Yeast handling in the brewery

During the interval between cropping from one fermenter and re-pitching into the next, yeast must be stored (see Quain, 1990 and O'Connor-Cox, 1998a for reviews). The yeast must be held under conditions that prevent contamination and minimise any changes in physiological condition, which might compromise the next fermentation into which the yeast is to be pitched. With regard to yeast physiology, the storage phase is a period of starvation and can only be prolonged for a certain length of time. The duration of this period is influenced by the storage conditions and the state of the yeast at the time of cropping. The latter parameter is itself a function of the conditions the yeast was exposed to in the preceding fermentation. It is possible, therefore, to distinguish the 'storage potential' of a particular batch of yeast and the ways in which this potential is modulated by the conditions of storage.

In order to survive periods of starvation the yeast relies on endogenous carbohydrate reserves, which are laid down when exogenous carbon is plentiful. In yeast, two types of storage carbohydrate are recognised, glycogen and trehalose (see Section 3.4.2). Of these two, glycogen appears to be a genuine storage reserve, which is accumulated during mid-fermentation and is utilised during starvation (Quain & Tubb, 1982). Starvation occurs both during storage and, of course, in late fermentation when growth has ceased. It follows, therefore, that the storage potential of the yeast will be compromised if yeast is held in fermenter for a long period, prior to cropping, especially if chilling has not been applied.

Utilisation of glycogen reserves during storage is not a linear event; rather, there is a period of slow dissimilation followed by rapid breakdown phase. Pickerell el al. (1991) concluded that the rapidity of glycogen dissimilation was correlated with the temperature and duration of the storage phase. Most importantly, subsequent fermentation performance was impaired (slow attenuation, poor yeast growth, high S02, acetaldehyde and VDK, high residual a-amino nitrogen ) if the glycogen content of the pitching yeast fell below a critical value. In the fermentations described, this was 15% of the yeast dry weight, at pitching or 380 mg glycogen per litre of wort (16°Plato).

The pattern of glycogen dissimilation is not linear with time. At first, the rate of breakdown is slow and then there is a second phase, in which the dissimilation rate accelerates. This pattern correlates with changes in yeast viability during storage. The duration of the slow initial phase and the onset of the phase of rapid decline are related to storage temperature. The data given in Fig. 7.9 shows the changes in viability, measured by methylene blue staining, of identical samples of yeast slurries stored at various temperatures. It may be seen that at low temperatures this strain of yeast was remarkably resistant to storage. However, at higher temperatures the onset of the decline in viability occurred after a relatively short period. It is undoubtedly the case that before there was a detectable decline in viability, other physiological changes would have occurred which would certainly have had an adverse impact on fermentation performance, should the yeast have been pitched. Thus, in another report McCaig and Bendiak (1985b) observed that pitching yeast stored at temperatures of 5°C, or below, dissimilated little glycogen and maintained high viability. This yeast

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).

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