Chapter 19

Beer Spoilage Organisms

Wild Yeast

Wild yeast is any yeast other than the pitching yeast. Wild yeast can be isolated at all stages of the brewing process from raw materials, wort, pitching yeast, and fermenting beer, through to the packaged product and the dispense system. Wild yeast can produce unintended flavors because of differences in ester, fusel alcohol, and diketone production. They are particularly known for producing phenolic or medicinal notes. In the presence of air, some wild yeast can grow rapidly and form a film on the surface of the beer, which can cause haze. Other effects may include primary yeast fermentation and separation difficulties, significantly lower terminal gravities, and a higher, alcohol content in the finished beer. The lower terminal gravities are due to the ability of wild yeast to ferment sugars (such as maltotetraose and dextrins) not used by the primary yeast.

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