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Taxonomy of beer-spoiling yeast species

As described by Sampaio et al. (see Chapter 4) and below (see ‘Molecular methods for identification of beer-spoiling yeast species'), yeast species are increasingly classified according to DNA sequence homology. However, fungi were historically organized and named based primarily on structures associated with sexual reproduction (i.e. spore morphology), along with physiological characteristics, including cell size and shape, and nutritional requirements. While much of this information remains useful today, it has inevitably led to confusion in naming and identifying species, since the

predominant mode of replication for many yeasts associated with industrial processes is asexual (i.e. non-spore forming). Furthermore, those species that do undergo sexual reproduction only do so under certain conditions and only one method of reproduction is typically observed at a specific point in time. Hence, in practice, many fungi have multiple names based on their sexual state when they were first isolated, a phenomenon particularly relevant to beer-spoiling yeasts. For example the genus names Brettanomyces and Dekkera refer to the same organism; Brettanomyces refers to the asexual (anamorph) form, while Dekkera is the name used to describe the sexual form (teleomorph). Current consensus within the world of yeast taxonomy is that fungal species should be referred to by a single name and that teleomorphic names cannot have priority over anamorphic ones (McNeill et al., 2012; Gams, 2016). However, it is also acknowledged that the designated genus name will often be guided by common usage. Hence, within the brewing industry it is still acceptable (if not desirable) that certain nomenclatures will continue to be used. It is anticipated that this rule should lead to greater clarity in the future. Previously there has been a degree of confusion within the brewing industry, especially with regard to spoilage yeasts that were originally classified as separate species, or those that have undergone a number of re-classifications. To illustrate this point, Pichia anomola (previously known as Hansenula anomola) is a teleomorph of Candida peliculosa; each form has different characteristics and exerts potentially different impacts on the product and process. Other yeasts have been described as having a ‘tortuous history' due to nomenclature confusion caused by repeated reclassification. One particular example is the yeast Candida utilis, a

Table 11.1 Typical points of contamination for beer-spoiling yeasts

Process stage

Species

Raw materials Pitching yeast High sugars

Aerobic stages of fermentation Fermentation Bottle conditioning Draft beer

Various locations (non-spoilers)

Candida, Cryptococcus, Debaryomyces, Pichia, Rhodotorula

Saccharomyces, non-Saccharomyces

Zygosaccharomyces, Kluyveromyces

Candida, Hanseniaspora (Kloeckera), Pichia

Saccharomyces

Brettanomyces

Candida, Brettanomyces, Torulaspora, Hanseniaspora (Kloeckera), Pichia Rhodotorula, Cryptococcus

potential beer-spoiler and an organism used in feed production, which has been known by five different names: Torula utilis, Torulaspora utilis, Hansenula jadinii, Pichia jadinii and C. utilis (Barnett, 2004). Within this chapter, the anamorphic names of species are utilized; where common usage dictates that the teleomorphic name is applied, the corresponding anamorphic term is added in parentheses.

 
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