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Fig. 7.9 Effect on viability of storing a slurry of lager yeast (40% wet w/v) at the temperatures shown in the legend. Samples of slurry (1 litre aliquots) were stored under nitrogen gas with gentle continuous stirring (C.A. Boulton, unpublished data).

produced standard fermentation performance and beer quality. However, similar yeast stored at 10, 15, 20 and 25°C for the same period showed a progressive reduction in viability and decline in glycogen reserves. This correlated with increasing impaired fermentation performance when the yeast was pitched into wort.

Others have disputed the importance of maintaining yeast glycogen levels in pitching yeast. Cantrell and Anderson (1983) concluded that no correlation existed between fermentation performance and yeast glycogen content. Sail et al. (1988) also reported that the fermentation performance of yeast was independent of glycogen content. In this case, however, the study was performed over just three days, during which time the yeast was stored at 1.6°C. In this time viability remained between 90 and 96% but glycogen levels declined from the relatively low initial concentration of 15.6 to just 9% of the yeast dry weight. These authors suggested some brewing yeast strains were capable of accumulating only low concentrations of glycogen, and, in such strains, this parameter had no relation with fermentation performance. Nevertheless, for most strains the need to conserve glycogen is well established.

Glycogen breakdown in yeast during storage is accompanied by an increase in the extracellular ethanol concentration (see Section 6.4.2.2). Thus, the ethanol concentration in the barm ale of stored yeast correlates with holding time. It has also been demonstrated that the viability of stored yeast correlates with the ethanol concentration in barm ale (Fig. 7.10).

The signal for rapid breakdown of glycogen during fermentation is exposure of yeast to oxygen. In this circumstance, glycogen dissimilation is linked to sterol synthesis (see Section 6.4.2.2). Limited sterol synthesis may also occur if stored pitching yeast is exposed to air. This is of no consequence in traditional fermentations. However, where rigorous control is required as in the case of large volume fermentations, this should be prevented in order to avoid excessive yeast growth. Where control is of importance, pitching yeast should be stored under an inert gas.

Trehalose is accumulated by yeast in response to stress (see Section 3.4.2.2). Starvation is such a stress and indeed during storage some trehalose synthesis occurs at the expense of glycogen dissimilation. For example, the data presented by Sail et al.

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