Heat. Acids. Denaturation. And what happens to chlorophyll during cooking.
We can keep green vegetables green by boiling very briefly (also known as blanching). This removes the air pockets that might mask the green of the chlorophyll we see. The vegetables thus look brighter (explained in this 1-Minute NomNom “It’s not easy being green, unless you let go of some hot air“).
However, leave them in the boiling water for too long and the vegetables turn a drab green (see examples of overcooked broccoli and long beans). Why does this happen?Chlorophyll has a magnesium ion right in the middle. When vegetables are heated, the cell membranes denature (i.e. their structure breaks down). The acids (a substance that has hydrogen ions, H+, to donate to others) in the cells leak out. The hydrogen ions replace the magnesium ions. This creates a new molecule that is drab green (this new molecule is called phenophytin).
This also happens when we put vegetables in an acidic solution. The water might be slightly acidic, or the foods the vegetables are mixed with are acidic. The former could be due to the water supply; the latter could be salad dressings that contain vinegar or citrus juice.
But wait. If prolonged heating turns vegetables a drab green, what happens after we blanch them? After we fish them out, don’t they continue cooking because of conduction (see the 1-Minute NomNom “Pleased to heat you”)?
Yes, they do! That’s why recipes suggest that we dunk the cooked vegetables into iced water immediately after blanching (see video below). The cooking process stops, and the magnesium ions are not replaced by the hydrogen ions. Chlorophyll does not change, keeping its bright green.
Like this? Be ice-cool and like me to discover more! All you need is a minute a day to explore the world’s marvels through the phenomenon of food!
photos: in order – depositphotos/fotaww; zigzagmtart; photon; marcomayer