I'm out of mayo and decide that rather than make a trip to town, I'll just make some. So I pop open the cookbook and read a list of stuff on "danger! bacteria! ..." It seems that for the stuff from the store, it sits on a shelf for six months, then we bring it home where it sits for another two months. Then we open in and pop it into the fridge where it sits for a month. Does the store bought stuff have a bunch of preservatives? How long does home-made stuff last? Can you can it?