Growth characteristics of Yerinia [i.e., Yersinia] enterocolitica in selected food products

Growth characteristics of five strains of Yersinia enterocolitica in pasteurized milk inoculated with either 10 or 1,000 CFU/ml Y. enterocolitica and stored at 4^° C for up to 21 days was determined. All five strains of Y. enterocolitica competed very well with background microflora of pasteurized milk and reached level of log 5.0 to 8.0 CFU/ml after 14 days at 4^° C. The initial level of inoculation had little or not effect on the total number of Y. enterocolitica after 14 or 21 days in pasteurized milk at 4^° C. An improved isolation technique based upon the alkalotolerance characteristics ofY. enterocolitica was developed and compared with conventional direct plating (DP) for recovery and enumeration of Y. enterocolitica from pasteurized milk. The pre-plating alkaline diluent (PPAD) technique couple with plating on Cefsulodin-Irgasan-Novobiocin (CIN) agar virtually eliminated all interference from background microflora and allowed easy isolation and enumeration of Y. enterocolitica PPAD technique was shown to be an effective method for recovery and enumeration of Y. enterocolitica from pasteurized milk stored at 4^° C, when Y. enterocolitica reached late log phase growth or stationary phase growth. The ability of Y. enterocolitica to penetrate and invade shell eggs and broken-out liquid eggs was investigated. Y. enterocolitica was recovered from shell eggs after 3 to 7 days using both the temperature differential (TD) and the pressure differential (PD) method of inocula-tions. The rate and extent of infection was substantially increased by supplementation of suspension medium with 20 ppm iron. Y. enterocolitica grew rapidly in egg yolks and whole eggs and survived and grew somewhat slower in egg whites at 4^° C. Supplementation of egg whites with 10 ppm iron improved growth of Y. enterocolitica in egg whites. Additionally, it was shown that grow patterns of Y. enterocolitica in raw liquid eggs were not significantly different (P<0.05) from those of pasteurized liquid eggs.