Originally Posted by FryBoy
Anything will soften if you soak it long enough, especially in something a little acidic, like most marinades.
If it's acidic enough, yes, but not in acid levels that still resemble food.
Acid actually toughens onions. I'd give the Harold Mcgee cite, but my copy is on loan. So if you want tender onions, sweat them. Once you add something acid (tomatoes, vinegar, wine, lemon), they won't get any tenderer.