|Cray, William - Food Safety Inspection Service (FSIS)|
|Hinton, Jr, Arthur|
|Meinersmann, Richard - Rick|
|Jin, Zhonglin - Tony Jin|
Submitted to: International Journal of Food Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/20/2015
Publication Date: 5/26/2015
Citation: Gurtler, J., Bailey, R., Cray, W.C., Hinton Jr, A., Meinersmann, R.J., Ball, T.A., Jin, Z.T. 2015. Salmonella spp. isolated from ready-to-eat pasteurized liquid egg produce: thermal resistance, biochemical profile, and fatty acid analysis. International Journal of Food Microbiology. 206:109-117.
Interpretive Summary: Pasteurized liquid egg products are ready-to-eat and are sometimes contaminated with Salmonella at low levels. A study was conducted to examine Salmonella isolates that were found in liquid egg products. The goal was to determine if the isolates were heat-resistant (thus being able to survive the commercial thermal pasteurization process) or if they were heat sensitive (which suggests that they probably contaminated the egg products after the pasteurization process). Seventeen Salmonella isolates found in liquid egg were tested for thermal inactivation at 60 deg C. It was found that four isolates had higher heat resistance indicating the potential to withstand the pasteurization process in the egg factory. Nine other isolates were not heat resistant, which may indicate that these isolates contaminated the liquid egg products only after the commercial pasteurization process. This study may help shed light on the source of Salmonella contamination.
Technical Abstract: The Egg Products Inspection Act of 1970 requires that egg products in the U.S. must be pasteurized prior to release into commerce. The USDA Food Safety and Inspection Service (FSIS) is responsible for regulating egg products. Salmonellae are infrequently isolated from pasteurized egg products by food manufacturers or the FSIS and may be present as a result of either pasteurization-resistant bacteria or post-processing contamination. In this study, seventeen strains of Salmonella isolated from pasteurized egg products and three heat-resistant control strains were compared for the following attributes: thermal resistance in liquid whole egg (LWE) at 60 degrees Celsius, enzymatic profiles, and serotyping and phage typing, antibiotic susceptibility, fatty acid analysis and strain morphological variation evaluated by scanning electron microscopy. Isolates were serotyped as Heidelberg (4 isolates), Widemarsh, Mbandaka, Cerro, Thompson, 4,12:i:-, and Enteritidis (8 isolates). All 20 isolates were sensitive to all 14 antibiotics tested for. The D60 values in LWE ranged from 0.34 – 0.58 min. All 20 strains were recovered from LWE inoculated with 8.5 log CFU/ml of Salmonella and pasteurized at 60 degrees Celsius for 3.5 min; however, some isolates were not recovered from pasteurized LWE that had been inoculated with only 4.5 log CFU/ml Salmonella and treated at 60 degrees Celsius for 3.5 min. Although some strains exhibited atypical enzymatic activity (e.g., reduction of adonitol, hydrolysis of proline nitroanilide or p-n-p-beta-glucuronide, and nonreduction of melibiose), differences in biochemical reactions could not be correlated with differences in thermal resistance. Furthermore, fatty acid analysis revealed that differences insaturate/unsaturated profiles may be correlated with differences in heat resistance, in two instances. One heat resistant strain (number 13, Enteritidis) had the statistically lowest unsaturated/saturate ratio at 39%. However, one heat sensitive strain (number 3, serovar 4,12:i:-) had the highest unsaturated/saturate ratio at 81%, and also the lowest concentration of stearic acid. This data represents the first steps in determining whether Salmonella contamination in pasteurized egg products may be the result of either thermally-resistant isolates or post-processing contamination. Contamination of LWE by Salmonella strains with higher heat resistance, (e.g., isolate numbers 2, 6, 10 and 12) may indicate the ability of Salmonella to survive pasteurization, while contamination of LWE strains with lower heat resistance (e.g., isolate numbers 1, 3, 5, 7, 8, 11, and 15) may indicate post-processing contamination of LWE by this foodborne pathogen.