What I learned from a chemist. Ed is a food chemistry whiz. During a discussion, someone asked if we really needed to worry about mayonnaise. People are always worried that mayonnaise will spoil. Ed’s answer will surprise you. Ed said that all commercially-made mayo is completely safe. "It doesn't even have to be refrigerated. No harm in refrigerating it, but it's not really necessary." He explained that the pH in mayonnaise is set at a point that bacteria could not survive in that environment. He then talked about the summer picnic, with the bowl of potato salad sitting on the table, and how everyone blames the mayonnaise when someone gets sick. Ed says that, when food poisoning is reported, the first thing the officials look for is when the 'victim' last ate ONIONS and where those onions came from (in the potato salad?). He says it’s not the mayonnaise (as long as it's not homemade mayo) that spoils in the outdoors. It's probably the ONIONS, and if not the onions, it's the POTATOES. He explained onions are a huge magnet for bacteria, especially uncooked onions. You should never plan to keep a portion of a sliced onion.. He says it's not even safe if you put it in a zip-lock bag and put it in your refrigerator. It's already contaminated enough just being cut open and out for a bit, that it can be a danger to you (and doubly watch out for those onions you put in your hotdogs at the baseball park!). Ed also says if you take the leftover onion and cook it like crazy you’ll probably be okay, but if you slice that leftover onion and put on your sandwich, you're asking for trouble. Both the onions and the moist potato in a potato salad, will attract and grow bacteria faster than any commercial mayonnaise will even begin to break down. Also, dogs should never eat onions. Their stomachs cannot metabolize onions Please remember it is dangerous to cut an onion and try to use it to cook the next day, it becomes highly poisonous for even a single night and creates toxic bacteria which may cause adverse stomach infections because of excess bile secretions and even food poisoning. ( I am so guilty of doing this..no wonder we have stomach problems and blame it on other things!) Forget the onions on the salad bars for me!!! ugh!