A simple presence-absence test for detection of small numbers of osmotolerant yeasts in foods was developed. Yeast extract glucose 50 broth [consisting of 0.5% (w/w) yeast extract and 50% (w/w) glucose] was used as enrichment medium and was incubated with agitation at 30°C. The detection was done by (a) microscope and (b) streaking 0.03 ml of enrichment culture on selective yeast extract glucose 50 agar and incubation at 30°C for 5–7 d. If no yeast cells were observed under the microscope within 10 d of incubation, the product sample was judged as “free from osmotolerant yeasts.” In accordance with this method 28 strains of osmotolerant yeasts were isolated from 27 spoiled high-sugar products. Twenty-four strains were identified as Zygosaccharomyces rouxii, 2 Zygosaccharomyces bailii and 1 each as Torulaspora delbrueckii and Debaryomyces hansenii.
The heat resistance of vegetative cells and asci of two osmotolerant yeasts (Zygosaccharomyces rouxii and Z. bailii) was investigated in two different broths of aw 0.963 and 0.858, respectively. The highest heat resistance was observed with asci of Z. bailii LMZ 108, showing a decimal reduction time (D-value) at 60°C and aw 0.858 of 14.9 min. Asci of Z. rouxii LMZ 100 were less heat resistant (D60°C-value at aw 0.858 = 3.5 min). The heat resistance (D-values) of asci at aw 0.963 proved to be 20- to 50-fold and 5- to 8-fold higher than the D-valucs of the corresponding vegetative cells of Z. rouxii and Z. bailii, respectively. However, the lower the aw of the heating broth, the smaller the differences between heat resistance of asci and that of vegetative cells. Morever, different preparations of the same cell material were found to lead to different heat resistances.
All three cardinal temperatures (Tmin, Topt and Tmax) for growth of 6 strains as well as Tmin and Tmax for growth of an additional 23 strains were determined in solutions of 10, 30, 50 and 60% (w/w) glucose at aw (20°C) of 0.990, 0.970, 0.922 and 0.868, respectively. The Topt for growth of Zygosaccharomyces rouxii and Z. bisporus were 24–28.5°C at aw >0.990 and 31–33°C at aw in the range of 0.922–0.868. Z. bailii showed Topt for growth of 29–31°C and 33–35°C at aw >0.990 and aw <0.922, respectively. The Topt for growth of Torulaspora delbrueckii was 27–28.5°C at aw <0.990 and 31–33.5 at aw in the range of 0.922–0.868. Debaryomyces hansenii showed a Topt of 24°C and 27–29.5°C at aw >0.990 and aw <0.922, respectively. The Tmin and Tmax for growth were also shifted toward higher values as the aw decreased; at aw<0.922 none of the tested strains grew at 4°C within 30 d. Several strains could grow at 42°C only in the presence of high sugar concentrations.
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