Broccoli, like other green vegetables, gets greener when you start cooking it. Why is this, and why does this green colour fade to a grey-green the longer it’s cooked? This graphic looks at the compounds produced when preparing broccoli to find the answer.
Green vegetables get their colour from chlorophyll, a pigment in the chloroplasts of plant cells. Usually, gases in the gaps between plant cells slightly cloud chlorophyll’s green colour. During cooking this air expands and escapes, making the green colour more vibrant.
As soon as the green colour has intensified, it starts to fade. Why? Well, cooking causes plant cells to start to break down, releasing organic acids. Hydrogen ions from these organic acids react with chlorophyll. They take the place of the magnesium at chlorophyll’s centre, forming pheophytins.
Pheophytins cause the grey-green colouration of overcooked green vegetables. Adding baking soda to the water stops their formation, as it reacts with hydrogen ions. You can also just cook the vegetables for a shorter time (5-7 minutes) in boiling water. These short times are not enough for the cooking process to break down the plant’s cell walls.
Chlorophyll isn’t the only compound we don’t want to destroy when cooking broccoli. Like other cruciferous vegetables, broccoli contains compounds called glucosinolates. It contains particularly high levels of a glucosinolate called glucoraphanin.
When you chop broccoli, you release an enzyme called myrosinase from the plant cells. This enzyme reacts with glucosinolates, including glucoraphanin. The reaction creates several products, including a compound called sulforaphane.
Sulforaphane interests scientists, as studies have shown it can kill some types of cancer cells. It’s thought to increase the production of a group of enzymes that break down carcinogens. Results from trials in humans have been variable but investigations are ongoing.
How you cook broccoli affects the amount of sulforaphane present. This is because the myrosinase enzyme is heat-sensitive. If you cook broccoli too soon after chopping, the enzyme breaks down. As a result little sulforaphane forms and other sulfur-containing compounds are produced.
These compounds include hydrogen sulfide and dimethyl sulfide. They give broccoli (and other cruciferous vegetables) an unattractive odour. Their reactions also lead to the brown-grey colour of overcooked vegetables, which is best avoided!