The Combined Effects of Humectant, Preservative and pH Value on the Survival and Growth of Clostridium botulinum.

The survival and heat resistance of Clostridium botulinum was studied in conditions which could he encountered in some Intermediate Moisture Foods (lMF's). Type A spores remained viable for up to 74 days in medium systems containing various combinations of humectants and preservatives and at various pH values. Loss of viability of type B spores was observed in the presence of propylene glycol but the observation was not repeatable. Sensitivity to heat was indicated when type B spores were pasteurised in systems containing propylene glycol and propionate. Survival of vegetative cells at reduced Aw levels could not be assessed successfully due to rapid sporulation after inoculation. The heat resistance at 80° and 90°C, as expected, was dependent upon the humectant type and the pH value. The presence of propionate did not reduce the D values significantly. The D values increased at Aw 0.80 in the presence of sucrose and HaCl but in propylene glycol-containing systems, the D90°C values did not increase significantly. The D80°C values in the presence of propylene glycol were lowered than in the absence of humectant. Interactions of humectant (NaCl, sucrose, glycerol or propylene glycol) and preservative (sorbate, benzoate or propionate) capable of inhibiting growth and toxin production by Cl. botulinum at high Aw were also investigated using Tryptone Soya Broth systems. Combinations of NaCl/sorbate, propylene glycol/sorbate and propylene glycol/benzoate were more inhibitory than either the humectant or the preservative alone. The pH value of the system was found to have a significant effect on the anti-botulinal activity of the combinations. The interaction was shown to he dependent upon the humectant type and not the Aw of the system. Unexpectedly, growth of Cl. botulinum was not inhibited at pH 6.5-7.0 by up to 35% (w/v) glycerol (Aw 0.90). The results from these experiments are discussed in relation to the formulation of IMF's, particularly those intended for subsequent rehydration or for consumption by infants, without increasing the risk of botulism