Is it possible to preserve color in steamed vegetables?

Question

When I steam green vegetables, like broccoli, they come out bright green. A few minutes later though, and they have darkened and dulled. Is this a symptom of cooking them for too long or is there some other way to preserve the bright green color?

Answer 1

Typically, vegetables will loose their colour if they are over-cooked, so it's probably worth while cooking them for a shorter period of time.

Usually steaming is a great way to preserve the nutrients and colour of vegetables, as is stir-frying rapidly.

Different kinds of vegetables contain various pigments in their skins. Green vegetables contain chlorophyll; red and white vegetables contain flavonoids and orange vegetables contain keratin. Each of these pigments requires a different approach when cooking to preserve colour.

When cooking green vegetables, never add a acid, such as lemon juice or vinegar as this will cause the vegetables to discolour quickly. Always cook green vegetables rapidly and preferably with out a lid.

With red and white vegetables, which contain flavonoids, the converse to green vegetables is true when cooking these. Adding an acid during the cooking process will retain and even restore the colour. For red vegetables use a red or white wind vinegar. For white vegetables use a slice of lemon.

Orange vegetables, which contain keratin and are generally fairly robust. For the most part they can be cooked with or without the addition of an acid and they will usually retain their colour.

Another way to preserve the colour is to blanch the vegetable in boiling water for a minute or so, then plunge them into ice water. Doing this stops the cooking process instantly. Using the method is useful if you intend to freeze vegetables.

Answer 2

you need a big pot of water to blanch small amount of vegetable, small portion at a time.

theoretical background:

1. there is gas in the immediate layer of the vegetable, which makes the vegetable appear a little bit dull. you want to drive this gas out.
2. the color is provided by chlorophyll, which is destroyed by heat, acidity and vegetable enzymes.
3. the enzymes works at room temperature, works quicker when slightly above room temperature, doubling their speed for every 10 degree C elevation of temperature, but is quickly deactivated (thus, become non-functional) at temperature near boiling point
4. the acid is released into the cooking solution, and the action of acid is dependent on the concentration of acid.
5. when temperature goes to near freezing point, all reaction slows tremendously

thus, use large amount of water and small amount of vegetable, then stop the cooking by quenching the reaction using ice cold water:

1. the large amount of water relative to the vegetable ensures the constant boiling-point temperature of the blanching water - this ensures fast cooking with little time for enzymes to work. if you use too much vegetable at a time then the temperature will drop due to transfer of heat from water to vegetable. this allow (1) driving out of gas quickly, (2) cooking of vegetable quickly without yellowing (caused by degradation of chlorophyll), (3) quick deactivation of vegetable enzymes
2. the large amount of water allows quick dilution of acid released from the vegetables
3. when you see the much-needed green colour you want, you want to stop the cooking process immediately to "freeze the moment" and what you want to do is to let the ice cold water stop the cooking reaction from outside to inside (especially outside, since color's on the outside)

that's basically it