580 Produce Degradation: Reaction Pathways and their Prevention
The pectate lyases (poly(1,4-α-galacturonide), EC 4.2.2.2) belong to the lyase
group of enzymes, which do not involve water as a substrate. These enzymes work
by splitting the glycosidic bond via trans elimination of hydrogen from the C-4 and
C-5 positions in the sugar molecule (Figure 19.15). This group of enzymes could
utilize either pectin or pectic acid as a substrate and could operate in both exo and
endo modes. These enzymes have a pH range between 8.5 and 9.5 and appear to
require divalent cations such as Ca+2 as a cofactor for optimal efficiency. EDTA
inhibits the enzyme activity completely.
The knowledge gained from the chemistry of pectin polymers and the role
enzymes play in their degradation has transformed the fruit and juice industry
worldwide. For example, fungal pectinases from Aspergillus spp. are commercially
used for partial degradation of pectin in fruit processing, leading to the clarification
of fruit juices and wines. More recently, microbial propectinases from Geotrichum
penicillatum and Geotrichum candidum have been discovered [96]. They are very
specific endo-polygalacturonases that solubilize all the pectin in a protopectin during
extraction from citrus waste. This has replaced a chemical (acid) extraction proce-
dure, which constitutes a significant breakthrough for the citrus industry in terms of
food safety and cost saving. The possibility exists to use pectin in biocatalytic
reactions exactly as envisioned for starch and cellulosics in health and environmental
protection.
19.4 BIOCHEMICAL AND STRUCTURAL CHANGES
IN PRODUCE IN RESPONSE TO POSTHARVEST
STRESSES AND ITS RELATIONSHIP TO
CARBOHYDRATE POLYMERSFruits and vegetables are subjected to a wide variety of stresses from the time of
harvest through postharvest handling and processing. In response to these stresses,
tissues and cells in plants undergo numerous physiological and biochemical changes
FIGURE 19.14Action of polygalacturonases (poly-α-1,4-galacturonide glycanohydrolase
(EC 3.2.1.15) on the hydrated pectic substances where it cleaves the glycosidic bonds.
FIGURE 19.15The pectate lyases (poly(1,4-α-galacturonide), EC 4.2.2.2) work by trans
elimination of hydrogen from C-4 and C-5 positions in the pectin molecule.
OHOMeOOC
OHOO
O
HOHOOC
OHO hydrolysisH 2 OOHOMeOOC
OHO
HO
HOHOOC
OH+ OHOOHOMeOOC
O
OHO
O
HOHOOC
OHO eliminationOHOMeOOC
OHOHOHOOC
OH+ OHO