MadSci Network: Microbiology

Re: Mayonaise and Food Poisoning

Date: Wed Aug 2 16:06:38 2000
Posted By: Charlene Wolf-Hall, Faculty, Food Science
Area of science: Microbiology
ID: 963421951.Mi

Hi Steven and Lindy,

Usually I try to stay out of marital disputes, but I will try to bring some 
peace to your conflict.

Let's think about what mayonnaise is: vegetable oil,  egg and vinegar.  Now 
letís think about what disease causing bacteria really like to grow in: a 
nutritious medium with a fairly neutral pH, enough water and at warm enough 

The vegetable oil doesnít make a particularly good medium for growth 
because the bacteria prefer simpler things to eat than fatty acids and 
there is little to no water present.  The egg is an excellent growth medium 
and can even be a source of Salmonella (however the egg usually gets cooked 
in the processing of commercially produced mayonnaise, killing any 
Salmonella from the raw egg).  The vinegar has some nutrients, but is so 
acidic that most bacteria would not even survive in vinegar alone.

Now if we mix these ingredients together we have a nutritious medium with 
enough water mixed into it, but the pH is acidic enough that most disease 
causing bacteria would not grow in it.  Note that I say grow and not 
survive.  There are some disease causing bacteria that are showing 
resistance to acid levels traditionally thought to prevent any harmful 
bacteria from being a problem.  This means that if enough of these bacteria 
are contaminating the mayonnaise, they could still potentially cause 
disease, even if they donít grow in number in the mayonnaise.

So, in a way, you are both right.  There is an ingredient in mayonnaise 
(vinegar) that kills many types of bacteria, but this does not make 
mayonnaise completely safe. Especially when other food ingredients start to 
be mixed in with the mayonnaise.  For example, potato salad can contain 
quite a bit of mayonnaise, but once combined with potatoes, onions, eggs, 
etc. to form the salad the over-all pH of the salad is increased from that 
of the mayonnaise alone.  So, if disease causing bacteria are present, the 
pH is high enough, and the temperature is in the danger zone (40 - 140 F), 
then you could have the right conditions for a foodborne disease outbreak.

I hope that sheds some light on your disagreement.  

The best food safety advice I like to give is to always wash your hands and 
cooking utensils thoroughly and keep your hot foods hot and your cold foods 

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