Combined effects of water activity, pH and temperature on the growth and spoilage potential of fungi.

Summary. An arbitrary parameter ‘rejection time’, i.e. the time required for a fungal inoculum to form a 2 mm diam. colony, was used to express the shelf life of jam after unsealing and exposure of the contents to airborne contamination. Individual and combined effects of water activity (aw), pH value and temperature on rejection time of low sugar jam were estimated from the radial growth on agar of colonies of 9 fungi. The decreases in aw (0·94–0·90) and temperature (25–15°) practicable for low sugar jam were more effective in increasing rejection time than the feasible decrease in pH value (3·7–2·9). The interaction between aw and temperature was significant. The effect of the changes in aw, temperature and pH on rejection time was broadly similar for media adjusted by either sucrose or glycerol. At a given aw, moulds were slightly more tolerant of glycerol than sucrose but yeasts, except for the osmophile Saccharomyces rouxii, were markedly more tolerant of glycerol than sucrose.