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The effect of pH on the thermal inactivation of staphylococcal enterotoxin A was investigated. Analysis of heated toxin by immunodiffusion in gel indicated that enterotoxin A in beef bouillon was inactivated faster at pH 5.3 than at pH 6.2. The z values (slopes) for the heat inactivation curves at pH 6.2 and 5.3 were 49.5 and 55 F (about 27 and 30 C), respectively. Enterotoxin produced and heated in dialyzed Casamino Acids medium and assayed by monkey feeding was more easily inactivated by heat at pH 5.3 than at pH 7.8. Thermal inactivation curves for enterotoxin A in beef bouillon (5 ,ug/ml, pH 5.3) were determined by two methods, monkey feeding and serological assay. The z values for the curves obtained by these two methods were both 55 F, although loss of biological or toxic activity of the enterotoxin occurred before loss of serological activity.

SUMMARY A large batch of enterotoxin A from Staphylococcus aureus strain 196‐E was prepared in easamino acid medium and concentrated 13.5 times by dialysis under polyethylene glycol. Concurrent tests on heat inactivation of this toxin were conducted with monkeys and cats. The heat inactivation curve based on the cat emetic reaction to intraperitoneal injection ean be expressed as a straight‐line semi‐logarithmic curve with a slope (z) of 48° F to traverse one log cycle of time and a resistance at 250°F of 11 min (F49250=11 min). The heat inactivation curve based on the monkey emetic reaction to oral feeding was F49250= 8 min. The heat resistance tests based on the two test animals are in remarkable agreement. Cats were found to be considerably more susceptible to the toxin by injection than were monkeys by feeding. The implication of these tests in relation to heat processing of foods is discussed, and preliminary indications are that the processes given commercially canned foods in which the organisms will grow are sufficient to destroy the toxin.

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