Metabolic activity and interactions between two strains, Saccharomyces cerevisiae r.f. bayanus (SBC2) and Saccharomyces cerevisiae r.f. uvarum (S6u), in pure and mixed culture fermentations

We have studied the metabolic activity of and interactions between two strains of Saccharomyces cerevisiae r.f. bayanus (SBC2) and Saccharomyces cerevisiae r.f. uvarum (S6u), fermenting in a synthetic must medium in pure culture, co-inoculation and sequential inoculation. The second strain was added with or without sterile filtration. We monitored the rate of fermentation; at the end, total and viable cells, percentage of each strain, alcohol, volatile acidity, total sulphur dioxide, glycerol, acetaldehyde and volatile compounds were determined. The rate of alcohol production was different during fermentation: at the onset, S6u was faster than SBC2, while lateron it was inverse. When fermentation was stopped simultaneously, S6u showed the highest total cell number when grown in pure culture and the highest percentage of viable cells in mixed culture fermentation. Moreover, S6u produced low amounts of alcohol, but more glycerol and volatile compounds (i.e. 2-phenylethanol, acetates, ethyl esters, and fatty acids) than SBC2. The co-inoculated and the sequentially inoculated sample, in which S6u was the first strain, gave values similar to the pure S6u culture. Hence, we conclude that S6u prevails over SBC2 when both strains ferment in a medium. It seems that sequential inoculation of SBC2 as the second strain is of advantage only with regard to the relatively fast ethanol production.