Combined effect of water activity and pH on inhibition of toxin production by Clostridium botulinum in cooked, vacuum-packed potatoes

The effects of water activity (aw, 0.955 to 0.970), pH (4.75 to 5.75), and storage time (up to 60 days) on toxin production by Clostridium botulinum in cooked, vacuum-packed potatoes were studied by using factorial design experiments and most-probable-number methodology. Samples were inoculated with 10(3), 10(4), or 10(5) spores of a mixture of five type A and five proteolytic type B strains, incubated at 25 degrees C, and analyzed for toxin production. Toxin was produced at pH levels of greater than or equal to 4.75 when the aw was greater than or equal to 0.970, pH greater than 5.25 when the aw was 0.965, and pH greater than or equal to 5.75 at an aw of 0.960. No toxin was detected when the aw was 0.955. The probability of toxigenesis was significantly affected (P less than 0.0001) by storage time, aw, pH, and the interactions aw.pH and aw.storage time. The response to a decrease in pH was linear, while the response to a decrease in aw was curvilinear. Using multiple linear regression, equations were derived which could predict the length of time until toxin production and the probability of toxigenesis by a single spore under defined conditions.