HYDROPHOBIC MEMBRANE FILTRATION METHOD FOR THE SELECTIVE RECOVERY AND DIFFERENTIATION OF ZYGOSACCHAROMYCES-BAILII IN ACIDIFIED INGREDIENTS

A single-step enumeration and identification pour plate medium was developed for Zygosaccharomyces bailii that totally inhibited environmental nonspoilage yeast and mold growth and supported high (93.9-97%) Z. bailii recovery rates. It consisted of Sabouraud dextrose agar supplemented with fructose (3%), tryptone (0.5%), trypan blue (0.025%), and yeast extract (0.25%) to stimulate Z. bailii growth rate. Selectivity was based on the synergistic combination of NaCl (2.5%), acetic acid (0.5%), and potassium sorbate (0.01%). The recovery efficiency and specificity of the medium were significantly improved when it was adapted to the Hydrophobic Grid Membrane Filtration surface plating procedure. Incubation time was reduced from 5 to 2-3 d, and Z. bailii was visually differentiated from oxidative, acetic acid tolerant yeasts (Hansenula and Pichia spp.) by surface colony morphology. The optimized method assisted in successfully identifying and correcting Z. bailii contamination problems in acidified ingredients including pickled vegetables and pasteurized fruit juice concentrates.