Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

THE BREAKDOWN OF CELL WALL COMPONENTS 167
Level
Arabinose
loss
Galactose
loss
Pectin
solubilization
Pectin
depolymerization
Absent
Apricot
Plum
Watermelon
Cucumber
Plum
Apple
Watermelon
Apricot
Plum
Watermelon
Some
Pepper
Tomato
Apricot
Blueberry
Apple
Pepper
Tomato
Intermediate
Avocado
Peach
Kiwifruit
Apple
Melon
Peach
Banana
Plum
Strawberry
Tomato
Avocado
Peach
Kiwifruit
Extensive
Pear
Blueberry
Muskmelon
Pepper
Tomato
Avocado
Blackberry
Kiwifruit
Pear
Blueberry
Fig. 8.1 Classification of fruits based on extent of cell wall polysaccharide modifications during ripening.
Shown are ripening-associated losses of arabinose and galactose and solubilization and depolymerization of
pectic component from cell walls of fruits of various species. Levels represent: Extensive >70%, intermediate
25–70%, some ≤25%, and absent undetectable. (Redrawn from Brummell, 2006.)
changes. Substantial differences in proportion of pectin solubilization exist between species;
greatest solubilization occurring in ripening avocado followed by kiwifruit and blackberry,
whereas solubilization was very low or absent in apples and watermelon (Brummell, 2006).
The increased pectin solubilization is correlated with a swelling of cell wall (Redgwell
et al., 1997a). In persimmon, avocado, blackberry, strawberry, and plum fruit that ripens to
a soft melting texture, wall swelling was pronounced, particularly in vitro. In vivo swelling
was marked only in avocado and blackberry. Fruit that ripened to a crisp, fracturable texture
such as apple, pear, and watermelon did not show either in vivo or in vitro swelling of the
cell wall. There is a correlation between swelling and the degree of pectin solubilization,
suggesting that wall swelling occurred as a result of changes to the viscoelastic properties
of the cell wall during pectin solubilization. The pectin that is solubilized during ripening
generally has a low Gal (Redgwell et al., 1997b) and Ara content (Brummell et al.,
2004a). This indicates that either solubilization and Gal/Ara loss largely affects different
pectic molecules (Redgwell et al., 1997b) or loss of almost complete side chain results in
polyuronide solubilization. Swelling was associated with movement of water into voids left
in the cellulose–hemicellulose network by the solubilized pectin (Redgwell et al., 1997a).
These processes combined with the loss of pectic side chains increase wall porosity that
may allow increased access of degradative enzymes to their substrates later in ripening
(Brummell, 2006). The walls of fleshy fruits become much more open, and hydrophilic environment
exists as ripening progresses. The increased accessibility of hydrolytic enzymes
to their substrate further promotes polymer dismantling.