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How much oxygen should be deployed during fermentation?

Oxygen cannot be eliminated from the brewing process, at least if serial repitching processes are desired. However, the 1 : 1 theory lacks merit and indeed supporting data. Maemura et al. (1998) found that the performance of yeast during small- scale wort fermentation was unimpaired when yeast had been propagated with only limited aeration (one hour at the beginning of propagation) compared with yeast exposed to a continuous supply of air. Little difference was seen in terms of cell density, carbohydrate reserves, or unsaturated fatty acid (UFA) level (Maemura et al., 1998).

The presence of O2 in wort at the beginning of fermentation allows yeast cells to synthesize lipids, thereby revitalizing the sterol-deficient cell population and ensuring that fermentation can proceed efficiently (see Chapter 1). An alternative approach involves oxygenation of the stored yeast prior to pitching, thereby reducing the O2 concentration necessary in the fermentation wort (Boulton et al., 2000; Depraetere et al., 2003). In this case, UFA synthesis occurs prior to pitching and the pitched yeast, being sterol-replete, has a reduced requirement for wort oxygenation. Trials have found that pre-oxygenated yeast in unoxygenated wort performs as well as normal yeast in oxygenated wort in terms of fermentation profile, ester synthesis, and alcohol production; the only apparent difference in this investigation was a reduced yeast growth in the unoxygenated wort (Boulton et al., 2000). While yeast cells are still exposed to O2, exposure is more readily controlled in the storage vessels than in larger, industrial-scale fermentation vessels and the use of excess O2 can be avoided (Boulton et al., 2000). The potential reduction in fermentation rate caused by the reduced cell density may be overcome by adjusting the pitching rate (Boulton et al., 2000). The reduction in cell growth in that investigation may have been due to excessive O2 consumption, which can result in depletion of trehalose. Optimum aeration of yeast prior to pitching has been shown to increase cell growth in unoxygenated wort (Fujiwara and Tamai, 2003).

It has also been suggested that the cellular requirement for O2 can be reduced by supplementation of stored yeast or wort with unsaturated fatty acid (UFA) or sterol (David and Kirsop, 1972; Taylor et al., 1979; Moonjai et al., 2003). Moonjai et al. (2003) have, for example, proposed the use of linoleic acid supplements as an alternative to wort oxygenation and demonstrated that pre-conditioning yeast in this fashion removed the requirement for wort oxygenation. Viability and fermentation performance of supplemented cells in non-aerated wort were similar to those of unsupplemented cells in aerated wort (Moonjai et al., 2003). Consequently, such supplementations may have potential in industrial fermentations by obviating the requirement for O2, thereby mitigating the effect of oxidative stress to yeast cells. It should, however, be noted that the ^-oxidation of fatty acids within yeast cell's peroxisomes can generate ROS such as H2O2. Koercamp et al. (2002) detected an oxidative stress response in yeast cells in chemostat cultures when the carbon source within the growth medium was switched from glucose to the fatty acid oleate.

Potential oxidative stress during brewing may be reduced by delaying the introduction of oxygen to the fermentation vessel. It has been found that, at least in small-scale fermentations, the fermentation performance of yeast was improved when oxygenation began 4 hours after pitching compared with oxygenation prior to pitching (Lodolo and Cantrell, 2005). Delayed oxygenation also resulted in a higher UFA and ergosterol synthesis, reduced levels of off-flavour compounds, and reduced free radical activity. It was hypothesized that the improved fermentation performance was due to improved pro-mitochondrial development in yeast cells not exposed to potentially toxic O2 at an early stage in their lifecycle (Lodolo and Cantrell, 2005). Whether delayed oxygenation is practical during industrial scale (3000 hl) fermentations has yet to be demonstrated.

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