Some time ago I boiled some blue/purple potatoes and discovered the water had turned bright green. Not being sure what that could possibly be (on store-bought spuds) I threw it all out.
This summer, I grew my own blue potatoes, and the same thing happened. I know these are clean and chemical free, since I grew them myself. What caused the water to turn green, and is it safe?
— Jennifer Armstrong
Saratoga Springs, NY
Ah, welcome to the fascinating (I’m not being fascicious, I truly think it’s fascinating) world of food science.
Questions like yours are also great because they help me realize that the mandatory Introduction to Food & Food Science I had to take during my college education does come in handy!
So, why did your blue potatoes yield green water? Nope, it wasn’t a mutant Monsanto potato. This just comes back to a very basic science concept — pH levels.
Most tap water is slightly alkaline, which doesn’t jive well with the more acidic potato environment. Alas, the blue pigment (caused by the presence of antioxidants known as anthocyanins) left a green tint in your water.
If you’d like to prevent this next time, add a small amount of vinegar to your cooking water.
While we’re on the subject of potatoes and the color green, I think it’s worth reminding everyone that while this example is no cause for concern, a potato with a green tint on it is.
A green potato has high levels of solanine, which can result in unpleasant symptoms when consumed.