Differential medium for the detection of the food and beverage spoilage yeast Zygosaccharomyces bailii

Abstract

Zygosaccharomyces bailii is a frequent food and beverage spoilage yeast, which is able to survive at low pH in the presence of weak organic acid preservatives. A collection of yeasts, isolated mostly from spoiled wines, was used in order to develop a differential medium for Z. bailii.. The strains selected differed in their origin and resistance to preservatives, belonging to the species Pichia membranaefaciens, Pichia anomala, Torulaspora delbrueckii, Dekkera anomala, Dekkera bruxellensis, Debaryomyces hansenii, Saccharomycodes ludwigii, Issatchenkia orientalis, Kluyveromyces marxianus, Kloeckera apiculata, Lodderomyces elongisporus, Schizosaccharomyces pombe, Rhodotorula mucilaginosa, Saccharomyces cerevisiae, Saccharomyces pastorianus, Saccharomyces bayanus, Zygosaccharomyces rouxii, Zygosaccharomyces florentinus and Z. bailii. The design of the culture media was based on the different ability of the various yeast species to grow in a mineral medium containing a single substrate (weak carboxylic acid) or mixed substrate (a sugar plus a weak carboxylic acid) as the only carbon and energy sources. When the assays were carried out in liquid medium with single substrate most of the strains displayed ability to utilize at least one of the several weak carboxylic acids tested. The nature of the acid and its concentration as well as the manipulation of the pH of the medium, associated to the incorporation of an acid-base indicator, allowed to select conditions where only Z. bailii strains gave rise to change in colour of the medium (positive response). However, these positive responses were only obtained after about 115 to 168 h. When the mixed substrate medium (weak acid plus sugar) was used, all the Z. bailii strains gave positive response after a considerably lower time (about 48 h) when compared with the single substrate medium for the same inoculum density and incubation conditions. Following these results the mixed substrate medium was tested for the other species using either microplate wells or solid medium. In both cases, only the Z. bailii strains gave positive response during the first 48 h of incubation. We concluded that such mixed substrate medium could be considered as a differential medium to distinguish Z. bailii from other dangerous yeast in food and beverages, with potential application.