On completion of fermentation, yeast that has sedimented to the cone at the bottom of the cylindroconical vessel is removed (cropped) from the base of the cone and a portion of this yeast is stored under beer at low temperature (3-4°C) until required for use in subsequent fermentations (Briggs et al., 2004). Before repitching, yeast can be washed with food-grade acids (pH) in order to remove bacterial contaminants (Simpson and Hammond, 1989), though the deployment of this practice is not consistent with best-practice hygiene and yeast management, which should obviate the need for this step. Yeast repitched into a fermentation vessel is subjected to the same procedures and indeed stresses in a cyclic manner. Serial repitching, whereby yeast cropped at the end of the fermentation is re-used in subsequent fermentations, is a process unique to brewery fermentations and should only be conducted a finite number of times to prevent yeast quality deterioration and the consequent fermentation performance compromise.

One of the key drivers for large-scale brewing is the requirement to improve the efficiency of wort fermentation, leading to a reduction in fermentation time and/or an increase in yield. The latter requirement has been addressed by the use of high- gravity worts, produced via the addition of sugar adjuncts that result in higher ethanol concentrations towards the end of fermentations (Casey and Ingledew, 1983; Casey et al., 1984; Stewart et al., 1988; Stewart, 2001). Traditional and more recent innovative developments in practice are usually adopted to improve product quality and/or process efficiency; however, the fermentation of wort by yeast and the handling of yeast between fermentations results in exposure to numerous stresses (Gibson et al., 2007).

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