Chlorophyll Leaching...From My Dinner

02/02/2009

As I was preparing dinner the other night, I was steaming some bok choy - a leafy green vegetable sometimes called Chinese cabbage or napa cabbage. I placed a small amount of water in the bottom of a sauce pot, over which was a colander holding the bok chow. The flame under the saucepot heated and vaporized the water, cooking the vegetable. After dinner whilst cleaning up I poured the leftover water into the sink and noticed that is had a pale green tinge to it. Immediately I assumed this was color that was leached out of the bok choy, but what exactly? Was it from the outside of the leaves or the inside of the stalk? Did it even come from the vegetable or did I not wash it enough? Was I taking flavor out of my food? What was responsible for this colored liquid?

Upon further research I learned that a near-ubiquitous plant pigment was leached out: chlorophyll. Chlorophyll is what gives plants their green color (in an obfuscated way, discssed below) and also aides in their energy production.

I recall from high school that a red car is actually every color except red, because it's paint absorbs all but one color from the light that hits it and bounces that rejected color back to our eyes. Chlorophyll is to plants what paint is to a car; all colors except green are absorbed by chlorophyll, green is bounced back, and the plant appears that color. Green light in the visible spectrum does not have as much energy as other wavelengths, so it is rejected by the plant.

Chlorophyll is present in nearly every part of the plant and can be broken down easily, which is why some of it leached into water. The pigment can actually be isolated in an experiment for children, found in this PDF. Beyond chlorophyll, other chemicals and vitamins are leached out of a plant during cooking. Assuming that the rate of leaching is roughly constant for all chemicals, it follows that the greener the water after cooking, the more vitamins the vegetables have lost. In future dinners, I'm going to steam the vegetables just enough to retain as much nutrition as I can.