Effect of heat treatment on survival of, and growth from, spores of nonproteolytic Clostridium botulinum at refrigeration temperatures

Abstract

Spores of five type B, five type E, and two type F strains of nonproteolytic Clostridium botulinum were inoculated into tubes of an anaerobic meat medium plus lysozyme to give approximately 106 spores per tube. Sets of tubes were then subjected to a heat treatment, cooled, and incubated at 6, 8, 10, 12, and 25°C for up to 60 days. Treatments equivalent to heating at 65°C for 364 rain, 70°C for 8 min, and 75°C for 27 min had little effect on growth and toxin formation. After a treatment equivalent to heating at 80°C for 23 min, growth occurred at 6 and 8°C within 28 to 40 days. After a treatment equivalent to heating at 85°C for 19 min, growth occurred in some tubes at 6, 8, 10, or 12°C within 28 to 53 days and at 25°C in all tubes within 15 days. Following a treatment equivalent to heating at 95°C for 15 min, growth was detected in some tubes incubated at 25°C for fewer than 60 days but not in tubes incubated at 6 to 12°C. The results indicate that heat treatment of processed foods equivalent to maintenance at 85°C for 19 min combined with storage below 12°C and a shelf life of not more than 28 days would reduce the risk of growth from spores of nonproteolytic C. botulinum by a factor of 106. If foods intended for refrigerated distribution and storage are liable to be maintained at temperatures higher than 10°C, growth and toxin production by proteolytic strains of C. botulinum must also be prevented.