I've noticed that if I puree raw onions before sauteeing them they turn increasingly green as they cook - and they have a harsh smell until they brown (with garlic, cooked in a stainless steel pan). If I chop them fine, they don't go green, nor do they smell strange. I'm guessing that pureeing them breaks down the cells and some chemical is released that causes the colour change?
And today I made an aubergine (eggplant) puree with mint, garlic, fresh chillies and tahini - it was a dull brown-green-purple sludgy colour till I added lime juice - then it went a quite vivid pink. I'm guessing a reaction to the acid - but I've no idea what turns pink in acidic conditions.
Anyway, it got me wondering if there are any recipes that depend on a chemical reaction. I can think of cinder toffee (I'm not sure what that is called in the US) - and fermentation (cheese, alcohol) - but are there other ways to play chemistry sets in the kitchen? Is it possible to make something toxic by cooking unusual combinations?