198 Produce Degradation: Reaction Pathways and their Prevention
7.3 CHLOROPHYLLS
Chlorophylls are green pigments of ubiquitous occurrence in nature. It is estimated
that plants, algae, and some microorganisms produce over 1 billion tons of chloro-
phylls per year (Schwartz and Lorenzo, 1990). Chlorophylls in photosynthesis allow
the sun’s energy to be utilized to synthesize carbohydrates and in this way change
light energy into chemical energy that is indispensable for the survival of all living
organisms. In higher plants chlorophylls a and b are imbedded in the highly struc-
tured lamellae of chloroplasts. The chlorophyll molecules interact with each other,
as well as with proteins and lipids. Most of the research on chlorophylls is concen-
trated on the biochemistry of photosynthesis and is beyond the scope of this review.
The chlorophylls are important constituents of produce. In fruits they usually degrade
during ripening and allow other colors to be unveiled or formed. In green vegetables
retention of color is of paramount importance for consumer acceptance.
7.3.1 CHEMICAL DEFINITION AND STRUCTURE
Porphin can be considered a basic structure from which chlorophylls may be derived.
Porphin consists of four pyrolle rings linked by carbon atoms. According to the
IUPAC nomenclature there are four pyrolle rings, A, B, C, and D, in the porphin
molecule. The carbon atoms are numbered from 1 to 20 starting from ring A. Fisher
proposed the most commonly used nomenclature and numbering of carbon atoms
(see Figure 7.3). The pyrolle rings are numbered I through IV. Peripheral carbon
atoms on the porphin rings are numbered from 1 to 8. The bridging carbons are
designated as Alpha, Beta, Gamma, and Delta. Alternatively, the pyrolle rings may
be assigned letters A, B, C, and D. When a fifth isocyclic ring is added to porphin
the resulting structure is called phorbin. The name porphyrin is used for substituted
porphins. Chlorophyll is considered a porphyrin with a centrally located Mg2+ ion.
In position 7 of the fourth ring there is a propionic acid substituent esterified with
phytol, a C-20 monounsaturated isoprenoid alcohol, that is responsible for the
hydrophobic nature of the chlorophyll molecule. In positions 1, 5, and 8, methyl
groups are present. A vinyl group is located in position 2. In position 9 there is a
carbonyl group and in position 10 an acetyl group. The isomer chlorophyll a contains
a methyl group in position 3, whereas an aldehyde group occurs in this position in
chlorophyll b. Isomers of chlorophyll a and b are found in products that undergo
heat treatment. These isomers are formed by inversion of the carbometoxy group at
C-10. Some trivial names are widely accepted in chlorophyll chemistry and were
adopted here from Jackson (1976):
Phyllins: chlorophyll derivatives containing magnesium
Pheophytins: the magnesium-free derivatives of chlorophylls
Chlorophyllide: the acid derivative resulting from enzymic or chemical
hydrolysis of the C-7 propionate ester
Chlorophyllase: the enzyme present in leaves that catalyzes hydrolysis of the
C-7 propionate ester
Pheophorbides: the products containing a C-7 propionic acid resulting from
removal of magnesium and hydrolysis of phytyl ester