Resistance of Yersinia enterocolitica, Escherichia coli O157:H7 and natural microflora against acidic conditions and freezing-thawing in fresh sausages

The resistance of Yersinia enterocolitica O:9, Escherichia coli O157:H7 and natural microflora against lactic acid (LA), ascorbic acid (AA), and freezing-thawing in noninoculated and inoculated fresh sausages was studied. Samples were stored at -18 degrees C for 28 days and thawed in microwave (MW), at room temperature (RT), in refrigerator (R) and under flowing of tap water (F) on days 7, 14, 21 and 28. Plate Count Agar (PCA), Sorbitol Mac Conkey agar (SMC) and Mac Conkey agar (MC) were used for microbial counts. A maximal reduction of 1.57 log in mesophilic aerobes and no significant changes in total and fecal coliform levels with respect to the initial counts in natural microflora were observed along storage. In inoculated fresh sausages, reductions of 1.37 log on PCA and 2.17 log on SMC were obtained in E. coli O157:H7 populations as compared to the control groups on day 0. Similarly, reductions of 1.69 log on PCA and 2.79 log on MC as compared to the initial level were observed in counts of Y. enterocolitica inoculated samples. Salmonella Anatum, P. aeruginosa, Y. enterocolitica B1A O:7,8-8-8,19 and E. coli non O157:H7 strains were recovered from the natural microflora by enrichment techniques. Thawing in refrigerator was more frequently related to the best reductions of total mesophilic aerobe, E. coli O157:H7 and Y. enterocolitica O:9 counts than the other thawing methods. Reductions of microbial populations observed in LA treated samples were similar to those observed in AA treated samples. Although the acidic and freezing treatments might reduce the microbial levels in natural microflora of fresh sausages, they appeared to be ineffective in the total elimination of high inocula of pathogens like E. coli O157:H7 and Y. enterocolitica O:9.