Fig. 6.40 (a) Readout from a permittivity biomass meter, (b) Profile of total solids and viable yeast count measured in-line during initial run-down of an 8 hi pilot scale cylindroconical fermenter (redrawn from Boulton & Clutterbuck, 1993).

'racking'. Management of this process is dependent on the type of fermentation and the beer quality. The essential features of this stage are control of yeast count and temperature. In addition, it is important to prevent increase in oxygen concentration in the beer as it passes onto the next phase of processing.

In traditional fermentation systems the style of vessel, the yeast strain used and the way the process is managed ensure that the yeast count at the end of fermentation is that which is required. In particular, there has usually been an element of 'natural selection' such that a yeast strain with appropriate properties has been chosen. The essential flexibility of modern fermentation systems, which allow the production of many beer types, is countered by the need to provide a means of controlling the yeast count during racking.

In the case of traditional top-fermented beers, moderate cooling is applied at the end of primary fermentation, usually to between 10 and 15°C. During vessel rundown, the aim is to achieve a suspended yeast count in the region of 1 x 106 cells ml 1

and a residual fermentable extract equivalent to roughly 0.75°Plato. Occasionally silica-based auxiliary finings may be added to fermenter, after cooling, to precipitate proteinaceous materials and promote better fining activity in cask. In some cases, the beer may be fully attenuated, in which case priming sugars are added later in the process to provide fermentable carbohydrate to fuel secondary fermentation. Some breweries reduce the yeast count to a minimum value during racking by the use of a continuous centrifuge, the operation of which is described subsequently. In this case, a further inoculum is made to provide yeast for secondary fermentation. This approach has the advantage that another yeast strain may be selected for use in the secondary fermentation, different to that used in primary fermentation.

For traditional lager fermentations, the required residual extract is 0.5-l°Plato, similar to that for top-fermented ales, however, a yeast count of 2-4 x 106 cells ml Ms required. Racking temperatures are lower than for ales, typically 4-8°C. During transfer to the lagering tanks, it is essential to ensure that the process is anaerobic. Thus, the connecting main and destination tank must be purged with carbon dioxide to remove air before transferring the beer. Where modern vessels are used for the production of chilled and filtered beers, where there is no secondary fermentation, it is necessary to reduce the residual yeast count as much as possible. This is achieved by passing the beer through a continuous centrifuge of a similar type to that shown in Fig. 6.41. This is a cylindrical device constructed of hygienic stainless steel. The cylinder contains a vertically arranged assembly of rapidly rotating discs. Green beer is introduced into the base of the centrifuge where it is thrown onto the discs. Yeast cells and other solids migrate across the surface of the discs from the centre to the outside, under the influence of the centrifugal forces, whilst the clarified beer passes through and exits from the top of the centrifuge. At the outer edge of the discs, the solids are thrown against the inner wall of the centrifuge bowl. At regular intervals,

Solids discharge

Green beer inflow

Solids discharge

Green beer inflow

Fig. 6.41 Cross-sectional view of a continuous intermittent discharge centrifuge.

the two halves of the centrifuge bowl open automatically and the accumulated solids are discharged into a separate collection main.

Centrifuges operate at a constant speed. The efficiency of separation of solids from beer is controlled by adjustment of the number of discs and regulation of the flow rate. The solids discharge is directed towards storage tanks, prior to being sent to a press, where the beer is recovered and the yeast disposed of. Rarely, the yeast may be recovered and utilised for re-pitching (Donnelly & Hurley, 1996).

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