It is frequently quoted that upwards of 80% of our taste is made up by smell.
So if you plug your nose and cover your eyes, the taste between an apple and onion should be indistinguishable, right?
The logic makes sense. When I'm home sick, congested and mucus-ridden, eating is often a mundane, muted task. All potato chip flavors taste the same. Chicken noodle soup reminds me of tea with chunks in it. When things are really misty in my nasal cavities, well, you can literally substitute apples and onions and I won't be able to tell the difference.
In order to test the myth thoroughly, I blindfolded a few of my friends and had them plug their noses while I fed them apples and onions. To everyone's surprise, the flavors were indistinguishable. When bit into without the onion's outermost skin, apples and onions share an extremely similar crunch.
If you're looking to indulge in this experiment at home, after you've gone through your trials, I highly recommend throwing in a slice of apple and onion into your mouth at the same time. We had an eerily interesting radish-like experience that I'm curious if anyone else shares as well. The flavors of the apple neutralize the onions pungency.
So there you have it, if you plug your nose, close your eyes and bite into an onion...it is indistinguishable from an apple...until your nostrils open again.