Keeping Vegetables Green

July 23, 2008

Have you ever wondered why cooking green vegetables leads to a loss of their bright green color? If so, read on. If not…well, read on anyway. It might be interesting. This is adapted from the always excellent Harold McGee.

There are two primary ways that veggies can lose their green. First, chlorophyll can lose its long carbon-hydrogen tail. This makes the molecule smaller and water-soluble. When in a liquid and lacking its tail, the chlorophyll will leak out of the vegetable, causing it to lose its green. The loss of its tail can be caused either by an acid or base liquid, or by an enzyme that functions best between 150° - 170° F (66° - 77° C).

The second method that causes a green vegetable to dull is the loss of a magnesium atom from the center of the chlorophyll molecule. This can be caused by heat, an enzyme, or acidic conditions. Without its magnesium, the chlorophyll turns dull.

Adding baking soda or a metal such as copper or zinc can prevent this discoloration, but too much baking soda will leave your vegetables mushy and too much metal is toxic. To preserve the color, first you want to keep the cooking time short - hopefully under seven minutes. An effective method is boiling them in a large amount of water, because the water will dilute the cells’ own acids. Additionally, McGee explains that most tap water is slightly alkaline (base), which also helps preserve the color. Either serve your vegetables immediately after cooking or dump them in a pot of ice water to stop the heat continuing to rob the chlorophyll of its magnesium.

If you dunk your green vegetables into boiling water, you don’t actually see a loss of color, but rather the brightness of the green increases.  The heat causes the gases between cell walls to expand and escape.  These gases, having previously blocked the green of the chlorophyll are now gone, leaving the bright green color.  If you plunge the vegetables into ice water after a brief boil, also known as blanching, the green will keep longer.

There you have it - the reason why green vegetables dull in color when cooking and what you can do to prevent it.

Posted in Food Science |