200 Produce Degradation: Reaction Pathways and their Prevention
was detected in citrus peels with a concurrent increase in chlorophyllase activity. A
different path of chlorophyll degradation, with formation of pheophytin, was
observed in parsley leaves. According to the older publications reviewed by Gross
(1991) the most common degradation of chlorophyll during thermal processing,
freezing, and storage of green vegetables is the conversion of chlorophyll to pheo-
phytins with a color change to olive-brown. The duration of treatment, temperature,
and concurrent release of acids accelerates degradation. In frozen-pack peas and
string beans 60 to 85% of chlorophyll changed to pheophytin. In canned green beans
all chlorophyll was converted to pheophytin. Several studies demonstrated that
blanching, which involves various forms of heat treatment to inactivate enzymes,
has a beneficial effect on retention of chlorophyll during processing and storage.
Dehydration causes chlorophyll degradation. During dehydration of spinach 26% of
chlorophyll was converted to pheophytin. Blanching before dehydration increased
the loss to 44%. Sterilization of canned green produce has a detrimental effect on
chlorophyll. After sterilization of green peas for 25 min at 120°C all chlorophyll
was degraded to pheophytin.
The initial change in the chlorophyll molecule subjected to heat is isomerization
by inversion of the C-10 carbomethoxy group (vonElbe and Schwartz, 1996). The
chlorophyll a and b isomers can be separated on a C-18 reverse-phase HPLC column.
Heating spinach leaves for 10 min at 100°C leads to conversion of 5 to 10% of
chlorophyll a and b to pheophytin a and b (Schwartz et al., 1981). The olive-brown
pheophytin is formed by magnesium ions’ being displaced by two hydrogen ions.
Formation of pheophytin from chlorophyll a occurs more rapidly than that from
chlorophyll b. This effect is attributed to the electron-withdrawing effect of the C-3
formyl group in chlorophyll b. Pheophytin is subject to further change by the
replacement of the C-10 carbomethoxy group with a hydrogen atom, leading to
formation of pyropheophytin. Reverse-phase HPLC allows separation of pyropheo-
phytins a and b from corresponding pheophytins. It was demonstrated that pyropheo-
phytins a and b are responsible for the olive-green color of many commercially
canned vegetables (Schwartz and Lorenzo, 1991). The hydrogen ions of pheophytins
and pyropheophytins are easily displaced by zinc or copper ions. The green-colored
complexes are stable in acidic solutions. Copper complexes of pheophytin and
pheophorbide are available commercially but are not allowed for food use in the U.S.
7.4 BENZOPYRAN DERIVATIVES
7.4.1 CHEMICAL DEFINITION AND STRUCTURE
Benzopyran derivatives in produce include compounds containing a central pyran
ring and two adjacent aromatic rings. Selected basic structures [see Figure 7.4 (a)-(g)]
include flavan, flavan-3-ol, flavanone, flavanonol, flavone, flavone-3-ol, and chal-
cone. By adding functional groups to these structures at various positions, numerous
compounds may be derived. Flavonoids derived from these structures play an impor-
tant role in the physiology and postharvest natural and man-made changes in pro-
duce. Many of these polyphenolic compounds have strong antioxidative properties
and are postulated to be beneficial for human health. Although anthocyanins are