Heat Resistance of Vegetative Cells and Asci of Two Zygosaccharomyces Yeasts in Broths at Different Water Activity Values

Abstract

The heat resistance of vegetative cells and asci of two os-motolerant yeasts (Zygosaccharomyces rouxii and Z. bailii) was investigated in two different broths of a w 0.963 and 0.858, respectively. The highest heat resistance was observed with asci of Z. bailii LMZ 108, showing a decimal reduction time CD-value) at 60°C and a w 0.858 of 14.9 min. Asci of Z. rouxii LMZ 100 were less heat resistant (D 60 =c-v alue at a w 0.858 = 3.5 min). The heat resistance (D-values) of asci at a w 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 a w of the heating broth, the smaller the differences between heat resistance of asci and that of vegetative cells. Morcver, different preparations of the same cell material were found to lead to different heat resistances. Processing of foods by heat remains the most important method in modern food technology, and the most effective one when safety is the main aim of processing. Hence the heat resistance of microorganisms is one of their most important attributes. According to Schmidt (38) and Hansen and Riemann (22) the heat resistance of microoganisms is affected by a large number of factors including the inherent resistance (e.g. differences among species of a certain genus, strains of a certain species and cells versus spores), environmental factors which are operative during growth and during cell or spore formation and environmental factors which are operative during the time of heating. In addition, several authors pointed out that recovery conditions after heat treatment also play an important role (6,20, 23,29,30,31,41,42). It is difficult to make an accurate appraisal of the factors affecting the heat resistance of osmotolerant yeasts from a survey of the literature. Moreover, data regarding the thermal destruction of osmotolerant yeasts, especially Zygosaccharomyces bailii, are limited. Vegetative cells of Z. rouxii and Torulopsis globosa obtained from lyophilized cultures, showed enhanced heat resistance in