Physiological characterization of spoilage strains of Zygosaccharomyces bailii and Zygosaccharomyces rouxii isolated from high sugar environments.

Two isolates of spoilage yeasts Zygosaccharomyces bailii and Zygosaccharomyces rouxii were obtained from a high sugar environment, in a factory producing candied fruit and nougat. Other strains, Z. bailii from other environments and other isolates from high sugar/salt environments were obtained for comparison (Zygosaccharomyces lentus, Candida magnoliae, Candida halophila and Pichia guilliermondii). A full physiological assessment of these isolates was carried out, of preservative and biocide resistance, osmotolerance, ethanol-tolerance, low pH resistance, degree of fermentation, and growth temperature and survival to pasteurisation. Results showed that the strains isolated from high sugar environments did not show extreme physiology. These were robust strains but within the normal parameters expected for the species. One exception to this was that the Z. bailii strains were abnormally able to grow at 37 degrees C. In all strains other than C. magnolia and C. halophila, cells were able to adapt to high levels of sugar. Cultures grown in high glucose concentrations were subsequently able to tolerate higher concentrations of glucose than previously. Similarly, high sugar was found to confer a degree of protection against pasteurisation, enabling survival in what would have otherwise been a lethal treatment. Isolates of Z. bailii showed a high level of resistance to preservatives such as sorbic acid, benzoic acid, acetic acid, cinnamic acid, and ethanol, and also to heat. Unexpectedly Z. bailii isolates were not exceptionally resistant to biocides such as peracetic acid, or hypochlorite. These results indicate that spoilage by yeasts such as Z. bailii may be better prevented by use of biocidal cleaning agents in the factory, rather than treating the food with preservatives.