Polyphasic identification of a Zygosaccharomyces rouxii isolated from grape juice concentrate and its control using thermal processing

A yeast, isolated from grape juice concentrate by a grape juice processing company in central Washington, was subjected to thermal processing in 52 & DEG; Brix grape juice concentrate, commercial grape juice, and Yeast-Mold (YM) medium to determine D-values and z-values in the search for an effective means of control. At 50 & DEG;C in grape juice concentrate the D-value was 33.44 minutes, dropping rapidly as the temperature increased to 52.5 & DEG;C, 55 & DEG;C, 57.5 & DEG;C, and 60 & DEG;C. From this data, a z-value was determined to be 4.38 & DEG;C. Grape juice and YM broth were similarly tested. Commercial grape juice at 45 & DEG;C, 47 & DEG;C, and 50 & DEG;C showed D-values of 9.80, 7.68, and 1.42, respectively, resulting in a z-value of 5.69 & DEG;C. The isolate heat treated in YM broth at 40 & DEG;C, 45 & DEG;C, 46 & DEG;C, 47.5 & DEG;C, and 50 & DEG;C showed no effect at 40 & DEG;C but at higher temperatures, with a D-value of 2.28 at 45 & DEG;C, 1.61 at 46 & DEG;C, 1.05 at 47.5 & DEG;C, and with a z-value of 7.46 & DEG;C. This data suggests that an increase in the concentration of sugars in the environment has a protective effect on the yeast against elevated temperatures. Even so, pasteurization at 90 & DEG;C for thirty seconds appears to be an effective means of control for the yeast investigated. A yeast with the identical colony and cell morphology was isolated from 52 & DEG; Brix grape juice concentrate from the same grape juice processing company. Both isolates were subjected to 28S DNA sequencing and FAME analysis for identification and determined to be Zygosaccharomyces rouxii, a known problem yeast in the grape juice and wine-making industry, to this point unknown in the central Washington region.