Performance of selective media for enumerating Zygosaccharomyces bailii in acidic foods and beverages

Abstract

A study was undertaken to evaluate the performance of yeast- malt extract agar (YMA) (control medium) and three selective media, acidified YMA (AYMA), acidified tryptone-glucose-yeast extract agar (TGYA), and Zygosaccharomyces bailii selective agar (ZBA), for detecting and enumerating nine strains of Z. bailii grown in six commercial food products with a(w) and pH values ranging from 0.82 to 0.99 and 2.99 to 6.47, respectively. These media were also evaluated for their suitability for enumerating two strains of Z. bailii grown in blueberry syrup containing 0, 300, and 600 μg of sodium benzoate ml-1 and subsequently stored at 1 and -19°C. The nonselective enumeration medium (YMA) supported significantly (P ≤ 0.05) higher recovery of all strains of Z. bailii from the six food products compared to the three selective media; TGYA was the best selective medium, followed by ZBA. The performance of the selective media was dependent on the strain of Z. bailii and the food type. Recovery of cells of Z. bailii from blueberry syrup before storage or after storage at 1 or -19°C was equivalent on YMA, TGYA, and ZBA but inferior on AYMA, regardless of the benzoate concentration in the syrup in which the cells had grown. Cells in blueberry syrup held at -19°C exhibited a higher sensitivity to the acidic environment imposed by selective media compared to cells held at 1°C. Sensitivity to selective media was more apparent in cells grown in syrup containing no sodium benzoate compared to cells grown in syrup containing 300 or 600 μg of sodium benzoate ml-1. It is recommended that TGYA be used for enumerating Z. bailii in acidic foods with reduced a(w), regardless of the presence of sodium benzoate in these foods or the reduced temperature at which they are held before being analyzed.