Demonstration and Isolation of Clostridium botulinum Types from Whitefish Chubs Collected at Fish Smoking Plants of the Milwaukee Area

Abstract

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.

Sensitivity of an Enrichment Culture Procedure for Detection of Clostridium botulinum Type E in Raw and Smoked Whitefish Chubs

“…Plates were enclosed in phosphorus ignition jars (9). Colony counts were made after 48 hr (i 6 hr) of incubation at 28 C. Persistence of toxigenic C. botulinum type E was unfailingly corroborated by selection of isolates to CLM, followed by neutralization tests as previously described (6).…”

Interrelationship of Heat and Relative Humidity in the Destruction of Clostridium botulinum Type E Spores on Whitefish Chubs

“…Both type B and type E C. botulinum spores have been isolated from raw and smoked whitefish chubs at this laboratory (13). However, the latter type was observed to occur much more frequently.…”

“…Brined whitefish chubs previously had been found to yield a contamination rate of 20% (13). The earlier surveillance employed five fish per culture as opposed to the present which employed one per sample.…”

“…Persistence of C. botulinum type E spores through the process had not been expected because of the organism's widely reported low tolerance to heat (6,11,12,16,17). Therefore, the possibility of (i) incomplete heating and (ii) postprocessing contamination also had to be considered (13).…”

“…S-32,7 (type E) and M- 17,5 (type B) strains of C. botulinum were employed. The former was isolated from commercially smoked whitefish chubs and the latter, a nonproteolytic type B, from commercially brined whitefish chubs (13,14). Stock spore suspensions were prepared and assayed by a procedure described previously (15).…”