A differential medium for the enumeration of the spoilage yeast Zygosaccharomyces bailii in wine

A collection of yeasts, isolated mostly from spoiled wines, was used in ord er to develop a differential medium for Zygosaccharomyces bailii. The 118 s elected strains of 21 species differed in their origin and resistance to pr eservatives and belonged to the genera Pichia, Torulaspora, Dekkera, Debary omyces, Saccharomycodes, Issatchenkia, Kluyveromyces, Kloeckera, Lodderomyc es, Schizosaccharomyces, Rhodotorula, Saccharomyces, and Zygosaccharomyces. The design of the culture medium was based on the different ability of the various yeast species to grow in a mineral medium with glucose and formic acid (mixed-substrate medium) as the only carbon and energy sources and sup plemented with an acid-base indicator. By manipulating the concentration of the acid and the sugar it was possible to select conditions where only Z. bailii strains gave rise to alkalinization, associated with a color change of the medium (positive response). The final composition of the mixed mediu m was adjusted as a compromise between the percentage of recovery and selec tivity for Z. bailii. This was accomplished by the use of pure or mixed cul tures of the yeast strains and applying the membrane filtration methodology . The microbiological analysis of two samples of contaminated Vinho Verde s howed that the new medium can be considered as a differential medium to dis tinguish Z. bailii from other contaminating yeasts, having potential applic ation in the microbiological control of wines and probably other beverages and foods.