Effects of Different Killer Yeasts on Wine Fermentations

¹ Department of Microbiology, University of Stellenbosch, Stellenbosch 7600, South Africa

Four K₂ killer yeast strains isolated from grapes and grape skins were compared in respect of their fermentation rates in Chenin blanc must, as well as their ability to produce toxin. Significant differences were detected in the amounts and/or activity of toxin produced, as well as in the fermentation rates. Two wine yeast strains (Vin 7 and GS-1) available commercially were tested for toxin sensitivity; GS-1 was found to be much more susceptible to the killer toxin than Vin 7. Mixed culture fermentations indicated that lagging fermentations were dependent on the killer yeast population, the amount of toxin produced by the killer, fermentation ability of the killer yeast, as well as the degree of sensitivity of the wine yeast strain. The dark green colony color displayed by killer strain K-103 on Modified Wallerstein nutrient agar medium was used to distinguish quantitatively between killer and sensitive yeasts during mixed culture fermentations. Results indicated that K-103, even at an initial concentration as low as 0.1% of the total population, eliminated the sensitive wine yeast.

Key words: killer yeasts, wine, fermentations

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