Submitted to: Food Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/14/2010
Publication Date: 8/20/2010
Citation: Jordan, J.S., Gurtler, J., Marks, H.M., Jones, D.R., Shaw, W.K. 2010. Inactivation kinetics of a four-strain composite of Salmonella Enteritidis and Oranienberg in commercial liquid egg yolk. Food Microbiology. 28:67-75. Interpretive Summary: The USDA requires that all liquid egg products be heat pasteurized for the destruction of the bacterium Salmonella prior to distribution to the public for consumption. These guidelines were first established in 1970 and are currently being reviewed by the United States Department of Agriculture Food Safety and Inspection Service. The goal of the present study was to determine the efficacy of thermal treatments to inactivate Salmonella in liquid egg yolk. Four strains of Salmonella were grown and added to commercially-processed liquid egg yolk at a level of approximately ten million Salmonella cells per milliliter of yolk and thoroughly mixed. This egg yolk was then placed in thin glass tubes, sealed, and heat treated at temperatures ranging from 58 to 66 deg C and cooled in ice water. The level of Salmonella in the egg yolk was then determined by established microbiological sampling methods. The data from these experiments were statistically analyzed and used to create a mathematical model that can be used to predict the reduction of Salmonella in liquid egg yolk pasteurized at temperatures ranging from 58 to 66 deg C. The results of this study provide information that can be used by liquid egg processing companies to aid in producing safe pasteurized egg yolk products and for satisfying pasteurization performance standards and industry guidance.
Technical Abstract: The goal of this study was to develop a general model of inactivation of salmonellae in commercial liquid egg yolk for temperatures ranging from 58 to 66 deg C by studying the inactivation kinetics of Salmonella in liquid egg yolk. Heat-resistant salmonellae (three serovars of Enteritidis [two of phage type 8 and one PT 13] and one Oranienburg) were grown to stationary phase in Tryptic Soy Broth and concentrated ten-fold by centrifugation. Each inoculum was added to liquid egg yolk and mixed thoroughly, resulting in a final population of ca. 7 log CFU/ml egg yolk. Inoculated yolk was injected into sterile glass capillary tubes, flame-sealed and heated in a water bath at 58, 60, 62, 64, and 66 deg C. Capillary tubes were ethanol sanitized, rinsed, and contents were extracted. Yolk was diluted, surface plated onto Tryptic Soy Agar + 0.1% sodium pyruvate and 50 ug/ml nalidixic acid and incubated at 37 deg C for 24 h before colonies were enumerated. Decimal reduction values were calculated from survivor curves with a minimum inactivation of 6 log CFU/ml at each temperature. Survival curves (except for 66 deg C) featured initial lag periods before first order linear inactivation. Estimated asymptotic D values were 1.83 min at 58 deg C, 0.69 min at 60 deg C, 0.26 min at 62 deg C, 0.096 min at 64 deg C and 0.036 min at 66 deg C. The estimate of the z-value was ca. 4.7 deg C with standard error of 0.07 deg C. A linear relationship between the log10 of the lag times and temperature was observed. A general kinetic model of inactivation was developed. The results of the study provide information that can be used by processors to aid in producing safe pasteurized egg yolk product and for satisfying pasteurization performance standards and industry guidance.