Originally Posted by mbasiszta
Thanks. I too was always wary of anything with old mayo. I was taught that long ago. I am still a sceptic.
A few points:
Health departments do not "look at onions first". They don't look at any food first. They develop what is called an "attack rate" chart, where the names of all served foods are lined up across the top and all the names of the sick are lined up along the left side. Each food a sick person ate is then checked off. The food with the most checks becomes the suspect food. Sometimes it is obvious as one food will have many more check marks. Sometimes it is not, as in some cases, almost everyone ate almost everything (for example a fish fry)
Step two is to get samples of all food if available as well as stool samples from as many of the sick as possible. Then both the food and the stools are tested for for the suspected bacteria (based on the symptoms and period of onset). This often resolves the case. Sometimes when nothing is available, it just becomes a best guess.
Commercial Mayo is relatively safe, and in fact the health dept doesn't even require it to be refrigerated on a sandwich bar in a restaurant. The reason, as has been pointed out is the PH. Once you add potatoes and other items you raise the PH of the entire product and this allows bacterial growth. It is a misconception to think that the bacteria grows only on the potato in this case. Bacteria like protein and mayo whose PH has been raised is a good source.
Finally, food poisoning is caused by pathogenic organisms given the proper temperature and time for growth. Some foods, particularly those that are most likely to be improperly handled, may be implicated in a larger percentage of food poisonings, however, almost any food with the right PH and a high enough water activity can be a host for growth.