Saccharomyces

It can reasonably be argued that the yeasts belonging to the genus Saccharomyces are among the most important of all organisms used in fermented foods, perhaps more so than even the lactic acid bacteria.These yeasts are required, after all, for the production of beer, wine, and spirits (not to mention bread), products that have a combined, world-wide economic impact in the trillions of dollars. In addition, the main species, Saccharomyces cerevisiae, is widely used as a model organism in biology and genetics, and its physiological and biochemical properties, as well as its genome sequence, have been well studied. At the gene level, Saccharomyces and other yeasts are much more complicated than are the bacte-ria.They have sixteen chromosomes, they can be diploidal or polyploidal (i.e., more than one set of chromosomes), and the size of their genomes is several times larger (more than 6,000 protein-encoding genes compared to about 2,000 in most lactic acid bacteria).

The taxonomy and nomenclature of Saccharomyces has been subject to rather regular and frequent revisions for more than a century. Although S. cerevisiae has long been one of the major species used in wine, brewing, and bread-making applications, the specific strains involved in the manufacture of these products are clearly different from each other and from laboratory strains of S. cerevisiae .The frequent changes in nomenclature have made it even more difficult to keep track of the particular species associated with a given fermentation.

Several examples illustrate this point. Prior to the 1970s, Saccharomyces ellipsoideus was recognized as one of the primary wine yeasts and was accorded species status. It was then reclassified as S. cerevisiae (and demoted to "synonym" status), although the earlier name continues to be used. Similarly, another group of wine strains classified as Saccharomyces uvarum were reclassified as Saccharomyces bayanus (or sometimes listed as Saccha-romyces bayanus subsp. uvarum). Finally, for many years, the species used in the lager fermentation (where lager is one of the two styles of beer) was classified as Saccharomyces carls-bergensis, which was also referred to as its synonym, Saccharomyces uvarum. Both species then merged as S. uvarum, only to be reclassi-fied yet again, first as Saccharomyces bayanus and then as Saccharomyces pastorianus, which is now the valid species name for the lager yeast. In the most recent volume (1998) of The Yeast, A Taxonomic Guide, there were fourteen accepted species of Saccharomyces. However, in a 2003 report (FEMS Yeast Research 4:223-245), only seven species were recognized: S. bayanus, Saccharomyces cario-canus, S. cerevisiae, Saccharomyces kudri-avzevii, Saccharomyces mikatae, Saccha-romyces paradoxus, and S. pastorianus.

Distinguishing between species of Saccharomyces is based primarily on morphological, physiological, and biochemical properties. These yeasts usually have a round spherical or ovoid appearance, but they may be elongated with a pseudohyphae. The sugar fermentation patterns and the assimilation of carbon sources are key factors for speciation (Table 26; also see Chapter 9 for beer yeast speciation). Other specific diagnostic tests include hyphae formation, ascospore formation, resistance to cycloheximide, and growth temperatures. Several physiological traits vary among the Sac-charomyces and are useful not only for classification, but are important for strain selection. Some strains, for example, are very osmophilic and halotolerant, and can grow in foods containing high concentrations of carbohydrates (e.g., high sugar grapes) or salt (soy sauce).

Nowhere, however, does the species of yeasts influence the fermentation as much as in brewing.As noted above, there are two general styles of beer, ales and lagers.Each requires a specific yeast, S. cerevisiae for ales and S.pas-torianus for lagers.These yeasts vary in several respects (reviewed in Chapter 9), but mainly on the basis of where they settle or flocculate in the beer. Hence, ale yeasts are referred to as top-fermenting yeasts because they tend to flocculate at the top of the fermentation vessel, and lager yeasts are referred to as bottom-fermenting because they settle at the bottom of the fermentor. It is important to note that however useful these phenotypic characteristics are for classification, they are not always consistent with species assignments based on the sequences of 18S rRNA and other regions.

Ecologically, Saccharomyces and other yeasts are ubiquitous in foods. In some fermented

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