The present invention relates to the unexpected discovery of a novel yeast culture medium comprising a concentration of cupric sulfate sufficient to promote growth of strains of Saccharomyces cerevisiae var. diastaticus while inhibiting growth of other varieties of brewing yeast. In certain embodiments, the invention provides a yeast culture medium. In other embodiments, the invention provides methods of using the novel yeast culture medium to test yeast slurries for contamination with Saccharomyces cerevisiae var. diastaticus.