Microbial Metabolites in Fruits and Vegetables 509
preserving flavor and color. A more indirect effect is derived from the enzyme found
in mycorrhizal fungi that is thought to contribute to nutrient recycling in the soil,
thereby enhancing plant development (Burke and Cairney, 2002).
17.2.3 PECTINASES
Pectic substances, commonly called pectins, are a heterogeneous group of polysac-
charides composed of a partially methylated polygalacturonic backbone with pos-
sible rhamnose, arabinose, and xylose side groups. Protopectin is a water-insoluble,
high-molecular-weight molecule with numerous branches. It is usually referred to
as a “parent” molecule since it is the principal form that occurs in nature. Pectic
substances, mainly protopectins, form middle lamella in plant tissues, which keeps
cells together. Pectins also make up about one-third of the dry weight of plant cell
walls, where they occur as amorphous, gel-like structures and serve as cementing
material between cellulose and hemicellulose fibers. During industrial extraction of
pectins, side chains of protopectin are eliminated; commercial pectins are mainly
linear α-1,4-galacturonans with defined degree of esterification.
Microorganisms do not contain pectins in their cell walls regardless of the
species. However, in the absence of other carbon sources, microorganisms, both
saprophytes and parasites, may hydrolyze pectins and utilize them as nutrients.
Furthermore, enzymatic degradation of plant cell walls is the main pathway of attack
by plant pathogens.
Pectin-degrading enzymes are widespread in nature (reviewed by Gummadi and
Panda, 2003). Endogenous plant pectinases have an important role in fruit growth
and ripening. Depending on the reaction they catalyze, pectin-degrading enzymes
are categorized as hydrolyses (e.g., polygalacturonase, polygalacturonosidase),
lyases (e.g., pectin lyase, pectate lyase), and esterases (e.g., pectin methylesterase,
pectin acetylesterase).
Protopectinases (PPases) are hydrolytic enzymes that solubilize protopectin.
Plant endogenous protopectinases are triggered during maturation and result in
softening of the fruits. Microbial protopectinases, isolated from yeast and fungal
species, mainly Aspergillus, Geotrichum, and Trichosporon, exhibit two forms of
action. Protopectinase A-type acts as an endoenzyme, splitting glycosidic bonds
within the polygalactronic region, while B-type PPase hydrolyze side chains and
cleave the link between protopectin molecule and other constituents in the wall
(Sakai, 1992).
Pectinesterase (EC 3.1.1.11; pectin pectylhydrolase, pectin methylesterase; PE;
PME) release methanol from methylated galacturonase units. Pectin methylesterase
is a common enzyme in plants and microorganisms. However, plant PMEs remove
methyl groups in blocks, leaving “pouches” of deesterified galacturonic acid resi-
dues, while fungal PMEs cause random deesterification, resulting in random distri-
bution of unmethylated galacturonic acids along the pectin chain (Benen and Visser,
2003). This enzymatic reaction is being used in industry to improve the consistency
of tomato products. After deesterification by fungal pectinesterases, carboxylic
groups at galacturonic acid residues became available to bind divalent calcium ions
and increase, for example, viscosity of tomato paste (Voragen and Pilnik, 1989).