Before you make that potato salad for your next cookout, be sure to read this: According to cookbook author Zola Gorgon, the onions you use could be a possible source of food poisoning.
Zola recently toured Mullins Food Products, the makers of mayonaise. Her tourguide was a chemistry expert named Ed, who develops most of the sauce formula (he even developed one for McDonalds!) He addressed the common idea that mayonaise will spoil, and his answer will surprise you...
Ed said that commercially made Mayo is completely safe. There is no harm in refrigerating it, but it is not really necessary. The pH is mayonaise is set at a point that 20 bacteria could not survive. He addressed the quintessential picnic, with the bowl of potato salad on the table and how everyone blames the mayonaise when someone gets sick. Appartently the first thing the that should be looked at are the ONIONS in the salad, and then possibly the POTATOES.
Onions, especially uncooked ones, are a huge magnet for bacteria. Both the onions and the moist potato in the potato salad will attract and grow bacteria faster than any commercial mayonaise will even begin to break down. He also advised that you should never keep a portion of a sliced onion, even in a zip-lock bag in our refrigerator, saying that it's already contaminated enough just be being cut open and out for a bit, that it can be a danger. Cooking the onion may be okay, but the slices on a leftover sandwich could be asking for trouble. Dogs should never eat onions, as their stomachs cannot metabolize them. Another note: watch out for the onions on your hot dogs at the baseball park!