Outgrowth of proteolytic Clostridium botulinum type A and B spores in pasteurized process cheese products was assessed to acquire data for improved models of botulinum stability. High-moisture (58.5%) products were made with different levels of pH (5.45 to 5.9), sodium chloride (1.1 to 2.8%, wt/wt) and citrates or phosphates as emulsifying salts (1.5 to 2%, wt/wt), and held at 15 to 30°C. Supplemental experiments were carried out to address the effect of lactic acid concentration originating from the nonfat and 50% fat dry basis (FDB) cheese raw materials, of moisture (50 to 69%), and of total fat (0.1 to 41%, wt/wt). Colony counts were recorded as substitutes for the traditional times to toxin formation. In the last experimental series a polyclonal ELISA against type A and B toxins was carried out as an alternative to the mouse challenge test. Very low spore levels could lead to detectable toxin formation. Temperature strongly influenced outgrowth. At 18°C outgrowth only occurred in 3 months at favorable aw (0.966) and pH (5.9). At 25°C, outgrowth occurred within one week under favorable conditions. No growth occurred within 3 months when aw and pH were 0.95 and 5.55 respectively. Polyphosphate appeared to be more inhibitory than citrate. Moisture is a frequently used indicator of botulinum stability, but when the FDB deviates from 50%, moisture is actually a poor indicator. Components such as NaCl, emulsifying salts, and lactic acid determine stability. Fat does not contribute to stability. Increased fat levels can reduce moisture without a concomitant increase in stability.
