by Di Johnson

Created on: May 22, 2008

A classic food poisoning incident could follow the following process. A customer sneezes into mayonnaise left for several hours at an unrefrigerated salad bar in a restaurant. The bacteria multiply and in doing so give off a toxin. This toxin is a noxious substance to the diners. One, or several customers, eat the mayonnaise and are ill within one to 6 hours with vomiting, nausea and headache. Often put down to over enthusiasm of the wine bottle or "something I ate"... Indeed!

Mayonnaise is a product which may be freshly produced in a commercial or domestic kitchen or purchased as a preserved product off the shelf.

All food products have a finite shelf life and recommended storage procedure which is decided by ongoing tests as part of a HACCP (Hazard Analysis) that the food manufacturer is obliged to conduct. It may consist of live tests of that particular product, or used and known parameters of a product.

In the case of commercially produced mayonnaise it has gone through a bottling process which produces high temperatures that will kill all harmful (pathogenic) bacteria. Once opened the mayonnaise is vulnerable to various contaminants. If the acid level supplied by the lemon juice or other acidic additions is such that it inhibits the growth of bacteria you may store it at ambient temperature. However, I have yet to see a jar or tube of mayonnaise from a commercial company that recommends not refrigerating their product.

Some products may not be terribly acidic which immediately puts those who ignore the manufacturers' instructions at risk.

If the product is homemade there are recommendations in place from various agencies such as the FDA and the FSA to use pasteurised eggs, or to allow sufficient acidic addition via wine vinegars or lemon juice to create a pH of 4.1. In addition to this low pH it is recommended to allow the mayonnaise to stand for 72 hours in order to destroy any potential contaminants present. It really is not straightforward!

My attitude is less complicated. I buy good quality mayonnaise where there are only three ingredients egg yolk, oil and lemon juice, a wholesome product. Once opened I make sure it is refrigerated. Peace of mind.

Where mayonnaise isn't refrigerated and has, due to a food poisoning incident, been proved to support bacterial growth, it is often due to Staphylococcus Aureus. This is a nasty bug which is found naturally on a large proportion of people. It is known as a commensal bacterium as it does no harm to the individual (host). It lives on a person's skin, in noses, mouths and throats, and there are hundreds of strains. Generally one strain is found within a family minimising risk to other family members. However this is why catering for others requires a higher standard of hygiene than perhaps some people would exercise when cooking only for the family. If someone else ingests these bacteria you may make them ill. Hence the basic rules about licking spoons and reinserting them into food, not sneezing over food and the opening paragraph illustrating this scenario.

Mayonnaise is a condiment which is handed around and may or may not contain the required acidic pH level to be stored ambiently - whether it is commercially produced or homemade.

Why take the risk? Refrigerate!

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