Microbial Metabolites in Fruits and Vegetables 511
and solubilization in common solvents, while noncrystalline regions are easily acces-
sible and represent the point of enzymatic and chemical attack.
A group of enzymes that degrade cellulose are referred to as cellulases and
enzymes that hydrolyze hemicellulose are called hemicellulases. Cellulases include
numerous endo- and exoglucanases, but none of them can individually degrade
crystalline cellulose fibers. The degradation starts with the C 1 -enzyme, which “opens
up” the cellulose matrix, making it more amorphous and accessible to hydrolysis
by the Cx-enzymes (Reese, 1956; Mansfield et al., 1999). C 1 -Cellulase is thought
not to be a hydrolytic enzyme but rather to be involved in the disruption of inter-
cellular hydrogen bonds of cellulose fibers. On the other hand, Cx-cellulases are true
hydrolases with affinities towards different substrates. Endo-Cx-cellulase attacks the
long-chain glucans and has no activity on disaccharide cellobiose, while exo-Cx-
cellulases show greater activity towards short oligomers. Thus, exo-β-1,4 glucan
glucohydrolase (glucohydrolase; EC 3.2.1.74) hydrolyzes single glucose units from
the nonreducing end of soluble cellulose, while exo-β-1,4 glucan cellobiohydrolase
(cellobiohydrolase; EC 3.2.1.91) catalyzes the liberation of cellobiose from the
nonreducing, and possibly the reducing, end of the polymer (Wood and McCrae,
1979). Finally, β-glusosidase or cellobiase (β-1,4 oligoglucan glucohydrolase;
EC 3.2.1.21) produces monomeric glucose from cellobiose. Interestingly, those cel-
lulases exhibiting an exoglucanase mode of action generally have a tunnel-shaped
molecular structure, while the more randomly acting endoglucanases have a more
“cleft-shaped” catalytic domain (Warren, 1996).
Cellulases, especially Cx-cellulases, occur in numerous higher plants, and their
activities increase during fruit ripening. Nevertheless, the extracts of the enzymes
cannot degrade cellulose fibers and therefore endogenous cellulases are not considered
to be a significant factor in tissue softening and degradation, especially compared to
pectinases. However, various microorganisms, aerobic and anaerobic, produce cel-
lulolytic enzymes. Plant pathogens, for example, secrete high levels of cellulases
along with pectolytic and hemicellulolytic enzymes. This combination of enzymatic
activity presents an excellent tool for degradation of plant cell walls and invasion
of plant tissue. Besides degrading the support and protective structure in the plant
cell, these enzymes completely hydrolyze the polysaccharides into soluble sugars,
providing nutrients for all microorganisms present and resulting in extensive soft-
ening and spoilage in fruits and vegetables (De Vries and Visser, 2001). On the other
hand, bacteria such as Bacillus and Pseudomonas secrete mainly endoglucanases
that randomly cleave cellulose. In order to be completely utilized, produced oligo-
mers have to be further processed by the wall-bound or intracellular enzymes in the
bacteria, which, consequently, make them less available to competing microorgan-
isms (Shallom and Shoham, 2003). Although bacteria are not considered to be as
efficient cellulose decomposers as fungi, there are some exceptions. Clostridium
thermocellum possesses several very efficient endocelluloses, exocelluloses, and
hemicellulases (Beguin et al., 1988; Lamed and Bayer, 1988). However, it can grow
only on cellobiose and cannot utilize xylose, arabinose, mannose, and other sugars
derived from hemicellulose. One possible explanation for this is that the hemicel-
lulases degrade surrounding polysaccharides, making cellulose more accessible to
cellulose-degrading enzymes (Lamed and Bayer, 1988).