The effect of acid pH on the probability of growth of proteolytic strains of Clostridium botulinum

Abstract

In low-acid foods, where the pH is not sufficiently acidic to prevent growth of Clostridium botulinum, sub-optimal pH may nevertheless contribute to preservative systems based on combinations of inhibitory factors. The effect of pH in the range 4.5–7.0 on the probability, P, of growth of single vegetative bacteria of proteolytic strains of C. botulinum at 30°C has been determined by comparison of the most probable number (MPN) count of the bacteria in nutrient medium adjusted to the required pH with HCl, and in the basal medium at pH 6.8–7.0. The medium was maintained in strictly anaerobic conditions at a redox potential equivalent to lower than −300 mV at pH 7. At pH values between 5.0 and 7.0 log 10 P for growth of strain ZK3 (type A) in 3 days was between 0 and −1. Between pH 4.6 and pH 4.9 growth was delayed and incubation for up to 14 days was required; log 10 P decreased sharply with decrease in pH to a value of −5.6 or lower at pH 4.60 for growth within 14 days. For an inoculum containing a mixture of 4 additional type A strains and 5 type B strains log 10 P for growth within 14 days at pH 4.66 was −5.6. A mathematical model was developed that related the effect of acid pH and incubation time to the probability of growth of strain ZK3 in the above conditions.