564 Produce Degradation: Reaction Pathways and their Prevention
19.1 INTRODUCTION
Nature utilizes sophisticated and ingenious biochemical pathways in developing
plants and/or fruits to synthesize simple carbohydrates such as monomeric sugars
and assemble them into simple to complex polysaccharide structures to perform
various physiological functions. Many of the molecules that make up the living
tissue in plants are polymers, which include cellulose, complex carbohydrates of
starchy foods, protein molecules, lipid, and DNA. Additionally, polysaccharides are
further complexed or linked with polymers such as protein, lipids, and lignin to
construct complex supramacromolecular structures and to confer tailored properties.
For example, in woody plants, lignin, a polyphenolic polymer, is complexed with
waxes that confer a shiny and smooth surface coating on the exterior of the plant
cell walls, providing protection from infections and restricting the flow of moisture
across the cell wall. In the absence of such coating, carbohydrate polymers will be
destined to rapid hydrolytic degradation and the plant will quickly dehydrate. In this
chapter we will only focus on carbohydrates such as starch, cellulose, hemicellulose,
and pectin, which play critical roles in providing the structural integrity and impor-
tant functionalities to plants, particularly in fruits and vegetables, during their growth
and maturity, as well as during their ripening, storage, and other postharvest pro-
cessing.
In general, polysaccharides in plants perform two main functions: they serve as
a powerhouse to store energy for carrying out various metabolic functions and they
provide structural integrity to plants. The simple distinction of α- vs. β-linkages
between the glucose monomer units determines the function of polysaccharides in
plant cells. Generally, the storage polysaccharides possess an α-linkage between the
1- and 4-positions of adjacent glucose units. In plant cells, this energy source is
starch, which is present as either amylose (linear molecules of glucose units) or
amylopectin (a branched glucose macromolecule). In animal cells, the energy source
is a highly branched arrangement of glucose units, called glycogen. The α-linkage
of storage polysaccharides imparts a helical conformation to the glucopyranosyl
chain. Because the helix inhibits extensive interchain associations, it is unfit as a
structural entity but excellent as an energy source because rapid degradation for
energy release is not hampered by the necessity of first breaking strong intermolec-
ular interactions.
On the other hand, structural polysaccharides provide the rigidity and elasticity
needed to protect cells. Such structural polysaccharides are characterized by the
β-linkages between glucopyranosyl units and, like storage polysaccharides, their
macromolecular structure varies with variation in life form. Unmodified cellulose is
the primary structural polysaccharide of plants. The^ β-linkage produces a nearly
linear, extended macromolecular conformation that permits close packing of polymer
chains; this close packing in turn encourages intermolecular hydrogen bonding (and
crystallinity) between adjacent chains to produce the rigidity required in a structural
material. The chemistry, structure, and functional properties of plant polysaccharides
will be covered in greater detail in the subsequent sections. Also included is a brief
discussion on enzymes, which are specific biological catalysts used in the synthesis
and assembly of carbohydrate polymers that provide useful functionalities to plants.