been added. This is because heating damages cells and may cause their contents to leak out. Among
those things that leak out is various of the vegetables own acids. Hydrogen ions from these acids react
with the chlorophyll molecules responsible for the green colour to give this greyish green colour.
How H+ ions are released during cooking
· How to keep “green” vegetables green?
1. The pan should not be covered - the acids that leak from the cells may then evaporate with
the water, reducing the chances of magnesium being removed from the chlorophyll molecule.
2. An alkali e.g. bicarbonate of soda, can be added to the cooking water to neutralise the leaking
H+ ions. This prevents them from being able to displace the Mg ions, so the green colour is
preserved. Unfortunately, cooking vegetables in the presence of an alkali rapidly breaks down
the pectin that holds cell walls together as explained above, so although vegetables will be
“greener”, they will also have a less desirable texture.
3. A copper coin can be added to the cooking water - copper coins and unlined copper pans
contain free copper or zinc ions that prevent the H+ from displacing the Mg2+ atom by
replacing it itself, without greatly affecting the colour. Chlorophyll containing a zinc atom at its
centre will absorb the same light wavelengths as when it contains a magnesium atom at its
centre. Therefore, the desired bright green colour is preserved. Unlike with the addition of
bicarbonate of soda, the vegetable texture is not affected. However, eating too much copper
sulphate can have various health implications.
4. Green vegetables should be cooked in large amounts of water. Cooking in a large amount of
water has two benefits – firstly, the heat is distributed much faster, so cooking times are
reduced and vegetables have less time to lose their colour. Secondly in a large amount of
water, the H+ ions released are much more diluted, so H+ displacement is less likely to occur,
reducing colour loss. Therefore, leafy green vegetables, which cook quickly (since the heat