Structure and Function of Complex Carbohydrates in Produce 583
enzymes are present in embryonic cells as well as in developing young plants and
play a critical role in fruit maturity and ripening, including storage and postharvest
deterioration [150]. Kim and Robyt have shown that the enzyme glucoamylase reacts
with starch inside the granules (waxy maize, maize, and amylomaize) to produce
D-glucose, which is subsequently retained within the granule for other functions
[151]. During the postharvest storage of ripened grape flesh, a 52% increase in the
α-galactosidase activity was observed over a period of 10 to 15 d [152]. Sugar beet
taproots could lose sucrose in storage as a result of mechanical wounding [153].
Interestingly, these enzymes have been shown to adapt well with respect to geneti-
cally modified plants. For example, transgenic potatoes exhibiting up to a 25%
increase in the degree of branching of amylopectin indicated no apparent change in
granular size or morphology, but an increased degree of branching produced 5 to
15% more short chains in the amylopectin [154].
Because cellulose has crystalline and amorphous domains and in plant cell walls
it is complexed with other polysaccharides, its degradation requires multiple
enzymes of variable specificities. Most of the knowledge pertaining to cellulose
degradation has been generated from in situ studies on cellulosic substrates and
microfibrils utilizing enzymes from a wide variety of sources [155–158]. Several
excellent reviews have been written detailing the cellulolytic enzymes and their
sources and reaction pathways [1,159,160]. Coughlan has also reviewed aerobic and
anaerobic fungal cellulases and their mechanisms of attack on the crystalline cellu-
lose [1,161]. In this regard, it has been shown that while some endoglucanases
decreased the average degree of polymerization and improved the alkaline solubility
of pure natural cellulose, some cellobiohydrolases exclusively degraded the crystal-
line portion of the cellulose structure [162,163].
Many soil-borne microbes such as yeasts, fungi, and bacteria are potentially
capable of degrading cellulose. During postharvest handling, fruits and vegetables
contaminated with cellulose-degrading microbes could cause minor to major damage
to the produce’s structural integrity. Spoilage of produce by yeasts is directly related
to degradation of cellulose, starch, sugars, and protein polymers [164]. During the
storage and aging process, exterior surfaces in produce are more susceptible to
deterioration. Scanning electron microscopy of the blueberry surface (Figure 19.16)
showed substantial changes in surface morphology of a fresh fruit relative to that
of old fruits bought at the local grocery store. The fresh surface appears to be quite
smooth without any fungal growth (Figure 19.16A), whereas the surface of an aged
blueberry had attracted fungal growth (Figure 19.16B) and developed cracks (Figure
19.16C). Further examination at higher magnification revealed that these surfaces
also included some areas with heavy damage to surface cuticle/waxes, and perhaps
to the cell wall (Figure 19.16D).
Additionally, many biochemical changes occur in fruits and vegetables exposed
to soil microbes during both pre- and postharvest [164–166]. For example, Maras-
miius quercophilus, a white-rot basidiomycete, produces lacases that can effectively
bleach the whole leaf [167]. The white-rot basidiomycete Phanerochaete chrysos-
porium also produces extracellular peroxidases that can destroy lignin, which affects
the integrity of the lignocellulosic cell walls by making them prone to electrolyte
leakage, infection, dehydration, and other postharvest disorders [168]. Johnson et