Chlorophylls are green pigments that contain a porphyrin ring complexed with magnesium. The two major chlorophylls, a and b, differ in that a methyl group in a is replaced by an aldehyde group in b. Chlorophyll a and b are degraded to Pheophytin a and b, respectively, by the replacement of magnesium with a proton. This degradation changes the colour of green-colored plants from bright green to dull olive brown. Acid conditions produced during thermal processing are responsible for this colour change. Figure 3 below shows the structure of both chlorophyll a and chlorophyll b. Chlorophyll is called a magnesium porphyrin because the four nitrogen atoms in the porphyrin ring are coordinated with a magnesium ion. Chlorophylls a and b occur together in about a 3:1 ratio. Chlorophyll a is blue green, whereas chlorophyll b is yellow green.

 

In the United States, purified chlorophyll is not an allowed colour additive. However, juices from green vegetables are sometimes used as colorants in pasta (spinach pasta) and other foods.