510 Produce Degradation: Reaction Pathways and their Prevention
Pectin depolymerizing enzymes, usually referred to as pectinases, include hydro-
lases that require water as a reactant and lyases that cleave glycosidic bonds by
β-elimination reactions. Both groups may act either as endo- or exoenzymes.
Pectinases are usually acidophilic, with optimum pH in the range of 3.5 to 5.5, and
have extensive application in the extraction and clarification of fruit juices and wine
(Pretel et al., 1997). Some alkaline pectinases have been isolated from Bacillus and
Pseudomonas spp. and are used in the pretreatment of waste water from vegetable
processing and degumming and processing of plant fibers (Tanabe et al., 1988; Cao
et al., 1992; Bruhlmann et al., 2000).
Polygalacturonase (PG) exist as endo-PG (EC 3.2.1.15; poly(1,4)-α-D-galactu-
ronide glycanohydrolase) and exoPG (EC 3.2.1.67; poly(1,4)-α-D-galacturonide
galacturonohydrolase). PGs cleave the α-(1,4) glycosidic bond but require demeth-
ylated galacturonic acids in order to act. EndoPGs are widely produced by plants
and microorganisms but not by animals (except snails). Endogenous plant endoPGs
are responsible for softening during fruit maturation, while fungal growth on fruits
and vegetables results in localized softening and spoilage due to rapid mercerization
and liquefaction of the product. ExoPG, although it occurs in various fruits and
vegetables, has only been detected in a few fungi and bacteria such as Clostridium
thermosaccharolyticum, Erwinia chrysanthenu, and Ralstonia solancearum (Huang
and Allen, 1997; Shevchik et al., 1999; van Rijssel et al., 1993). ExoPGs attack the
pectin molecule from the nonreducing end, liberating one galacturonic acid molecule
at a time.
Pectic lyases (trans-eliminases) cleave α-1,4 glycosidic bonds but do not involve
water. The catalyzed reaction occurs by the mechanism of β-elimination. Pectin
lyases (EC 4.2.2.10; PL) act as endoenzymes, preferentially splitting highly esterified
pectin, while pectate lyases (EC 4.2.2.2; PAL) act on nonesterified or low esterified
pectate and exist in both endo and exo forms. Lyases have not been found in plants
but are common in microorganisms. Pectin lyases are predominantly of fungal
(e.g., Asperigillus spp.) and pectate lyases of bacterial (e.g., Bacillus and Erwinia
spp.) origin. Since both polygalacturonases and lyases result in softening of fruits
and vegetables or decreased viscosity of pectin-rich products, determination of the
4,5 unsaturated end products of β-elimination indicate microbiological sources of
the enzyme.
17.2.4 CELLULOSE- AND HEMICELLULOSE-DEGRADING ENZYMES
Cellulose is a major structural component in plant cell walls. It is a large, linear
polysaccharide made exclusively of β-1,4 linked glucose units. Long cellulose mol-
ecules, being inherently rigid and extended, arrange themselves in a parallel mode
and develop extensive hydrogen bonds. This results in formation of crystalline
microfibrils with about 70 cellulose chains per fibril. However, small, noncrystalline
regions in the fibrils may exist due to the presence of other sugar residues entangled
within cellulose chains. In plant cell walls, the cellulose fibers are embedded in an
amorphous complex of branched and linear mannans, xylans, arabans, and galactans,
called hemicellulose, which provide support for the structural network. This structure
is particularly important since highly crystalline fibrils resist hydrolytic enzymes