Enzymes in Quality and Processing of Tropical and Subtropical Fruits - Tropical and Subtropical Fruits: Postharvest Physiology, Processing and Packaging - page 37

Nonthermal and/or minimal-thermal treatments have
also been used to preserve as much as possible the orig-
inal sensory and nutritional properties of the fruit (Leistner
and Gorris, 1995; Alzamora et al., 1998). A combination
of mild heat treatment and osmotic dehydration using dif-
ferent concentrations of additives has been shown to de-
crease PPO activity and reduce the browning in banana fruit
(Waliszewski et al., 2007). Also, packaging in controlled
atmospheres has also been shown as an effective method
to control the browning process by limiting or excluding
oxygen (Ahvenainen, 1996).

and also for increased pectin solubility (Carrington et al.,
1993). In avocado, experiments indicated that PG is a dom-
inant factor in pectin, or polyuronide, degradation dur-
ing ripening based on undetectable PG activity during the
preripe (preclimacteric) stage and dramatically increased
activity during the climacteric and postclimacteric stages
(Wakabayashi et al., 2000). Interestingly, tomatoes and av-
ocados contrast in the rate of pectin degradation, where
tomatoes have been shown to have more gradual pectin
degradation compared to avocados, in which nearly 90%
of the total cell wall uronic acid is affected (Sakurai and
Nevins, 1997; Wakabayashi et al., 2000).
Table 3.4 shows the optimal reaction conditions for PG
in some tropical and subtropical fruits. In papaya, PG activ-
ity was highest in the placenta with the activity decreasing
outwardly from the placenta to the exocarp (Chan et al.,
1981). PG with varying levels have also been isolated from
pear (Ahmed and Labavitch, 1980), mango (Singh and
Dwivedi, 2008), olive (Fernandez-Bola nos et al., 2001),
orange (Dong et al., 2008), and banana fruits (Pathak
et al., 2000).

POLYGALACTURONASE (PG)

Nomenclature and reactions catalyzed
Polygalacturonase (PG) catalyzes the hydrolytic cleavage
of glycosidic α -D-(1-4) bonds in cell wall pectin (pectic
substances). The endo-acting type (EC 3.2.1.15) hydrolyzes
α

-(1-4) linkages between adjacent galacturonic acid units
at random whereas the exo-acting type (EC 3.2.1.67)
releases monomers or dimers from the nonreducing end
of the chain (Brummell and Harpster, 2001). The basic
structure of pectic substances consists of galacturonic acid
residues linked by

Control of PG activity in tropical
and subtropical fruits
Pulp softening is one of the most remarkable and necessary
changes during ripening of fruits; however, overripening
or softening can be a major cause of postharvest deterio-
ration and losses. Continued solubilization and depolymer-
ization of pectic polysaccharides have been observed during
the postharvest ripening of many fruit types (Wakabayashi
et al., 2000; Fabi et al., 2009). Thus depolymerization of
pectin leads to a drastic decrease in viscosity during indus-
trial processing and, consequently, diminishes the quality
of some fruit products. This is particularly important where
consistency and texture are critical quality attributes. Inac-
tivation of PG is thus important in mitigating these quality
defects. Pathak and Sanwal (1998) showed complete loss

-1-4 glycosidic bonds (Van Buren,
1979). This enzyme works in concert with other pectolytic
enzymes, chiefly pectin methylestrase (PME), to hydrolyze
pectic substances (Fachin, 2003).

α

Occurrence and activity of PG in tropical and
subtropical fruits
Pectic substances (pectin) contribute to the integrity of the
cell wall matrix (Brummell and Harpster, 2001; Willats
et al., 2001). PG, along with other pectic enzymes,
is necessary for depolymerization and solubilization of
the pectin during the fruit-ripening process (Fischer and
Bennett, 1991).
In tomato fruit, transgenic studies demonstrated that
PG appears to be essential for pectin depolymerization

Table 3.4. Polygalacturonase (PG) in tropical and subtropical fruits.

Properties or Role in
Processing

Fruit Substrates Reference

−

Mango Rhamnogalacturonans Optimum pH range 4 7.5,

temperature around 40 ◦ C

Prasanna et al. (2006)

Avocado Polygalacturonic acid Optimum pH 6.0 Wakabayashia and Huber (2001)

Papaya Polygalacturonic acid Optimum pH 4.6 and

temperature 45 ◦ C

Chan and Tam (1982)

Orange Polygalacturonic acid Optimum pH 5.0 Hart et al. (1991)