512 Produce Degradation: Reaction Pathways and their Prevention
Microbial cellulases are inducible enzymes, which means that these enzymes
will be produced when cellulose is present in the medium (Reese et al., 1969;
Kubicek et al., 1993). However, this raises the question of how the crystalline, highly
insoluble cellulose can induce enzyme synthesis, which is a strictly intracellular
process, without entering the microbial cell. It is highly possible that microorganisms
constantly produce low levels of cellulolyitc enzymes. These enzymes produce
soluble cellobiose, cellobiono-δ-1,5-lactone, and short cellulose oligomers that can
enter the cell and promote cellulase gene expression (Kubicek et al., 1993). Hemi-
cellulases are a diverse group of enzymes and include xylanases (EC 3.2.1.8),
β-mannanases (EC 3.2.1.78), α-L-arabinofuranosidases (EC 3.2.1.55), and
α-L-arabinanases (EC 3.2.1.99), among others. Similar to degradation of cellulose,
hydrolysis of hemicellulose is carried out by microorganisms that can be found either
freely in nature, mainly plant pathogens, or as a part of the digestive tract of ruminant
animals. Although cellulose- and hemicellulolose-degrading enzymes are mainly
produced by microorganisms that grow and cause spoilage of fruits and vegetables,
their optimum pH is generally between 4.5 and 6.5. Their optimum temperature
range is between 35 and 50°C, but cellulases from many sources show thermosta-
bility, which may have detrimental effects on insufficiently processed plant tissues.
In the food industry, commercial cellulases are usually used in combination with
pectinases and hemicellulases. Their applications include the extraction and clarification
of fruit juices, extraction of oils from seeds, isolation of proteins from soybeans and
starch from corn, improving the rehydrability of dried vegetables, removal of cell walls
to facilitate the release of flavors and enzymes, and the production of soluble sugars
and alcohol from cellulosic wastes (Bhat and Bhat, 1997).
17.2.5 STARCH-DEGRADING ENZYMES
Starch is the main reserve polysaccharide in plants and presents the chief carbon
source for animals, plants, and microorganisms. However, having thousands of
glucose units in its structure, it is too large to be transported through the cell
membrane and enter the cell. When there are no other, preferable, carbon sources
available in the surrounding media, microorganisms begin to produce extracellular
starch-degrading enzymes to provide necessary nutrients. Depending on their activity
on starch molecules, these enzymes can be classified as endo- or exoenzymes.
Endoenzymes, such as α-amylase or pullulanase, hydrolyze glycosidic bonds ran-
domly within the starch molecule, while exoenzymes, such as β-amylase or glu-
coamylase, cleave molecule of maltose or glucose one by one from the nonreducing
end of the polysaccharide (Hyun and Zeikus, 1985). Generally, endoenzymes cause
rapid loss in viscosity while exoenzymes do not affect viscosity but increase sweetness.
α-Amylase (EC 3.2.1.1; α-1,4-D-glucan glucanohydrolase) is the main enzyme
in carbohydrate metabolism in microorganisms, animals, and plants. It randomly
hydrolyzes α-1,4 glycosidic bonds in amylase and amylopectin but has no effect on
α-1,6 branching points. The resulting molecules are α-limit dextrins of various
lengths, and, if the reaction is prolonged, maltose and even glucose. Due to the rapid
decrease of molecular weight of starch molecules, the viscosity of their dispersions
significantly decreases.