Chlorophylls

Figure 3 Structure of chlorophylls a and b. [From Aronoff, S. (1966). In “The Chlorophylls” (L. P. Vernon and G. R. Seeley, eds.), Academic Press, New York.]

Several chlorophylls have been described. Two of them, chlorophyll a and chlorophyll b, are of particular interest in food coloration because they are common in green plant tissues, in which they are present in the approximate ratio 3 : 1, respectively. Their structures resemble that of heme since they are all derivatives of tetrapyrrole. An important difference is that the central metal atom is iron in heme and magnesium in the chlorophylls. Another difference is that the pyrrole unit IV in the chlorophylls is hydrogenated. In addition, the chlorophylls contain a 20-carbon hydrophobic “tail,” the phytyl group (Fig. 3).

The chlorophylls are located in special cellular bodies, the chloroplasts, where they function as photosynthetic agents. As food pigments, chlorophylls impart their green color to many leafy (spinach, lettuce, etc.) and nonleafy (green beans and peas, asparagus, etc.) vegetables and to unripe fruits. They are not very stable pigments, however. Ethylene, a gaseous plant hormone, destroys chlorophylls, and it is occasionally used to degreen fruits. The acids naturally present, formed, or added to plant tissues during food processing convert the bright green chlorophylls to dull olive brown pheophytins by replacing the magnesium of the molecule with hydrogen. Unfortunately, no fail-safe procedure has been proposed for preventing this discoloration in heated and stored green vegetables. Freezing storage is an effective method of preserving the green color of vegetables.