Yeast species composition, effect

If a particular yeast strain is chosen for the production of single cell protein, it is desirable to grow the chosen microbe on this same yeast to induce lytic enzyme production. The rationale for this is the considerable variation in cell wall composition between yeast species, and the most effective lytic enzyme complex is obtained by using the same strain as enzyme inducer. 

To overcome these difficulties, researchers have developed a molecular taxonomy over the last 15 years based on the following tests DNA recombination the similarity of DNA base composition the similarity of enzymes ultrastructure characteristics and cell wall composition. The DNA recombination tests have proven to be effective for delimiting yeast species. They measure the recombination percentages of denatured nuclear DNA (mono-stranded) of different strains. An elevated recombination rate between two strains (80-100%) indicates that they belong to the same species. A low recombination percentage (less than 20% of the sequences in common) signifies that the strains belong to different and very distant species. Combination rates between these extremes are more difficult to interpret. 

The yeast responsible for alcoholic fermentation in winemaking is usually introduced into the must from the surface of the grapes, the surface of winery equipment, or from specifically prepared cultures. The fermentation process can occur either naturally, without inoculation, or by inoculating the must with selected starters. The use of locally selected yeast strains (usually belonging to the species Saccharomyces cereoisiae), with strain-specific metabolic characteristics can positively affect the final quality of the wine (Regodon et al., 1997 Romano et al., 2003). Several studies have clearly shown the effects of indigenous and inoculated yeast populations on the wine volatile composition (Mateo et al., 2001 Nurgel et al, 2003). 

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