Chapter 21

Beer Spoilage Organisms

Spoilage by Wild Yeast

Wild yeast is any yeast other than the pitching yeast. With the exception of lambic and other spontaneously fermented beers, wild yeasts are considered spoilage organisms in brewing and are avoided at all costs. 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.

Saccharomyces spp.

Saccharomyces spp. in general is considered to be the most widespread and the most hazardous and for that matter the greatest threat since they are similar to production strains. The majority of the Saccharomyces spp. brewing contaminants detected belong to S. cerevisiae but also other Saccharomyces spp. have been reported. The most problematic and much talked about Saccharomyces wild yeast is Saccharomyces diastaticus which has emerged since a few years as a spoiler and found worldwide in the brewery industry. S. diastaticus is capable of fermenting complex sugars—malto­tetraose and often dextrins—that brewer’s yeast cannot handle, and has been associated in cases of superattenuation in beer (Hough et al., 1982)

Non-Saccharomyces spp.

There are many non-Saccharomyces yeasts found in the brewery but most cannot compete with brewing yeasts. Non-Saccharomyces yeasts encountered in the brewery include representatives of the following genera—Brettanomyces, Torulopsis, Pichia, and Candida.

Brettanomyces spp.

Less frequently encountered than the genus Saccharomyces, but more troublesome as a producer of acetic acid off-flavor from acetaldehyde in the presence of oxygen is the genus Brettanomyces (Campbell, 1987). Brettanomyces (Brett) is capable of resisting high alcohol levels and can also grow in oxygen limited environments as well as a low pH environment which means it is able to thrive in alcoholic fermentation environments.

Torulopsis, Pichia, and Candida spp.

Turbidity may also arise from species of Torulopsis, which may consist of very small cells and are therefore slow in sedimenting (Fix, 1989). Breaking up of the film or pellicle produced by Pichia and Candida spp. may also produce turbidity, of which Pichia membranaefaciens and Candida vini are frequently recorded (Campbell, 1987). Pichia and Candida species will oxidize ethanol in a vessel exposed to air to produce acetic acid.

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