Brewers Yeast

Maintenace of Harvested Yeast

Yeast Concentration

The yeast slurry or "barm" removed directly from the fermenter usually has the consistency of thin cream, of which a major proportion is beer. There are a number of disadvantages in direct reuse of barm yeast for repitching. One problem is the possibility of carrying infection forward and another is that the yeast concentration is variable within the slurry (34). Consequently, the barm is washed and concentrated prior to its use as pitching yeast. Bacterial infection levels can be controlled by yeast washing. Rotary vacuum filters and simple mesh strainers can be used to separate yeast; however, the most common system of yeast separation is the plate and frame filter, or yeast press (34).

Culture Contamination

It frequently happens that brewing yeasts carry a persistent low level of contaminants such as Obesumbacterium proteus, acetic acid bacteria, and slow-growing Torula-type yeasts. These organisms are generally regarded as harmless because their numbers never reach a point where they are likely to have adverse effects on the beer. On the other hand, L. pastorianus, Z. anaerobia, and S. carlsbergensis are strains considered harmful at low levels. Lager fermentations seem to be more susceptible to bacterial contamination than do ale fermentations primarily because the pH drops more slowly for lagers than for ales and bacteria are suppressed by falling in pH.

Yeast Washing

Pitching yeasts collected from brewery fermentations are never absolutely free of microbiological infection. In spite of whatever care and sanitary precautions are taken, some bacteria and wild yeast will contaminate the pitching yeast. To minimize microbiological infection, brewery yeast can be washed using the following procedures.

Yeast Storage

In most breweries, yeast is stored during the period between cropping and repitching. Pitching yeast may be stored within the brewery as a slurry or as a barm in a yeast collection vessel, or it may be stored under a layer of water or beer, under a layer of wort, or as pressed cake.

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