Effect of diluent type on viability of yeasts enumerated from foods or pure culture

The effects of seven diluent types on the viability of yeasts enumerated from foods and in pure culture were studied. The diluents were laboratory glass distilled water; saline water (0.85% NaCl), sodium phosphate buffer (0.1 M, pH 7.0), 0.1% peptone, 0.1% yeast extract, 0.1% peptone in 0.1 M sodium phosphate buffer, pH 7.0, and 0.1% malt extract. For all foods studied, dilution in 0.1% peptone gave the highest counts, with saline and phosphate buffer diluents giving lower counts than those obtained with distilled water. When seven species of yeast were enumerated in pure culture, highest counts were obtained using 0.1% peptone as the diluent and, with three exceptions, all species gave higher counts when diluted in diluents other than distilled water. When yeast suspensions were held in diluents for up to 2 h before plating, cell death occurred. The extent of death was highest in distilled water, saline and phosphate buffer diluents. Cell death also occurred in 0.1% peptone, yeast extract and malt extract, but to a lesser degree.