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Aerobic (non-fermentative) beer- spoiling yeasts

The use of the phrase ‘non-fermentative' is slightly misleading, as many species of yeast within this category can ferment (albeit weakly) under certain conditions. However, others are unable to ferment complex sugars or only show limited replication capacity under anaerobic conditions; the fact that these yeasts require oxygen for robust growth suggests that the term ‘aerobic' may be a more appropriate descriptor. Aerobic yeasts are more likely to be found within areas of the brewery that are not associated with the fermentation step, for example raw materials, surfaces and equipment, and in final pack. However, it is important to note that even aerobic species are often able to survive at low concentrations during fermentations and persist through serial re-pitching. Generally their presence during the fermentation stage is unlikely to significantly affect the final product since they will most likely remain below a threshold cell number or be out-competed by the production strain. Of the aerobic beer-spoiling yeasts, Pichia and Candida species are arguably the most important since they are relatively prevalent ( Jespersen and Jakobsen, 1996); however, Kluyveromyces, Torulaspora and Brettanomyces can also act as opportunistic spoilers during the process, particularly during the aerobic phase of fermentation and in situations where oxygen ingress is difficult to prevent, such as in unpasteurized cask beers. A summary of the characteristics and spoilage potential of aerobic beer-spoiling yeast can be found in Table 11.3.

 
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