If, after peeling an onion then topping and tailing it, I wait before chopping it, the middle segments push up and outwards (see photo, right). Why?

Alan Middleton, Weymouth, Dorset, UK

I recently received an email which involves a person who went to a mayonnaise factory. This person writes that storing cut onions in a fridge is very bad as onions quickly become filled with bacteria and are then dangerous to eat. I looked on the internet and found at snopes.com the reference to the email in question. According to snopes mayonnaise can be stored in the fridge almost indefinitely due to its PH, however the mystery remains, are diced onions poisonous if held onto for a length of time (apparently even overnight) ?

http://www.snopes.com/food/tainted/cutonions.asp

On a similar topic I have been told the same thing about cooked rice, that it should be stored in the deep freeze due to bacteria contamination, even overnight,  in the fridge.

Thanks

Paul(South Africa)