Researchers at a university in Australia have found a way to prevent salmonellosis affecting eggs through surface contamination.

Flinders University scientists developed a shell egg decontamination method that removed Salmonella Typhimurium from the outside of an egg without impacting its usability.

The method adapted a temperature-controlled water bath common in kitchens and associated with the sous-vide technique. Raw eggs are used in some recipes for certain food products such as mayonnaise, mousse, eggnog, and artisanal ice cream.

All clear in less than 10 minutes
The outside of whole eggs were artificially inoculated with two Salmonella Typhimurium strains. Eggs were decontaminated by placing them in a sous-vide cooker with the water heated to 57 degrees C (134.6 degrees F). Complete decontamination was achieved in nine minutes, according to the study published in the journal Pathogens.

In Australia, Salmonella Typhimurium is most commonly linked with the egg-related outbreaks, while globally Salmonella Enteritidis is the main cause. The former is found on the outside of the eggshell with internal contamination uncommon while the latter primarily contaminates internal contents of the eggs.

Eggs were heat treated for 30 seconds, 1, 2, 3, 3.5, 6, 6.5, 7, 8, and 9 minutes. The experiment was conducted with three eggs at each time point and repeated three times for each time point and Salmonella strain. Artificially inoculated eggs that did not undergo heat treatment were used as the control.

The concentration of Salmonella cells inoculated on to the egg varied from 28 to 150 colony-forming units per milliliter (CFU/mL) with an average of 74 CFU/mL.

There was no growth of the standard strain after 3.5  minutes at 57 degrees C (134.6 degrees F) in the sous-vide water bath but enrichment confirmed survival of Salmonella. No growth was observed after six  minutes at 57 degrees C (134.6 degrees F) after enrichment.

The clinical isolate was not recovered from the eggs after eight  minutes but enrichment confirmed survival of Salmonella. Nine minutes at 57 degrees C (134.6 degrees F) confirmed complete loss of viability of this strain.

Minimal impact on egg quality
A blind control study was also conducted to assess the acceptability and usability of treated eggs by chefs and food handlers.

Decontaminated eggs were found by chefs, using measurements and acceptability scores, to have no significant difference in their quality or performance as an ingredient when compared with non-treated eggs.

Parameters tested indicated the decontamination method did not impact egg quality. There were no significant differences between treated and control egg measurements for albumen pH, yolk index, and Haugh unit.

Researchers said the method can be used to decontaminate eggs before preparation of raw egg products.

“Using this method immediately before the preparation of raw egg products could help reduce the burden of salmonellosis in Australia and protect public health. This method could also reduce the possibility of cross-contamination while processing raw egg products.”

Future work is needed to evaluate effectiveness of the method against other Salmonella strains in Australia and to explore effectiveness against heat resistance-induced strains of Salmonella, according to the report.

A second study by the Flinders research team examined effectiveness of Australian guidelines that recommend raw egg mayonnaise should be prepared and stored under 5 degrees C (41 degrees F) and adjusted to a pH less than 4.6 or 4.2.

Researchers found survival of Salmonella Typhimurium in mayonnaise is significantly improved at 4 degrees C (39 degrees F) and that lower temperatures protect the pathogen from the bactericidal effect of low pH.

“We found that the preparation of mayonnaise at pH 4.2 or less and incubating it at room temperature for at least 24 hours could reduce the incidence of salmonellosis,” said Thilini Keerthirathne, a researcher.

“But there is a risk of storing mayonnaise at 37 degrees C (98 degrees F). If the pH is not correctly measured, the warmer temperatures will promote the growth of Salmonella. As such it is crucial to ensure the pH of the mayonnaise is at pH 4.2 or less.”