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Fig. 7.10 Relationship between exogenous ethanol concentration and viability of yeast in storage tank (Loveridge et al., 1999).

(1988) in one case showed an increase during storage in yeast trehalose levels whilst glycogen was declining. However, trehalose levels at pitch do not appear to have a great deal of influence on subsequent fermentation performance. Thus, Guldfeldt and Arneborg (1998) reported that yeast trehalose content in pitching yeast had no effect on growth, attenuation and ethanol production during fermentation, although high trehalose levels did favour maintenance of elevated viability during the initial stages of fermentation.

Trehalose is known to exert protective effects by stabilising membranes. It seems likely, therefore, that during storage of pitching yeast some glycogen may be dis-similated and used to synthesise trehalose in an attempt to prolong survival. This event may not improve the performance of such yeast on pitching since glycogen levels may be reduced. It may be argued that yeast in this form has entered a resting or almost dormant stage in which re-entry into an active growth phase may be a prolonged process. From another point of view it is known that the stresses of very high gravity brewing induce relatively high levels of trehalose accumulation (Majara et al., 1996a, b). This would imply that such yeast when cropped would survive prolonged storage with ease. This possibility appears not to have been tested.

7.3.1 Storage as pressed cake

Yeast storage in the form of pressed cake is associated with traditional top-cropped fermenters and in many respects is the least satisfactory method from a quality standpoint, although it has the advantage of economy. Yeast is skimmed, as described in Section 6.7.1, and pressed on a plate and frame filter to recover entrained beer, which is returned to the process stream. Typically, the yeast cake is transferred to clean metal bins and then stored in a refrigerated room at 2 to 4°C until required. Alternatively, it may be removed from the plates of the filter and stored as thin slabs on metal trays.

Yeast storage in this form is prone to infection since the yeast is open to the atmosphere (see Section 8.1.4.2). Obviously, the yeast is also exposed to air with the consequences in terms of limited sterol synthesis, as described in Section 7.3. However, this effect will be limited to the surface of the yeast cake, and therefore some heterogeneity in physiological condition would be predicted. This can be exacerbated where the cold room is of the type that blows cold air over the surface of the yeast.

One of the problems of storing pitching yeast as pressed cake is the difficulty of temperature control. The data in Fig. 7.11 shows the temperature, measured at various depths, in a bin containing 90 kg of yeast cake stored in a cold room maintained at approximately 1°C. The difficulties of achieving adequate attemperation in this situation are highlighted. Thus, yeast cake has poor thermal penetration and this, coupled with exothermy due to the basal metabolism of the yeast, produces the results shown. Prolonged storage in this form may lead to excessive heat generation and autolysis. To avoid this problem it is better to store the yeast in thin sheets.

7.3.2 Storage as a slurry

In the majority of breweries, pitching yeast is stored as slurry. This may be pressed yeast removed from a traditional top-cropping fermenter and re-slurried in water. In

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