Cd

Fig. 7.11 Temperature profile measured at various depths in a bin containing 90 kg of pressed yeast cake. The bin was located in a cold room attemperated at c. 1 to 2°C (A.R. Jones, unpublished data).

some breweries, yeast is stored in the cones of cylindroconical fermenters. This obviates the expense of installing dedicated yeast storage vessels. Although an inexpensive approach, it is not ideal since attemperation of the slurry is impossible. However, it can be a useful temporary measure where the turn-round time between fermenting vessel emptying and refilling is very short.

More commonly, the slurry consists of yeast and entrained beer taken directly from fermenter without further processing. Occasionally the slurry may be stored in open troughs; however, closed vessels are more usual, especially in modern breweries. Storage of yeast in slurry form affords significant advantages. The yeast is normally contained within a vessel, and therefore there is an opportunity to control the risk of infection and provide an inert atmosphere. The yeast is in 'liquid' form, which facilitates transport via pumping. Most importantly, attemperation of yeast slurries is much simpler than with pressed cake. Storage of yeast slurries in closed tanks is particularly suited to breweries that handle several different yeast strains. Thus, it is much simpler to minimise the possibility of cross-contamination. On the debit side, installation of properly designed yeast storage tanks and associated plant is an expensive undertaking. However, when the importance of maintaining yeast stocks within a brewery is reflected upon, it should be considered money well spent.

A typical yeast storage or 'collection' vessel is constructed from stainless steel, with mechanical agitation and dished ends (Figs 7.12 and 7.13). This configuration assists circulation of the slurry and facilitates good draining. Capacities vary from 8 up to about 50 hi depending on the requirements of the brewery. Cooling jackets are usually provided to maintain the yeast slurry at a suitably low temperature. Occasionally, vessels may be unlagged and located in refrigerated rooms, although this is less satisfactory. Attemperation is assisted by provision of a mechanical agitator. As ever, hygiene is of paramount importance. Dedicated CiP is provided and a microbiological quality filter on the gas exhaust main is vital. As with any vessel the sample point must be designed and operated with thoughts of hygiene uppermost and preferably they should be of the steam sterilisable type. Vessels may be mounted on load cells to provide continuous information regarding pitching yeast stocks and usage.

Several yeast storage tanks are typically located in a separate room, for economy each being served by common mains and pumps. It is essential that such common mains are cleaned after every transfer in order to avoid cross-contamination of yeast

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