|Figure 3 Structure of chlorophylls a and b.
[From Aronoff, S.
(1966). In “The
(L. P. Vernon and G. R. Seeley, eds.),
Press, New York.]
Several chlorophylls have been described. Two of them,
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.