Demonstration and isolation of Clostridium botulinum types from whitefish chubs collected at fish smoking plants of the Milwaukee area.

A total of 1,071 whitefish chub samples were examined at eight stages of processing, including sampling aboard ship, various processing steps in the smoking plant, and display in retail cases. The frequency of Clostridium botulinum contamination of freshly caught and eviscerated chubs was approximately 13 to 14%. The highest percentage of contamination (20%) was noted among chubs sampled at the brining step of processing. The prevalence of contamination among chubs sampled at other processing stages prior to the smoking operation ranged from 6 to 14%. Of 858 freshly smoked chubs that had been processed at 180 F for 30 min (internal temperature of loin muscle), 10 were contaminated with C. botulinum (1 Type B and 9 Type E). The use of heat-shocked (60 C for 15 min) and nonheat-shocked enrichment cultures in combination yielded a greater number of positive samples than either method yielded when used alone. Each toxic enrichment culture obtained was subcultured to obtain isolation of the toxigenic organism. Toxigenic pure cultures of C. botulinum were obtained from 80% of the fish samples observed to produce toxic enrichment cultures.