Distribution of Spores in Foods

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Raw products are naturally contaminated with C. botulinum. Several studies have demonstrated that this organism is ubiquitous in soil. The factors determining type distribution are as yet unclear (62). Organic fertilizers of animal origin and sewage sediments contribute to soil contamination (154). It is therefore not surprising that foods of vegetable origin are contaminated with this microorganism. C. botulinum spores, generally types A and B, have been detected in red capsicum, apricots (155), carrots, onions (156), potatoes, parsley, spinach (157), garlic (158), cabbages (159), and cultivated mushrooms (160,161). Spore numbers in mushrooms range from <0.08-0.16/100 g (11) to 41/100 g (160).

Terrestrial sediments present in the aquatic environment may contaminate fish with nonproteo-lytic C. botulinum spores. However, the aquatic environment is optimal for the survival of the type E spore. The incidence of C. botulinum type E in acquatic sediments appears to result from aquatic carrion (162). Spores of nonproteolytic strains, type E in particular, have been found in bred trout (163-165), salmon (166), whitefish (167), catfish and sardines (168). The level of contamination in fish products can be quite high (169), estimated around 17 spores/100 g (170).

Meats can also be contaminated with C. botulinum spores, more frequently pork rather than beef, mutton, or poultry. This may be due to the ingestion by pigs of soil along with their feed.

Contamination levels for red meats, though based on limited data and methods, range from 0.04 to 2.2 spores/kg (171).

Clostridia are rarely present in raw milk. However, when the hygienic conditions of harvesting are poor and milk cows are fed on silage, their number increases considerably. The clostridia affecting the organoleptic quality of cheese have been widely studied, but little is known about the occurrence of C. botulinum spores in raw milk, probably because milk-derived products have rarely been associated with cases of botulism. The level of contamination of raw milk has been estimated at <1 spore/kg in Canadian milk (172); 1 spore type B/100 mL has been detected in 5.5% of the lots furnished by 35 Italian dairies (P. Aureli and G. Franciosa, 1998, unpublished data). Their milk produced the mascarpone (typical Italian spread process cheese) that was responsible for a botulism outbreak in Italy in 1996 (10).

Spores of C. botulinum types A and B occur naturally in other products meant for human consumption such as honey (85,171,173,174), natural sweeteners (87,175), and herbs for infusion (92), but none of these products have so far been involved in cases of foodborne botulism. The contamination levels for honey have been estimated between 55 and 60 spores/g (176).

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