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mutually exclusive applications. For example, strains of S. cerevisiae used in baking or winemaking may produce splendid bread or wine but quite unacceptable beers. Similarly, there are many environmental 'wild' yeasts, which, although taxonomically S. cerevisiae, result in the 'spoilage' of beer through the formation of unpleasant flavours or aromas. An excellent example of this is those strains of S. cerevisiae able to ferment non-fermentable oligosaccharides or starch (formally described by the synonym of S. diastaticus). These strains cause spoilage through fermenting further than brewing strains as well producing an unpleasant 'phenolic' type aroma. To complicate matters further, S. cerevisiae is not exclusively an industrial microorganism but is perhaps the most popular laboratory organism used world wide as a 'model eukaryote'. These strains differ significantly from brewing strains of S. cerevisiae in that they are genetically simpler and are well defined. Such strains lack the robustness and genetic complexity of their taxonomic relatives and are incapable of producing anything remotely like beer!

Whilst the taxonomic debate lumbers on, there is compelling evidence that ale and lager strains are different. As noted above, the genomes of S. cerevisiae and S. pastorianus have some commonality (e.g. the 'cerevisiae' genome) and fundamental differences (e.g. the S. bayanus element in the S. pastorianus genome). A fundamental point of difference well known to taxonomists and yeast physiologists is the capability of lager strains to utilise the disaccharide melibiose (a-D-galactose-(l->6)-a-D-glucose). Genetically, this capability is complex (Turakainen et al., 1994) involving up to ten MEL genes, which are exclusive to strains fermenting melibiose (Naumov et al., 1995). Lager yeasts do not grow on melibiose as such but on the products of its hydrolysis - galactose and glucose - via a-D-galactosidase (for a review see Barnett, 1981).

This has provided a simple route to the differentiation of ale and lager strains of S. cerevisiae (see Sections 4.2.5.2 and 4.2.5.6). The activity of the a-D-galactosidase is highly regulated being induced by galactose but repressed by glucose (Gadgil et al., 1996). In passing, 'melibiose utilisation' has been one of the targets of the genetic modification of bakers' yeast. Beet molasses, the feedstock for bakers' yeast, contains up to 3% raffinose which is normally unavailable to the 'highly specialised strains' used in the baking industry (Vincent et al., 1999).

Lager yeasts also exhibit a greater affinity for galactose than ale strains. In aerobic culture, lager strains metabolise galactose and maltose simultaneously whereas ale strains ferment the maltose preferentially (Crumplen et al., 1993). Lager strains also appear to form more sulphite than ale yeasts in wort fermentations (Crumplen et al., 1993). Other differences have been reported in the utilisation of the wort sugar, maltotriose which lager strains seem able to use more rapidly than ale strains (Stewart et al., 1995). As noted earlier (Section 4.2.1), bottom-fermenting yeasts (S. pastorianus) differ from ale strains (S. cerevisiae) in that lager yeasts transport fructose via a proton symport mechanism (Rodrigues de Sousa et al., 1995).

However, process development and change have undermined some of the more empirical distinguishing features between ale and lager yeasts. For example, as rule of thumb, ale yeasts were 'top-fermenting yeasts' and lager yeasts were 'bottom-fermenting yeasts'. Today, such a distinction has become blurred. The use of large cylindroconical fermenting vessels has resulted in ale yeasts - classically top fermenting - becoming through selection essentially bottom fermenting. Similarly, it is a fundamental 'rule' that lager yeasts perform ideally at low temperatures (8 to 15°C) whereas ale yeasts operate best at higher temperatures (approx. 20°C). Physiologically this is borne out by laboratory studies (Walsh & Martin, 1977) that show ale strains to have a higher maximum growth temperature (37.5 to 39.8°C) than lager strains (31.6 to 34.0°C). Similarly, there are differences in optimal growth temperature (Topt) with lager yeasts (S. pastorianus) having a Topt below 30°C and ale strains (S. cerevisiae) a Topt above 30°C (Guidici et al., 1998). These differences are used taxonomically to type yeast species within the Saccharomyces, where S. pastorianus never grows above 34°C whereas strains of S. cerevisiae happily grow at 37°C (Vaughan-Martini & Martini, 1998). Further, a detailed comparison (Walsh & Martin, 1977) of growth rates between 6 and 12°C, showed a lager strain to grow significantly more quickly than an ale strain. Consequently, it would appear that lager strains are better 'equipped' than ale strains to grow at lower temperatures. It is tempting to conclude that this reflects the contribution to the lager yeast genome of S. bay anus which is noted to be 'cryophillic' (Naumov, 1996). However, the view that lagers can only be fermented at low temperatures and are unacceptable when produced at ale temperatures (Lewis, 1974) has been challenged. Perfectly acceptable, and, in terms of product matching, indistinguishable lagers can be produced at ale fermentation temperatures (unpublished observations, David Quain & Chris Boul-ton).

