|MadSci Network: Microbiology|
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 temperatures. 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 cold.
Try the links in the MadSci Library for more information on Microbiology.