Improved Detection of Wild <i>Saccharomyces</i>

Abstract

Lin's wild yeast medium was modified to be specific for wild Saccharomyces. The addition of 0.6% ET80 (ergosterol, 4.2 mg/ml, in 50% v/v Tween 80 and ethanol) to Lin's medium allowed wild Saccharomyces to grow when plates were incubated under anaerobic conditions and effectively suppressed non-Saccharomyces yeast. To maintain effective suppression of pitching yeast, the crystal violet concentration was raised to 6 mg/L; equivalent suppression in unmodified Lin's medium incubated aerobically required a concentration of 1.2 mg/L. Adjustment of basic fuchsin did not improve selectivity. With the exception of Saccharomyces bayanus, all pure cultures of wild Saccharomyces could be differentiated, although the growth response of S. diastaticus was weak. All spoilage yeasts isolated from filled containers during two investigations grew well on crystal violet agar, suggesting that these media are well suited for routine use in the brewery. The modified media were suitable for use with polycarbonate membrane filters. Selective media containing ET80 could also be used with cellulose acetate membrane filters but not with any membrane filter containing cellulose nitrate.