How to prevent/slow down fava beans from turning to darker color

beans

I usually make fava beans from dry beans, I simmer them in plain water for hours.
Right after they are cooked they are bright green and have a very fresh delicious taste, but after letting it cool the color will change dramatically to a darker grey colour and as time goes the taste will change to the worse, Canned beans usually don't have this issue, I guess they add something to it, so what is causing this change and how can I prevent it?

Edit: I made an experiment by separating three bowls of beans and their water
one was topped with oil, one was rapidly cooled and then refrigerated and the last was the control left to cool down slowly in the open air, the rapidly cooled one was on the best in terms of colour then the oil covered one and last was the control

Best Answer

I think the answer lies in how you're cooling them (hence my earlier comment). Many vegetables need to be 'shocked' immediately after cooking in order to retain a vibrant color. As shocking stops the cooking process, it also contributes to maintaining a good texture and flavor.

Here is a very similar Q & A from The Globe and Mail:

The question: I love fava and green beans in the summertime and would love to serve them at dinner parties, but they always look dull and grey after I cook them. Whenever I have them in French restaurants, they're bright green and vibrant. My daughter insists the chefs are probably just using MSG. Is she right?

The answer: It's not likely MSG they're using - just a simple technique called a "big pot blanch and shock." The key is to cook your green vegetables as quickly as possible so the heat doesn't have time to release their pigment and then shock them in ice water as soon as they're done. Fill the biggest pot in your kitchen - I use an eight-litre stockpot - with cold water and bring it to the hottest boil your stove can muster. Add a cup of table salt for every four litres of water, then dump in only as many vegetables as you can add without stopping the boil. Cook them in batches if you must. When they're done, scoop them out and chill immediately in a big pot of ice water. And maybe wear some protective sunglasses. They're going to be that bright.

Since, at the end of your cooking, the beans have the nice color and flavor you like, I would suggest cooking as you normally do but use the shocking technique immediately after.

If you normally keep the beans in the cooking liquid, I would still shock the beans and refrigerate them separately from the liquid. You can refrigerate the liquid and, when cold, add the beans back in.

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