Yeast quality assessment, management and culture maintenance

GGStewart Associates, Cardiff, Wales, UK The International Centre for Brewing Distilling, Heriot-Watt University, Edinburgh, Scotland, UK 

The objectives of wort (unfermented beer) fermentation are to consistently metabolize wort constituents into ethanol, carbon dioxide and other fermentation products to produce beer with satisfactory quality and stability. It is also important to produce yeast crops that can be confidently repitched into subsequent brews. This is unlike distiller s yeast strains where the yeast culture is used only once it is not repitched. 

During the brewing process the overall yeast performance is controlled by a plethora of factors. These include  

All of these factors, individually or more often in combination with one another, permit the definition of the requirements of an acceptable brewer s yeast strain (Stewart Russell, 2009). To achieve beer of high quality, not only the yeast must be effective in receiving the required nutrients from the growth/fermentation medium (the wort), able to tolerate the prevailing environmental conditions (e.g. osmotic, temperature and ethanol tolerance) and impart the desired flavour to the beer, but the microorganisms themselves must be effectively removed from the fermented wort by flocculation, centrifugation and/or filtration after they have fulfilled their metabolic role. 

It has already been discussed that a major difference between brewer s yeast strains and other alcoholic producing yeasts is that brewers recycle their yeast cultures from one fermentation into a subsequent fermentation. It is therefore important to protect the quality of the cropped yeast culture because it will be used to pitch a later fermentation and will consequently have a profound effect on the quality of the resulting beer produced with it. Distillers (e.g. Scotch whisky producers) use a yeast culture only once. This introduces a separate series of selection criteria for a yeast culture that are beyond the scope of this chapter. 

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Yeast quality assessment, management and culture maintenance ... 

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