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Oxygen in the brewery

Interestingly, no definitive work concerning the required levels of oxygen for both propagation and fermentation has been published. In general terms, during fermentation some archaic ‘rules of thumb' are applied such that for every °P increase in wort gravity an additional 1 ppm of oxygen is required, which for the purposes of this review will be known as the 1 : 1 theory. The work of Ashraf and Smart, (in preparation), questions this guideline and the premise that the levels of oxygen applied are not attuned to the requirement of yeast generally. In very large-scale fermentations, the point of application of oxygen or air is also not uniform, and in any case oxygen uptake is not a metric for most full-scale fermentations. This is surprising because oxygen plays an essential role in the brewing process despite its potential toxicity. A supply of oxygen is necessary during brewery propagation and early fermentation to generate yeast biomass and ensure that yeast is in optimum physiological condition for effective fermentation (Hammond, 2000; Hulse,

  • 2003) . Oxygen is required for lipid synthesis, which is necessary to maintain plasma membrane integrity and function, and consequently for cell replication (Hammond, 2000; Briggs et al., 2004). Sterols are synthesized using carbon devolving from glycolysis via acetyl-CoA. The first part of the synthesis is anaerobic, involving the conversion of acetyl Co-A to squalene; the conversion of squalene to 2,3-epoxysqualene, the precursor to the formation of sterols, requires molecular oxygen. (Boulton and Quain, 2003). A lack of oxygen can lead to an increase in cellular acetyl coenzyme A which can, in turn, lead to increased levels of esters such as ethyl acetate that can affect beer flavour (Briggs et al.,
  • 2004) . Conversely, overexposure of yeast to oxygen in the fermentation vessel can result in excessive yeast growth at the expense of ethanol production (Briggs et al., 2004). Optimum oxygen levels are therefore necessary for successful beer production.

After fermentation is complete, the yeast cells that have sedimented out of the wort are re-used in subsequent fermentations. Serial repitching, a procedure unique to brewery fermentation, may have important implications for yeast physiological state and fermentation performance (Powell et al., 2003) (see Chapter 1).

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