Baker's yeast is traditionally used throughout Europe and the United States as starter culture for the production of a large variety of wheat-based breads. In Scandinavia and northern Europe, sourdough bread made from rye by back-slopping is also popular. The European tradition for production of bread seems to have spread now all over the world, including Southeast Asia and Africa (8). In all cases the dominant yeast used as starter culture is Saccharomyces cerevisiae Meyen ex. E.C. Hansen, as described by Vaughan-Martini and Martini (91) and Barnett et al. (79). As an alternative baker's yeast, Saccharomyces exiguus has been used (107).
Baker's yeast is produced as a bulk product. The global yearly production amounts to 2 million tons, and a yearly growth of approximately 4% has been reported (108). The propagation of baker's yeast is based on a fed-batch process characterized by aerobic conditions and low carbohydrate concentrations (109). Except for the utilization of carbohydrates, optimization of baker's yeasts seems to focus on aspects quite different from those of brewing yeasts. Also, the number of commercially available baker's yeasts appears to be limited compared to brewing yeasts. Besides efficient utilization of maltose, maltotriose, and other fermentable carbohydrates present in the dough, the important technological properties of baker's yeasts appear to be biomass yield, formation of CO2, influence on dough structure, aroma formation, and a high resistance to stress conditions such as oxidative stress, drying, freezing, and thawing (8,109-111). The latter results in a demand for high yeast concentrations of trehalose as a protective agent (112). Also, the ability of the yeast strains to adapt, from the conditions during propagation and production to the conditions in the dough, needs to be considered (109).
Types of fermented cereals other than traditional wheat bread are seen especially in areas where European-style bread is not traditionally produced, as in a great part of Africa for example. Examples of indigenous fermented cereals are products based on fermented maize, millet, and sorghum. These products are most often produced by spontaneous fermentation in which S. cerevisiae is the dominant yeast species, in coexistence with LAB, especially (9,50,74,113).
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