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Anaerobic (fermentative) beer- spoiling yeasts

Yeast contaminants that exhibit fermentative properties represent a serious problem within the brewery since they can compete directly with production strains. Furthermore, the similarity of these yeasts to brewing strains can make them difficult to detect. The characteristics and effects of fermentative beer-spoiling yeasts on the product and process are variable. Yeast species belonging to the genera Kluyveromyces, Saccharomyces, Torulas- pora, and Zygosaccharomyces can show an enhanced growth rate such that they have the capacity to displace the culture yeast over the course of serial re-pitching. Given that a brewing yeast slurry may be used from as few as 3-4 times to as many as >100 serial re-pitchings, it is clear that under certain circumstances this could lead to significant issues. With the exception of strains that exhibit killer activity (see section ‘Beer-spoiling yeasts and killer toxins'), fermentative yeasts do not harm the production strain but may compete for resources, reduce ethanol yield, and generate off-flavours. Although major changes to the end product can indicate their presence, often much less apparent and more subtle defects are caused. Fermentative beer-spoiling yeast strains often exhibit differences in sugar preference and patterns of nutrient utilization that can lead to variations in the concentrations of esters, higher alcohols, and VDK. In addition, many spoilage yeasts do not flocculate well and do not interact with finings, with the result that they can pass into conditioning where they can have negative sensorial effects on post-fermentation beer, as well as causing haze and turbidity. A summary of the characteristics and spoilage potential of aerobic beer-spoiling yeasts can be found in Table 11.2.

Table 11.2 Characteristics of typical anaerobic (fermentative) beer-spoiling yeasts

Genus

Common

species

Characteristics

Beer spoilage potential

Additional information

Hanseniaspora

(Kloeckera)

H. uvarum (K.

apiculata)

H.

valbyensis H. vineae

Apiculate (lemonshaped) yeast Fermentative yeast that prefers anaerobic conditions

Fermentation, turbidity, and off-flavours

Not typically associated with beer, but may be present due to crosscontamination, for example from wine barrels if used for conditioning

Ascomycete Hanseniaspora is the teleomorph, producing spores

Kloeckera is the anamorphic form.

Can be found in wine production and in certain lambic beers

Kluyveromyces

K.

marxianus

Various cell morphologies but typically ovoid/ ellipsoidal Fermentative yeast

Fermentation, turbidity and off-flavours

Ascomycete Found as a contaminant throughout the food industry, particularly in soft drinks and dairy products

Saccharomyces

S. bayanus S.

cerevisiae

S.

pastorianus

S.

unisporus

Spherical-shaped cells Includes nonproduction strains and variants

S. bayanus cells may be elongated Fermentative yeasts

Fermentation, turbidity, and off-flavours

Some strains have varying flocculation, which can interact with brewing yeast Some strains may be phenolic

Diastatic yeasts can yield especially low attenuation and reduced mouthfeel

Ascomycete

Cells often more irregular than brewing strains and size is typically smaller, which can result in haze/filtration issues

Some non-brewing strains will produce tetrahedralshaped spores

Schizosaccharomyces

S. pombe

Rod-shaped cells Fermentative yeast

Fermentation, turbidity, and off-flavours

Ascomycete

Divides by fission and can be readily distinguished from budding yeasts

Torulaspora

T.

delbreuckii

Spherical/ellipsoidal

cells

Fermentative yeast (some species are obligate fermenters) Grows poorly under anaerobic conditions

Fermentation, turbidity, and off-flavours

Associated with pitching yeast

Can also spoil unpasteurized beer

Ascomycete

Saccharomyces delbreuckii, Saccharomyces fermentati, and Saccharomyces rosei are obsolete synonyms T. delbreuckii is a teleomorph of Candida colliculosa Highly osmotolerant organism

Zygosaccharomyces

Z. bailii Z. bisporus Z. rouxii

Oval-shaped cells Fermentative yeast High sugar tolerance

Fermentation, turbidity, and off-flavours

Ascomycete

Stress-tolerant yeast found as a contaminant throughout the food industry Particularly associated with high-sugar products

 
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