Yeasts are involved in both spontaneous and controlled fermentations. For spontaneous fermentation processes, the yeasts are introduced by either the raw materials or via the process equipment (74-76). When yeasts are used as starter cultures, they are in general used as single cultures and may be introduced either to initiate the fermentation process or at a later stage in the fermentation to ensure optimal aroma production. Most yeast species are able to grow under both aerobic and anaerobic conditions. However, some yeast species are specifically respiratory yeasts whereas others are fermentative yeasts for which respiration is repressed even at aerobic conditions. Primarily, yeasts utilize carbohydrates as carbon sources, which are converted into alcohols and CO2 as well as a number of secondary metabolites such as esters, organic acids, aldehydes, and ketones (77).
Yeasts involved in the fermentation of foods and beverages belong primarily to the ascomycetous yeasts. Among these, the most well described yeast species is undoubtedly Saccharomyces cerevisiae. This species is used worldwide for the production of bread, wine, beer, cheese and so forth and is overall the predominant yeast starter culture in use. Other important yeast species are Saccharomyces pastorianus, used for production of lager beer, and Debaryomyces hansenii, used for production of cheese and fermented meat products. A microscopic picture of D. hansenii cells is shown in Fig. 2. Yeast species other than the above mentioned are potential starter cultures and do often occur in high numbers during spontaneous fermentations (Table 9). The evolvements of DNA technologies and molecular typing techniques have over the recent decades influenced significantly the taxonomic position of many yeast genera and further reorganizations are expected in the future. For a current taxonomic description of yeast species, the taxonomic keys of Kurtzman and Fell (78) and Barnett et al. (79) should be consulted.
The benefits obtained by moving from spontaneous fermentations to controlled fermentations are many and, therefore, there seems to be a growing interest in the use of yeasts as purified starter cultures, not only in the control of existing fermentation processes but also in the development of new food products. In controlled fermentation, the habitat of the yeast species as well as the various functions of different yeast species should be taken into consideration as well as any probiotic property (80,81) or possible pathogenic hazard (82,83). Also, the taxonomic position of the starter cultures must be clarified and methods for typing at subspecies level introduced.
Especially for brewing, wine, and baker's yeasts, efforts have been made to improve the technological properties of the strains by formation of genetically modified organisms (GMO). However, in most cases the GMOs have not been used in industrial food fermentations due to consumer resistance and statutory regulations (7).
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