Produce Degradation Pathways and Prevention

Structure and Function of Complex Carbohydrates in Produce 573

chains of other sugars protrude, and the variation in these sugar types determines
the type of hemicellulose. The backbone sugar molecules of the hemicellulose also
provide bonding between hemicellulose and other cell wall matrix components on
the cellulose microfibrils. Some of these hemicellulose molecules are cross-linked
in turn to acidic pectin molecules (e.g., rhamnogalacturonans) by short neutral pectin
molecules (e.g., arabinogalactans). Cell-wall glycoproteins are probably attached to
the pectin molecules. The structure of cellulose microfibrils and the associated matrix
are depicted in Figure 19.10.
Hemicellulose is composed of complex carbohydrate polymers with xylans and
glucomannans as the main components. In most xylans, the xylan backbone carries
acetyl, methylglucoronyl, and arabinofuranosyl side chains in varying proportions
[43]. The arabinofuranosyl groups can be further esterified by aromatic acids such
as ferulic and p-coumaric acid [44], which are thought to participate in lignin-
hemicellulose crosslinks involving ether linkages [45]. Lignin is a highly branched,
random polymer generated by the free-radical condensation of aromatic alcohols
[46].
The degradation of hemicellulose is performed by a complex set of enzymes
hydrolyzing the xylan or glucomannan backbone and the various bonds of the side
chains [47–50]. Lignin is highly resistant to biodegradation and protects cellulose
and hemicellulose against enzymatic hydrolysis. Enzymatic attack of lignin involves
radical oxidation by peroxidases [46,51].
Xylanases that break down hemicellulosic materials are currently being utilized
by industry in many processes, including the hydrolysis of agricultural waste for
further bioconversion into alcohol fuels, treatment of animal feed to release pentose
sugars, dissolving pulps to obtain cellulose for rayon production, and biobleaching
of wood pulps for paper production. Bacteria, yeasts, and fungi are all known to
produce xylanase enzymes. Of much interest are xylanases produced by Bacillus
thermotoga and Talaromyces byssochlamydoides because of their thermotolerance
(75 to 80°C) [52]. Xylanase breakdown of hemicellulose in wood pulp will produce

FIGURE 19.10The primary cell wall in plants represents a complex matrix where fibers are
interconnected to other polysaccharide components such as hemicellulose, pectin, and glyco-
protein. The microfibrils are usually embedded in a matrix of hemicellulose and lignin.

Glycoprotein

Cellulose

Microfibril

Acidic Pectin

Molecule

Hermicellulose

Molecule

Netural Pectin

Molecule