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° 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 ℃ in grape juice concentrate the D-value was 33.44 minutes, dropping rapidly as the temperature increased to 52.5 ℃, 55 ℃, 57.5 ℃, and 60 ℃. From this data, a z-value was determined to be 4.38 ℃. Grape juice and YM broth were similarly tested. Commercial grape juice at 45 ℃, 47 ℃, and 50 ℃ showed D-values of 9.80, 7.68, and 1.42, respectively, resulting in a z-value of 5.69 ℃. The isolate heat treated in YM broth at 40 ℃, 45 ℃, 46 ℃, 47.5 ℃, and 50 ℃ showed no effect at 40 ℃ but at higher temperatures, with a D-value of 2.28 at 45 ℃, 1.61 at 46 ℃, 1.05 at 47.5 ℃, and with a z-value of 7.46 ℃. 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 ℃ 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° 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.