Salmonella is a key cause of foodborne gastroenteritis in Australia and case numbers are increasing with most outbreaks linked to eggs, poultry meat, pork, dairy, nuts and fresh produce.

Now Flinders University researchers have found a simple solution for preventing salmonellosis affecting eggs through surface contamination, giving crucial help for Australia’s vast food services industry.

Raw eggs are used in many food products such as mayonnaise, mousse, eggnog, and artisanal ice cream. However, a problem is associated with eggshells being contaminated with the bacterium Salmonella enterica serovar Typhimurium (ST).

To address this issue, the Flinders research team aimed to develop a decontamination method that removed ST contamination from the eggshell without impacting the egg’s usability.

Using a method that employed equipment commonly found in commercial kitchens, the researchers decontaminated eggs by placing them in a sous-vide cooker with the water heated to 57C.

Complete decontamination of ST was achieved by treating eggs for nine minutes. The decontamination method uses kitchen equipment commonly used for sous-vide cooking.

The results, as published in the journal Foodborne Pathogens and Disease, is the first study to look at decontamination of ST on the eggshell.

The 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 nontreated eggs.

A preview of the paper, ‘A Successful Technique for the Surface Decontamination of Salmonella enterica Serovar Typhimurium Externally Contaminated Whole Shell Eggs Using Common Commercial Kitchen Equipment’ (November 2019) by Thilini Keerthirathne, Kirstin Ross, Howard Fallowfield and Harriet Whiley is online DOI: 10.1089/fpd.2019.2734

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

Despite these guidelines, a significant numbers of salmonellosis outbreaks continue to be recorded every year in Australia.

Over the past decade in Australia the incidence of salmonellosis has increased from 40.9 per 100,000 population in 2005 to 71.5 per 100,000 population in 2015.

One of the most common sources of salmonellosis has been identified as raw eggs and egg products.The researchers found that the survival of Salmonella Typhimurium in mayonnaise is significantly improved at 4C and that lower temperatures protected ST 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,” says Flinders environmental health researcher Ms Thilini Keerthirathne.

“But there is a risk of storing mayonnaise at 37C. 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 if at pH 4.2 or less.”

This study, ‘The Combined Effect of pH and Temperature on the Survival of Salmonella enterica Serovar Typhimurium and Implications for the Preparation of Raw Egg Mayonnaise’ (November 2019) by TP Keerthirathne, K Ross, H Fallowfield and H Whiley, has been published in Pathogens journal. DOI:10.3390/pathogens8040218

PhD candidate Ms Keerthirathne says these two studies will prove valuable in helping to decrease the current levels of foodborne salmonellosis outbreaks related to eggs and raw egg products in Australia.

In Australia it was estimated that annually there are 4.1 million cases of foodborne illness including 30,000 hospitalisations and 100 fatalities. One of the most prevalent causes of foodborne illness is salmonellosis.