Up to 550 ppm (550 micrograms/ml) of nisin in combination with 60 ppm (60 micrograms/ml) of nitrite failed to prevent outgrowth of Clostridium botulinum spores in pork slurries adjusted to pH 5.8. Reducing the pH enhanced nisin activity. Proteolytic and nonproteolytic type B spores were equally resistant to nisin.
The heat stability of staphylococcal enterotoxins A, B and C (SEA, SEB, SEC) in phosphate buffered saline solution at a concentration of 100 ng per ml indicated that normal cooking times and temperatures are unlikely to completely inactivate the toxins. The order of heat resistance of the three toxins was SEC>SEB>SEA.
One hundred and fifty-three samples of tofu, related products, and environmental samples comprising 346 sample units were collected from 14 manufacturers across Canada. They were analyzed for coliforms, Salmonella, Yersinia, Staphylococcus aureus, and psychrotrophs. Although S. aureus counts were generally less than 250 cells per g and Salmonella was not detected, levels of psychrotrophs exceeded 106 per g in more than 45% of finished tofu and okara samples, and levels of coliforms exceeded 103 per g in more than 35% of these samples. Yersinia enterocolitica was also isolated from four samples of finished tofu.
The performance of four selective media for enumerating Staphylococcus aureus in artificially contaminated samples of corned beef was strain-dependent. Baird-Parker (BP), Kranep (KR), Mannitol Salt (MS) and Staphylococcus medium 110 (S110) performed equally well in enumerating an enterotoxin A producing strain, but KR and BP were significantly better than S110 in enumerating an enterotoxin D producing strain of S. aureus. In naturally contaminated cheese samples which abound with competing microorganisms, BP performed significantly better than the other three media.
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