Chlorophyll, the large molecule with a long tail and a body holding Magnesium atom at its center, gives vegetables the Green color. Any change in this pigment during a cooking process will dull or lead to the loss of the bright green color.
Prolonged cooking, high temperature, acidic medium, and exposure of the chlorophyll molecule to the cell’s own enzyme will result in the loss of the Magnesium atom or the tail; a prime reason for losing the green color. Therefore, it is important to understand the right cooking technique to preserve the green color of the vegetables.
Chlorophyllase is the cellular enzyme that is most active at 66-77 °C (slow-cooking) and is responsible for the loss of the tail portion of the chlorophyll molecule. The loss of the long tail portion makes the chlorophyll pigment water soluble, leaking it out into the cooking medium, and further undergoing changes.
Cooking without water is not a solution nor is high heat cooking technique such as stir-frying or microwaving. This is because, with the rise in temperature, the cell membrane undergoes damage, exposing the chlorophyll molecule to its natural acids present in the plant. The best strategy is blanching as it keeps the cooking time short (5-7 min), and as blanching requires a large amount of water, it dilutes the cell’s natural acids to a large extent. As blanching is carried out in boiling water it destroys the chlorophyllase enzyme preventing the loss of the tail portion of the chlorophyll molecule.
We could see the result of blanching while cooking Palak Paneer where spinach leaves were blanched and cooked with cottage cheese with no loss of its bright green color. See the recipe for tips and cooking technique.
In slightly acidic cooking medium, hydrogen atoms replace the magnesium atom of the chlorophyll pigment, and you can see the change when the bright green in the vegetable changes to dull green.
One way to avoid losing the ‘bright green’ is to cook the vegetable in a slightly alkaline water adding a pinch of baking soda since the amount of hydrogen atoms required to displace the magnesium is less. There is one drawback - adding excess of baking soda can turn the vegetable mushy. To avoid this use a pinch of baking soda for 4 liters of water. The household tap water is slightly alkaline in nature (to prevent pipe corrosion) and using this water for blanching is better than adding baking soda.
Another strategy is to cook the vegetable in a vessel made of copper, brass or silver. The copper or zinc metal will replace the lost magnesium and avoid any change in color. These metals are beneficial in trace amounts but in large quantity, it can lead to poisoning.
If you are planning to consume raw leafy vegetables as salads, add the dressing in the end - right before serving. Most dressings use acidic ingredients like lemon juice or vinegar that can change the natural color of the vegetable. Another way is to coat the greens with a thin layer of butter or olive oil and then add the acidic dressing. The coat of oil prevents the direct contact of the acidic ingredient with the chlorophyll.
Dehydration, freezing, pickling, and other preservation techniques can damage the chlorophyll, and change the color of the vegetable.