Mayonnaise and Salmonella – an unhealthy mix

Mayonnaise, informally mayo, is a popular ingredient as it brings moisture and smoothness to foods and can carry lots of different flavours. From the food safety point of view, mayonnaise represents a problematic raw egg product, since several outbreaks have been linked to this product over time. While food safety authorities provide regulations on commercial mayonnaise preparation to ensure the safety of the product, many salmonellosis cases still occur as a result of catering and home-made mayonnaise consumption. Salmonellosis is the second most common foodborne illness in European Union, with almost half of the reported cases related to the consumption of contaminated eggs and egg-based products, mayonnaise being one of them.

SafeConsume approach to test salmonellae sensitivity to acids

Different Salmonella strains of serovars associated with eggs or broilers (Enteritidis, Infantis, Typhimurium, and a heat resistant Senftenberg 775W) have been tested for sensitivity to different combinations of acid, pH and temperature in laboratory broth and egg yolk. The strains’ tolerance towards both organic and inorganic acids was first evaluated in laboratory broth, to establish if there are differences in their behavior. Afterwards, in order to make recommendations to consumers on what acid type to use when preparing an acidified raw egg yolk recipe safely, two common acidulants were investigated with respect to their capability to inactivate Salmonella, namely vinegar and lemon juice.

Effectiveness of organic acids against salmonellae

Initial results in the laboratory broth showed that inactivation rate depends on the acid type rather than the strain of Salmonella. Notably, the heat resistant strain was not more tolerant towards the acids compared with other strains. However, if the repair systems associated with higher heat resistance were removed the acid tolerance decreased. Organic acids, such as acetic acid and citric acid, were more efficient in inhibiting growth compared to an inorganic acid such as hydrochloric acid, with acetic acid ensuring a faster inactivation rate than citric acid.

The results obtained from acidification with vinegar (5% acetic acid) of egg yolks artificially contaminated with Salmonella showed that inactivation of Salmonella occurs more rapidly at room temperature (25℃) compared to 8℃. Storage of the acidified egg product at 8℃ drastically decreased the inactivation effect of the organic acid, which may be due to the cellular changes that occur at low temperatures, influencing the bacteria sensitivity towards acid stress. Also the inactivation rate of salmonellae increases with decreasing pH.

Data on acidification of the raw egg with freshly squeezed lemon juice indicate that this acidulant cannot ensure the safety of the product per se, since the content of citric acid in the juice may vary widely between lemons depending on origin, season etc., and the concentration needed of this acid for a substantial reduction of Salmonella would not be acceptable for a mayonnaise. It would, however, be in the range of recipes for e.g. lemon mousse.

What scientists have found related to safe homemade mayonnaise?

The safest is to use pasteurized eggs or commercial eggs marked as Salmonella free. If eggs from back yard sources or eggs of unknown Salmonella status origin are being used, it is important that they have been kept cold since Salmonella may proliferate quickly in the eggs at ambient temperatures. If you don’t know the status of your eggs and plan to prepare acidified raw egg dishes such as mayonnaise the researchers identified at least four procedures which could ensure a safe recipe according to the flavour requirements of the final dish and the time and temperature available. Note that the recommendations are for egg yolks mixed with the acid and kept at the various temperatures before mixing with oil or other ingredients.

  1. In order to consume the raw egg product within the same day, the egg yolk should be acidified with vinegar (5% acetic acid) to pH 3.9, and stored for 2 h at room temperature before mixing it with oil.
  2. If the mayonnaise should not be consumed before the next day, the egg yolk should be acidified to a pH of 3.9 and stored at refrigeration temperatures for 24 h.
  3. If a less acidic version of the raw egg recipe is preferred (vinegar to a pH 4.2), the mixture egg yolk-vinegar should be stored for 24 h at room temperature. This is only advisable if the pH can be measured.
  4. If the egg yolk is acidified with lemon juice, the pH should be lowered to pH 2.9 and stored at room temperature for 24h.

What consumers can do to stay safe when eating homemade mayo?

As consumers do not have the possibility to measure pH, they can follow scientists’ recommendation based on the proportion between the acidulant and egg yolk. Thus, the following pHs can be attained:

  • pH 4.2, corresponds to a ratio of vinegar (5% acetic acid) to the egg yolk of 0.40, which means 1 part vinegar and 2.5 parts egg yolk
  • pH 3.9, corresponds to a ratio of vinegar (5% acetic acid) to the egg yolk of 0.82, which means 0.82 parts vinegar to 1 part egg yolk,
  • pH 2.9, corresponds to a ratio of lemon juice to the egg yolk of 1.23, which means 1.23 part lemon juice and 1 part egg yolk. The lemon juice amount is too high in this case to prepare mayonnaise but this recommendation can be applied for the preparation of dishes such as lemon mousse.

Decreasing the risk associated to salmonellosis

Study recommendations on safe raw egg yolk recipes may help reduce the salmonellosis risk associated with consumption of home-made mayonnaise, when eggs of uncertain Salmonella status are going to be used. In order to facilitate the application of the study’s recommendations, the pH required to inactivate Salmonella was translated into the ratio of acidulant to egg yolk volume.

Research never ends

The researchers suggest other research directions to be able to control salmonellae in mayonnaise:

  • To investigate the fate of Salmonella in the acidified egg yolk after addition of oil and other ingredients affecting acidification such a mustard
  • To investigate the fate of other microorganisms that may spoil the mayonnaise, such as acid tolerant bacteria and yeasts, after an extended storage of the yolk-acid mixture at room temperature.
  • Investigating the egg handling-practices of retailers and consumers with respect to storage temperatures and drivers to improve these.

Key researchers involved in these findings

The study was conducted by Line Nielsen (first author) and Susanne Knøchel (corresponding author), two researchers from the University of Copenhagen (Department of Food Science, Section of Microbiology and Fermentation).

Link to the article

Nielsen, L., Knøchel, S. 2020. Inactivation of salmonella strains in acidified broth and raw egg yolk as a function of pH and acid type. Food Microbiology, Volume 92: 103574,

Links to other similar research

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