Isolation and survival of "Yersinia enterocolitica" in ice cream at different pH values, stored at -18ºC

Pederiva, Norma B. Barbini deGuzmán, Ana M. Stefanini de

The presence of "Yersinia enterocolitica" was investigated in 203 samples of industrial (123) and non-industrial ice cream (80). Two "Y. enterocolitica" strains were isolated from non-industrial ice cream, which suggests the possibility of post-manufacturing contamination. One strain was typed as B:1A, O:3,50,51; lis Xz, while the other one was biotyped as B:1A but not serologically typed. Survival of "Y. enterocolitica" was investigated by inoculating nine samples of industrially manufactured ice cream to contain 20 CFU/ml of "Y. enterocolitica" and stored at -18ºC for 480 days. The inoculated samples were classified into three different groups according to their pH (Group 1: pH 4-5; Group 2: pH 5-6 and Group 3: pH 6-7). Viability was determined by a contamination of direct plating and enrichment. In Group 1, "Y. enterocolitica" was not detected after 150 days of storage, while in Groups 2 and 3, this microorganism was isolated until day 480 of storage. These fidings suggest that the survival time of "Y. enterocolitica" in ice cream stored at -18ºC is significantly (p<=0.05) lower at pH values under 5.