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Oxygen

As noted above, the generally accepted guideline for pitching rate is 1 x 106 viable cells/ml for every 1° Plato of wort. A similar metric is used for wort oxygen with 1 mg/l (ppm) for every 1° Plato of wort (Barnes, 2006). As with pitching, this is rule of thumb and a useful starting point for optimization. Functionally, as yeast has the requirement for oxygen, pitching rate is the real determinant of oxygen requirement. As ever, it may not be as simple as this! Early reports noted that both ale (Kirsop, 1974) and lager (Jacobsen and Thorne, 1980) strains can vary - from half air saturation to oxygen saturation or more - in the amount of oxygen they require for a successful fermentation.

In terms of process, oxygen is added under backpressure on the cold side of the wort cooler either as air or oxygen. Whilst the proportion of the oxygen in the gas phase is the bigger determinant, pressure together with wort temperature and solids

Key events during fermentation

Figure 3.2 Key events during fermentation.

determine solubility. The solubility of oxygen or air in water at 25°C and 1 bar pressure is, respectively, 40 or 8 mg/l. Accordingly, the concentration of oxygen in worts saturated with air at 15°P and 20°P is 6.3/5.5 ppm at 15°C and 5.5/4.9 at 20°C (Kirsop, 1974). Given this, it is not surprising that wort oxygenation - rather than aeration - is the norm with high-gravity wort. Although typically well controlled at the wort cooler, the actual concentration of oxygen in fermenter may be lower due to gas breakout, especially if the vessel is some distance from the point of addition. As with yeast pitching regimes, multiple batches of wort allow different ‘tunes' of oxygen addition. Addition to all or some of the wort batches may allow tuning but will inevitably add process complexity and opportunity for error.

Fundamentally, oxygen determines the extent of yeast growth by enabling the synthesis of sterols and mono-unsaturated fatty acids. Too little and there is insufficient lipid synthesis and consequently insufficient yeast growth and incomplete fermentation. Too much oxygen - should it remain available once the cells begin to divide - can result in unrequired additional lipid synthesis and unnecessary yeast growth. As yeast growth impacts negatively on the formation of ethanol, unnecessary yeast growth reduces the fermentation efficiency. These interactions are demonstrated experimentally in Fig. 3.3.

 
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