Five non-refrigerated, pasteurized process cheese spreads, considered shelf-stable, were studied for their ability to support growth and toxin production by spores of Clostridium botulinum types A and B. Based on pH and water activity (aw) Cheese with Bacon, Limburger, Cheese Whiz, Old English, and Roka Blue cheese spreads were selected for the study. The pH ranged from 5.05 to 6.32 and the aw from 0.930 to 0.953. Fifty jars of each cheese spread were inoculated with 24,000 spores each; an additional 50 jars of the Cheese with Bacon spread received 460 spores each. The inoculum consisted of five type A and five type B strains in 0.1 ml of 0.85% NaCl. At 35 C, 46 jars of Limburger and 48 jars of Cheese with Bacon spread, which received the greater inoculum, became toxic starting at 83 and 50 days, respectively. One jar of Cheese with Bacon spread which received 460 spores became toxic. The average toxicity of the Limburger was 3000 MLD/ml of extract as compared with 54 MLD/ml for the Cheese with Bacon spread. Results of this study will be considered in determining whether these cheese spread products should be treated as low-acid canned foods under the Good Manufacturing Practice Regulations of the Food and Drug Administration.

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