576 Produce Degradation: Reaction Pathways and their Prevention
cleaved by pectin methylestarases present in the cell wall [98,99]. Reduction in
pectin methylestarase and polygalacturonase activities in tomato fruits results in
pectins with a higher degree of esterification and higher molecular weights [100,101].
Reduced pectin methylestarase activity in tomatoes causes an almost complete loss
of tissue integrity during fruit senescence but has little effect on fruit firmness during
ripening [102]. Reduced pectin methylesterase activity also modifies both accumu-
lation and partitioning of cations between soluble and bound forms of pectin and
selectively impairs the accumulation of Mg2+ ions over other cations. High-resolution
images of the tomato cell wall suggest that it is constructed from at least two
independent networks, one based on cellulose/hemicellulose and the other on pectin.
Reduction in the cellulose/hemicellulose network does not affect the thickness of
the cell wall formed or the spacing of pectin molecules [103]. When tobacco cell
walls were grown under unadapted high-salt conditions, pectin molecules were
oriented within the wall in a manner similar to cellulose, whereas in an adapted cell
wall there was no clear orientation [104]. Esteban et al. [105] reported on the role
of pectic substances in the texture maintenance of eggplant fruit. Pectin esterification
is also reported to play a role in plant resistance to certain diseases [106].
Although proteins are not discussed in this chapter, it is important to note that
in addition to cellulose, hemicellulose, and pectin, the primary cell wall also contains
small amounts of proteins or glycoproteins that are enriched in hydroxyproline, an
amino acid. Hydroxyproline residues are also found in collagen, the extracellular
matrix protein in animals. In the case of plants, however, the hydroxyproline as well
as many serine residues are linked to short oligosaccharide side chains, thus forming
glycoproteins. Since it is difficult to extract the glycoprotein without destroying the
structure of the cell wall, it seems that these molecules, along with cellulose,
hemicellulose, and pectin, are tightly integrated in the complex polysaccharide
matrix of the cell wall.
For a plant cell to expand or bring about changes in its shape or morphology,
the cell wall has to be stretched or deformed. Because cellulose microfibrils exhibit
little elasticity, such changes must involve the movement of microfibrils past one
another. The possible types of microfibril movements allowed will depend on the
orientation of the microfibrils within the primary cell wall as well as on the bonding
interactions between matrix macromolecules and the cellulose microfibrils. Cells
not only take in nutrients and expel waste products across their plasma membranes,
but they also respond to chemical signals in their environment. In the case of plant
cells, such signal molecules must penetrate the cell wall. Since the matrix of the
wall is a highly hydrated polysaccharide gel (the primary cell wall being 60% water
by weight), water, gases, and small, water-soluble molecules penetrate rapidly.
19.3 MOLECULAR MECHANISMS OF CARBOHYDRATE
DEGRADATION MEDIATED BY ENZYMES AND MICROBESCell and tissue injuries in produce can be triggered or initiated by a single or a
combination of factors that include photo, thermal, mechanical and chemical expo-
sures. Once the deterioration is initiated, hydrolytic enzymes from foodborne