Thermal resistance of Salmonella spp. and Listeria monocytogenes in liquid egg yolk and egg yolk products

The effectiveness of various pasteurization procedures in destroying Listeria monocytogenes and Salmonella enteritidis in liquid egg products was evaluated. Survivor studies were performed on individual strains of L. monocytogenes and L. innocua in commercially broken raw egg yolk samples after heating at 61.1, 63.3, and 64.4 degrees C using submerged vials, and on Salmonella spp. at 60.0. 61.1, and 62.2 degrees C. Surviving bacteria were enumerated on TSA and results expressed as D-values. The influence of aw-lowering ingredients such as salt and sugar on thermal resistance in yolk was investigated using a five-strain mixture of L. monocytogenes or a mixture of Salmonella spp. (four strains of S. enteritidis, one strain each of S senftenberg and S. typhimurium) at 61.1 degrees C to 66.7 degrees C. At 61.1 degrees C (present minimum temperature for pasteurization of plain egg yolk), a 7-log-unit reduction of Salmonella took 1.4 to 2.4 min, whereas a 7-log-unit reduction of L. monocytogenes took 49 to 16.1 min. The D-value for L. monocytogenes at 64.4 degrees C increased from 0.44 min in plain yolk to 8.26 min after a 21.5-min lag (total time to achieve 1-log-unit reduction was 30.7 min) in yolk with 10% salt and 5% sugar, and 27.3 min after a 10.5-min lag (total time 37.8 min for l-log-unit reduction) in yolk with 20% salt. The D-value for Salmonella in egg yolk at 64.4 degrees C was < 0.2 min, but when 10% salt was added, the D-value was 6.4 min. Aw-lowering solutes in liquid egg yolk increased the thermal resistance of Salmonella and L. monocytogenes.