As noted above, lager yeasts are by no means as diverse as ale strains. In a fascinating account Casey (1990) notes that top-cropping yeasts had been in use - mostly unknowingly - for at least 3000 years (Samuel, 1997). Conversely, bottom-cropping lager yeasts were used exclusively by Bavarian brewers until the 1840s when they were smuggled to Czechoslovakia and Denmark. With increasing trade and travel, lager yeasts were soon being used worldwide. Consequently, compared to ale yeasts, the diversification of lager strains is relatively new. Indeed, Casey (1996) - using chromosomal fingerprinting (see Section 4.2.6.3) - has shown that a selection of lager strains from throughout the brewing world have essentially one of two basic fingerprints ('Tuborg' and 'Carlsberg') with small but reproducible differences between strains. Conversely, similar analysis of numerous ale yeasts failed to show any form of common fingerprint. Casey (1996) concludes that ale yeasts 'lack a common origin' with 'breweries in different regions' selecting 'unique yeast strains for each location'.

Casey's observations are notable as they strike at the heart of strain diversity. The lager strains originate from one or two common sources but although closely related are genuinely different. This clearly points at genetic change through environmentally driven adaptation or through ongoing chromosomal rearrangement. Whatever the driver, it is increasingly recognised that genetic change or 'instability' is far more common than previously thought in brewing yeast (see Section 4.3.2.6). Further complexity is introduced when considering the contribution of the environment to strain diversity. Not only would the indigenous population of S. cerevisiae be expected to be influenced by geography but - overlaid on top - are genetic changes that may (or may not) offer some selective advantage. Musing on the genetic diversity of brewing yeast strains is inevitably speculative. Perhaps, in the future, Casey's initial work will be developed and the genealogy of ale yeasts will begin to be unravelled.

There is no universal view as to the commercial need for a diversity of brewing yeast strains. Some breweries derive their portfolio of products from one or two yeasts. Other breweries maintain that a diversity of different brewing yeasts are necessary to produce a diversity of products. If anything, the 'reductionists' have gained the upper hand. Brewery closures and consolidation of brewery production within large efficient breweries have tended to result in the culling of yeast strains servicing relatively minor brands. Although doubtless necessary, this is unfortunate as the rich diversity of the currently unfashionable or process-difficult yeasts are being lost forever.

4.2.3.1 Culture collections of brewing yeasts. One solution to this loss of diversity is to purchase suitable yeast strains from a culture collection. Here, yeasts have been deposited - often anonymously - which are being, or more often, have been used in commercial fermentations. Most collections also maintain 'type' strains, genetic strains, genetically modified strains and general strains 'of interest'. Invariably, a small biography is attached which details what is known about the yeast strain (source, use, flocculence, genetic markers etc.). It is usual practice to store strains in, or above, liquid nitrogen or, occasionally, freeze-dried to assure long-term survival and minimise genetic change.

Arguably, the best-known brewing yeast culture collection is the UK's National Collection of Yeast Cultures (NCYC) which currently holds almost 700 strains. Deposited strains date back to 1910 but were presumably in use before that (Anonymous, 1995). Although selection of strains can be made conventionally via a hardcopy catalogue, the NCYC database can be searched via the Internet according to the desired strain criteria (attenuation, head formation, flocculence, rate of fermentation etc.). Other important collections that include brewing yeasts (often local) are the American Type Culture Collection (ATCC), the Centraalbureau voor Schimmel-cultures (CBS) in Holland, VTT Culture Collection in Finland and BCCM in Belgium. Details of how to contact these collections are given in Table 4.11.

Table 4.11 Commercial collections of yeasts.

Collection

Address

American Type Culture Collection (ATCC)

Centraalbureau voor Schimmelcultures (CBS)

National Collection of Yeast Cultures (NCYC)

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