To preserve high sugar products, the effectiveness of sodium benzoate and ethyl-paraben (para-hydroxybenzoic acid ethylester) against 18 osmotolerant yeast strains was investigated at different water activity values (aw). The influence of pH, acidulant, humectant as well as inoculum level on the tolerance limits for growth of selected strains has also been determined. The tolerance limits for growth of all 18 tested strains were only slightly affected by the aw of the substrate, provided that the preservative concentration reflects only the amount of water and not the whole volume of the substrate. At aw < 0.900 and pH<4.0 1500 ppm Na-benzoate was necessary to inhibit growth of all 18 tested osmotolerant yeast strains for 30 d, while in a similar medium but at higher pH-values Na-benzoate was less effective. Similarly, at aw <0.900 and pH 3.0 a 30-day-free shelf life was guaranteed by addition of 400 ppm ethyl-paraben, while a higher concentration of ethyl-paraben (900 ppm) was necessary if the medium was acidulated to pH 4.8 only. The activity of Na-benzoate or ethyl-paraben against osmotolerant yeasts was usually poor if the initial count of contaminants was high. Zygosaccharomyces bailii was the most preservative-resistant osmotolerant yeast among the tested genera and species.
1Present address: Fermentation Microbiology Laboratory, Department of Applied Biological Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139.