All breweries generate a surplus of yeast, which must be disposed of, preferably with some financial gain. Usually the yeast will be in the form of pressed cake after processing to recover barm ale. Spent yeast comes in various states of purity and physiological condition depending on which stage of the brewing process it is recovered from. Many breweries have dedicated storage tanks that are used for temporary holding of waste yeast. Associated with the tanks is specific plant for separating the yeast and beer. This may take the form of a large plate and frame filter (Fig. 2.12) or some other similar device.
Much of the yeast that is recovered from conditioning tank bottoms and similar sources is heavily contaminated with other solid materials and in order to facilitate efficient pressing has to be mixed with a filter aid such as kieselguhr. This material has no commercial value and has to be disposed of. In modern breweries, which do not require the presence of yeast during cold conditioning, it would be more appropriate to identify methods for avoiding the formation of yeast-containing tank bottoms. Possibilities would include the use of highly efficient green beer centrifuges capable of removing all yeast from the process stream during fermenter run-down and producing a dry yeast discharge which could be disposed of without further processing.
The highest quality yeast is that destined for use as pitching yeast but surplus to production requirements. Some of this may be sold for use in other fermentation industries, for example, to distillers (Bathgate, 1989). More commonly, such yeast is sold for further processing to produce yeast extracts, which are used in many food products. In addition, brewer's yeast is an excellent source of many vitamins and trace metals, and for this reason, it is used to prepare dietary supplements.
A common goal for many brewers has been to identify methods of increasing the value of spent yeast. Suggestions have included genetic modification to produce heterologous proteins to 'add value' (see Section 4.3.4). Other proposed routes have included the recovery and purification of fine chemicals for use in other chemical and pharmaceutical industries. This avenue has yet to be exploited because of the high initial investment and the uncertainty of finding a market.
Brewing Yeast and Fermentation Chris Boulton, David Quain Copyright © 2001 by Blackwell Publishing Ltd
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