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.” The pH in mayonnaise is set at a point that bacteria could not survive in that environment. What about the quint essential picnic, with the bowl of potato salad sitting on the table and how everyone blames the mayonnaise when someone gets sick.
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?). It’s not the mayonnaise (as long as it’s not home made Mayo) that spoils in the outdoors. It’s probably the onions, and if not the onions, it’s the POTATOES. Onions are a huge magnet for bacteria, especially uncooked onions.
You should never plan to keep a portion of a sliced onion. 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 by 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 hot-dogs at the baseball park!)
Finally, 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 it 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.
So, how’s that for news?