Challenge of Pasteurized Process Cheese Spreads with Clostridium botulinum Using In-Process and Post-Process Inoculation.

A study was done to evaluate the antibotulinal safety of pasteurized process cheese spreads and to compare two different published methods of inoculation of cheese spreads with Clostridium botulinum spores. Pasteurized process cheese spreads of various compositions were challenged with approximately 1,000 spores per g of C. botulinum types A and B. Two different methods of challenge were tested: (a) an "in-process" or "hot" inoculation in which a spore suspension was added to hot cheese spread in a cooker during agitation, and (b) a "post-process" or "cold" inoculation in which 0.1 ml of heat-shocked (80°C, 10 min) spore suspension was added to cheese spread already packed in glass jars and stirred. Certain products that were thought to have an adequate margin of safety by hot challenge studies became toxic when challenged by the cold method. Experiments to check localization of the spores in cold-inoculated cheese spread produced results suggesting that the concentration of the inoculum plus the localized diluting effect of added water in the cold-inoculated cheese spread probably account for the discrepancy between the two procedures.