578 Produce Degradation: Reaction Pathways and their Prevention
of degradation [109]. The development of genetically modified varieties of fruits
and vegetables with starch that is resistant to enzymatic breakdown could play an
important part in the diet of diabetic patients. In this regard, preliminary work in
our laboratory on the enzymatic modification of granular starch (pH 5.5 from 0 to
8 h) yielded granules that were morphologically coherent and had intact crystalline
structures but suffered a 5 to 6% loss of the amorphous region and indicated a slight
shift in the glass transition (Tg) value of the starch polymer. However, the most
dramatic effect of this treatment was on starch degradation behavior. With time, as
the amorphous portion of the starch polymer became more solubilized, crystalline
domains in the starch were increasingly resistant to degradation. Such modified
starches offer potentially interesting opportunities in the design of food products in
which delayed carbohydrate degradation is highly desirable.
19.3.2 CELLULOSE DEPOLYMERASES
The cellulose depolymerization process is somewhat complex due to the physical
heterogeneity of the cellulose substrate (degree of crystallinity, available surface
area, pore size, etc.), which requires the participation of several regio-selective
enzymes. For example, endoglucanases (EC 3.21.4) are inactive against crystalline
cellulose but hydrolyze the amorphous region by random hydrolysis of β-glucosidic
bonds. This treatment could result in the rapid decrease of viscosity relative to the
rate of increase in reducing groups. On the other hand, for cellobiohydrolases (exo-
splitting) (EC 3.21.91), which degrade amorphous cellulose by consecutive removal
of cellobiose from nonreducing ends, the rate of increase in reducing groups in
relation to the decrease in viscosity would be much higher than for the endogluca-
nases. The exoglucohydrolases (EC 3.2.1.74) consecutively hydrolyze glucose units
from the nonreducing end of cellodextrins, which contributes to a rapid decrease in
the chain length of the substrate, yielding a cellulosic material with a greatly reduced
rate of hydrolysis. Finally, β-glucosidases (EC 3.2.1.21) cleave cellobiose to glucose
and remove glucose from the nonreducing end of small cellodextrins. Unlike the
exoglucohydrolases, the rate of β-glucosidase increases as the size of substrate
decreases, cellobiose being hydrolyzed the fastest. These enzymes work best at pH
optimum between 5.5 and 6.0 and are stable at temperatures up to 60°C. These
enzymes are sensitive to heavy metals ions, sulfhydryl compounds, and oxidizing
and reducing agents, and some exhibit glucose repression. Possible scenarios of the
enzymatic breakdown of cellulose polymer are presented in Figure 19.9.
A wide variety of aerobic and anaerobic cellulolytic microorganisms are found
in nature, particularly in environments where cellulosic substrate is abundantly
available. Fruits and vegetables also produce cellulolytic enzymes, but mostly for
the purpose of fruit maturation [110–113]. Only a few microorganisms produce a
complete set of enzymes capable of degrading native cellulose efficiently. For effi-
cient cellulose degradation, cellulolytic enzymes usually work in conjunction with
the noncellulolytic enzymes to bring about the complete degradation of cellulose,
which is ultimately converted into carbon dioxide, water, and the residual biomass
under aerobic conditions and into carbon dioxide, methane, and water under anaer-
obic conditions. In woody plants, cellulose degrades very slowly due to its high