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Suggested Response - Q5

**Discuss the classification and effects of yeasts that are described as wild yeast contaminants in a lager fermentation.

Response:

A wild yeast is described as any other yeast in beer besides the pitched yeast.

Wild yeasts in beer are classified as either Saccharomyces or non-Saccharomyces species.

If a lager fermentation is contaminated by an ale yeast, such as S. cerevisiae, this too can be classified as a wild yeast.

Most wild yeasts in beer are those of the Saccharomyces genus, where the common wild yeast flavour produced is that of the phenol 4-vinylguiacol or 4VG – the spicy, clove-like flavour present in German weissbiers and some Belgian styles.

S. cerevisiae var. diastaticus is a wild yeast that has the ability to utilise dextrins as an energy source, by virtue of possessing the glucoamylase gene. Glucoamylase, otherwise called amyloglucosidase or AMG, attacks dextrins, thereby producing simpler sugars that are then fermented, causing super attenuation of the wort or beer.

The non-Saccharomyces group contains different genera such as Candida, Pichia, Torulaspora, and Zygosaccharomyces. These are opportunistic contaminants and can grow in aerobic conditions, although this is commonly in unpasteurised cask conditioned ales, so their occurrence in lagers would be unlikely.

Brettanomyces (Dekkera) produces the volatile phenols 4-ethylguaiacol and 4-ethylphenol, which are described as barnyard, horse sweat, plaster, and smoke. A number of other metabolites, including tropical fruit esters and acids, can be produced from glucose.

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