The heat resistance of ascospores of four Saccharomyces spp. isolated from spoiled heat-processed soft drinks and fruit products

The heat resistance ( D and z values) of four Saccharomyces spp., viz. S. bailii, S. cerevisiae, S. chevalieri, S. uvarum , was investigated by a previously described method (Put et al. 1977). The highest heat resistance was observed in ascospores of S. cerevisiae 175 showing a decimal reduction time or death rate constant at 60°C of 22.5 minutes. The average D 60°C values of the remaining species were: S. bailii 10 min; S. chevalieri 13 min; and S. uvarum 1.5 min; while z values varied between 4.0 and 6.5°C. It can be postulated that there may exist some comparison between the mechanism of yeast ascospore and bacterial endospore heat resistance. However, (i) washed Saccharomyces ascospores cannot be stored at 5°C for a longer period than 2–3 weeks without loss of heat resistance; and (ii) ascospore heat survival curves are essentially not linear over the whole range, which may be due to some heterogeneity of the ascospore population tested. Besides, it was observed that the heat resistance ( D 60°C values) of the Saccharomyces ascospores proved to be 50–150-fold higher than the D _60°C values of the corresponding vegetative cells.