Structure and Function of Complex Carbohydrates in Produce 575
middle lamella component involved in intercellular adhesion [57,64]. The overall
strength of the plant cell wall is ultimately determined by factors such as the
orientation, mechanical properties, and links between pectic substances and cellulose
fiber, etc. [65]. Some pectin molecules are glycosidically linked to xyloglucan chains
that can bind covalently to cellulose [58,60,66]. The firming effect of pectin in fresh
tissues involves the formation of free carboxyl groups, which leads to enhanced
calcium binding between pectin polymers but decreases the susceptibility of the
pectin to depolymerization by β-elimination [67]. It has been reported that in tissues
of apples and tomatoes the normal decrease in the degree of methoxylation (DM)
(increase in free carboxyl groups) is not accompanied by firming during ripening
[68,69]. Softening during the ripening of fleshy fruits is attributed to enzymatic
degradation and solubilization of the protopectin [70–79]. It is generally believed
that textural changes in some produce occur as cell wall pectins are hydrolyzed by
polygalacturonases, as evidenced by the softening in some fruits such as tomatoes
that exhibit high levels of polygalacturonase activity [80–82]. However, other inves-
tigators found somewhat different results when comparing the firmness of unpeeled
kiwifruit and its water-soluble, high methoxyl pectin content [78].
The blanching and degradation of carrot tissues, surprisingly, increased mole-
cular weights of both the water-soluble pectin and the EDTA-soluble pectin obtained
from these tissues [83]; however, dehydration without blanching drastically
decreased the molecular weight of pectin in both of these fractions. The observed
increase in molecular weight in blanched tissues is attributed to the inactivation of
pectolytic enzymes. In this regard, many species of plants have shown a preferential
loss of either galactose or arabinose during the ripening process [84]. A good portion
of these neutral sugar residues can come from pectin side chains, which could then
increase the susceptibility of pectin to polygalacturonases and pectin methylesterases
by making it more accessible to these enzymes. Loss of side chains would also
reduce the entanglement of the pectin molecule, increasing the slippage factor. A
variety of glycosidases have been found to remove neutral sugars from pectin side
chains [85]. Nonetheless, several other studies suggested that additional mechanisms
might be involved in tissue softening [86–91].
Besides hydrolytic cleavage, other nonenzymatic mechanisms have also been
proposed for the possible loss of cell wall integrity and pectin degradation [91]. For
example, pectin degradation due to alteration in the ionic strength of fluids that
solvate the cell well has been suggested. In a recent study, Batisse et al. [92] reported
that softening during ripening in cherry fruits does not depend upon pectin depoly-
merization.
Pectins are among the cell wall components whose collective ability to contain
the turgor pressure of the cell wall determines whether growth will take place [93].
As structural components of plant cell walls, native pectins play an important role
in many quality aspects of fruit and vegetable products [94]. The size, charge density,
charge distribution, and degree of substitution of pectin molecules can be biologically
or chemically altered [95]. Pectins are synthesized during the early stages of growth
in young, enlarging cell walls [96–97] and are present in various stages of molecular
development, growth, and maturity [57]. In the initial stages of synthesis, the car-
boxyl groups of pectins are highly methylesterified, but the ester groups are later