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The origins of brewing yeast

The majority of publications in the field have been focused on lager strains mainly because of the larger global volume associated with these beer styles. Although many of the observations concerning stress responses may also apply to ale and hefe ale yeast strains this is not always the case. One reason for these differences is that lager yeast strains are genetically different from their ale and wheat yeast peers and in fact from each other. Lager yeast strains are interspecific hybrids (meaning hybrids from two different species) but the exact parentage had been a matter of debate (Kodama et al., 2005) (see Chapters 4 and 6). Originally a model involving hybridization ofan ale strain of Saccharomyces cerevi- siae with a strain of the genetically complex species, Saccharomyces bayanus, was generally accepted. However, lager yeast also appear to contain DNA not common to either parent (Rainieri et al., 2006; Nakao et al., 2009), suggesting genetic input from an unknown ‘parental species'. This remained a matter of much debate until the discovery of a new species, Saccharomyces eubayanus, which has been revealed to be an exact genetic match to the non-S. cerevisiae parent in the complex (Libkind et al., 2011). The species was discovered associated with Nothofagus (southern Beech) in the forests of Patagonia and unpublished reports suggest it is also found elsewhere in the world but many workers in the field of yeast ancestry and diversity believe this to be but one example habitat from which such species have been derived.

The hybrid condition of S. pastorianus (S. cerevisiae x S. eubayanus) may be the reason for its psychrophilic nature, apparently inherited from the cold-tolerant S. eubayanus parent (Libkind et al., 2011). This characteristic has resulted in superior performance in the low-temperature environment of lager fermentations and confers resistance to cold during storage and yeast transfer in breweries.

Lager strains are categorized as Saaz and Froh- berg after the locations in Bohemia and Germany in which they were originally used. These two hybrid groups are used by modern brewers but are genetically distinct from one another (Liti et al., 2005; Dunn and Sherlock, 2008).

Ale brewing predates lager brewing, and although the yeast strains have been selected over time, their origins are less well documented or indeed studied. Ale strains belong to the species S. cerevisiae but strains can be really quite genetically distinct from one another.

Wheat beer yeast strains also belong to the species S. cerevisiae and are characterized by the occurrence of the PAD1 and FDC1 genes, encoding phylacrylic acid and ferulic acid decarboxylase, respectively. Both genes must be functional for the characteristic clove-like, phenolic character due to 4-vinylguaiacol (4-VG) to be produced.

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