Survival of Escherichia coli O157:H7 in set and stirred yogurt as influenced by an exopolysaccharide, colanic acid, production
Lee, Shiao Mei
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This study was conducted to determine the role of the exopolysaccharide, colanic acid (CA), produced by Escherichia coli O157:H7 in the survival of the pathogen in set and stirred yogurt. Pasteurized milk was inoculated with a wild type E. coli O157:H7, its CA-deficient mutant, and a 1:1 ratio mixture the two strains, respectively, before fermentation for both set and stirred yogurt. Stirred yogurt was also inoculated after fermentation. Samples were stored at 4° or 15°C with sampling done twice a week for 3 weeks. All samples were enumerated for total plate counts, population of E. coli O157:H7, and starter cultures. The results showed that CA plays a protective role in protecting E. coli O157:H7 from acid stress in set yogurt. However, the same conclusion could not be drawn for stirred yogurt. It was also found that survival of the pathogen is influenced by storage temperature and pH of the yogurt.