A biology colleague asked me yesterday if I knew why the British put sodium bicarbonate in the water when they boil their veggies. Leaving aside the question of why anyone would want to emulate the British when it comes to cooking, he thought it would have something to do with keeping the chlorophyll green. Chlorophyll is a little net with four nitrogens on the inside that grab hold of magnesium, which gives it the green color that lets it absorb light from our yellow sun so effectively. When you boil vegetables and jostle that magnesium around, sometimes it falls out. How does bicarbonate prevent that?
Well, I didn't know, but I had a book that did: On Food and Cooking by Harold McGee. This is a wonderful reference that I really need to read cover to cover sometime. But it had a whole section on this.
It turns out acid (positively charged) can displace the magnesium (positively charged), and it leaves a big hole in the chlorophyll, making it yellow. Extra bicarbonate absorbs extra acid and the magnesium stays put, and the chlorophyll stays green. An interesting twist is that copper can displace magnesium too, but because copper's a metal the chlorophyll stays nice and green, so some old cookbooks say you can keep your veggies green by boiling them in copper pots. Very cool and it does work ... except for the fact that copper is a poison!
So there you have it, British cooking is poisonous. (Well, the copper pot version. Actually, I shouldn't make fun of British cooking so much -- I absolutely love tea and bangers and mash!)