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?
Typically, vegetables will lose their colour if they are over-cooked, so it's probably worthwhile 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; orange vegetables contain keratin. Each of these pigments requires a different approach when cooking to preserve colour.
When cooking green vegetables, never add an acid, such as lemon juice or vinegar, as this will cause the vegetables to discolour quickly. Always cook green vegetables rapidly and preferably without a lid.
With red and white vegetables, which contain flavonoids, the converse to green vegetables is true. 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 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.
you need a big pot of water to blanch small amount of vegetable, small portion at a time.
- 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.
- the color is provided by chlorophyll, which is destroyed by heat, acidity and vegetable enzymes.
- 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
- the acid is released into the cooking solution, and the action of acid is dependent on the concentration of acid.
- 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:
- 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
- the large amount of water allows quick dilution of acid released from the vegetables
- 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
If you don't want cold veges but still want them as colourful as possible, I find the following technique works a treat, left until just before a meal will be served:
- If you have frozen veges, ideally thaw them first for best result. You can run them under cold water (cheapest) or warm water (quickest), or sit them in a bath of either (least attention required).
- As mentioned in @bubu's answer, it's important to bring a good amount of water to the boil in comparison to the weight of veges you are going to cook (say 3-4 times the weight at least). Keep the element on maximum.
- Add the veges, and set a timer for them. Choosing the appropriate time for each type of vegetable takes practice, and is a matter of preference anyway. When using veges that require different cooking times, time them to finish at the same time. eg if I'm cooking carrot, zucchini and broccoli, I'd add the carrot first, wait 2 mins, then add the broccoli & zucchini and cook another 60 seconds (for total cooking time of 3 mins for the carrot).
- Sieve immediately to avoid overcooking & serve immediately (leaving them sitting around while hot will make them lose colour).
Minimising the time the veges spend heated maximises their colour. The reason a good amount of water is required, in comparison to the weight of the veges, is because when the veges are added the temperature drops, but the more water there is the less it will drop. If you have too many veges, or they are frozen, the temperature drops too much and it takes longer for them to cook and dulls their colour.
One can use a similar technique with steaming, but if too many vegetables are used then the ones closest to the steam are overcooked before those furthest away are cooked at all. The only solution to this is to produce more steam somehow (not possible in some cases), or to cook less veges at a time.