THE EFFECT OF RECOVERY MEDIUM ON THE ESTIMATED HEAT-INACTIVATION OF SPORES OF NONPROTEOLYTIC CLOSTRIDIUM-BOTULINUM

Heating spores of non-proteolytic strains of Clostridium botulinum at 85-degrees-C, followed by enumeration of survivors on a highly nutrient medium indicated a 5 decimal kill in less than 2 min. The inclusion of lysozyme or egg yolk emulsion in the recovery medium substantially increased apparent spore heat-resistance, with as little as 0.1 mug lysozyme/ml sufficient to give an increase in the number of survivors. After heating at 85-degrees-C for 2 min between 0. 1 % and 1 % of the spores of 11 strains (5 type B, 4 type E, 2 type F) formed colonies on medium containing 10 mug lysozyme/ml. Enumeration of survivors on a medium containing lysozyme showed that heating at 85-degrees-C for 5 min resulted in an estimated 2.6 decimal kill of spores of strain 17B (type B). These findings are important in the assessment of heat-treatments required to ensure the safety with respect to non-proteolytic Clostridium botulinum of processed (pasteurized) refrigerated foods for extended storage such as sous-vide foods.