Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
to the combination of the decrease in the rate when a positively charged amino group
is present close to the glycosidic linkage to be hydrolyzed, and the substrate-assisted
mechanism by which a glycosidic linkage following an A-unit is cleaved.
Chitosan solutions can be sterilized by autoclaving (typically 120∞C for 20 min),
which may lead to depolymerization, depending on the pH and the chemical composition
of the chitosan. We have found that autoclaving of chitosan solutions (F As
of 0.01, 0.35, and 0.60) at pH 4.5 at 120∞C for 20 min does not reduce the intrinsic
viscosity of the chitosan with an F A of 0.01, whereas the intrinsic viscosities of the
other two chitosans are only moderately reduced, from 760 to 550 ml/g for the
chitosan with an F A of 0.35 and from 820 to 600 ml/g for the chitosan with an F A
of 0.60. 26 It seems therefore that autoclaving can be used to sterilize chitosan
solutions without severe depolymerization.
The stability of chitosan hydrochloride powders with different chemical compositions
has been studied at 60, 80, 105, and 120∞C. 27 It was concluded that the
dominant degradation mechanism in the solid state of these chitosan salts is acid
hydrolysis.
14.5 ENZYMATIC DEGRADATION
Chitosans may be enzymatically degraded by hydrolases widely distributed in nature.
Lysozyme can also, in addition to its natural substrate (the glycosidic linkage of
certain bacterial cell walls peptidoglycans), hydrolyze chitin and chitosans. 28,29
Two different approaches have been used to investigate degradation rates and
specificity of lysozyme-catalyzed degradation of chitosans. Viscometric studies, in
which chitosans with widely varying F As and known Bernoullian distribution of
monomers have been used, 30,31 offer an easy and convenient experimental technique
to study initial degradation rates, which avoids the transglycosylation reaction. Table
14.2 gives initial lysozyme degradation rates of chitosans with widely varying F A
TABLE 14.2
Relative Lysozyme Degradation Rates of
Chitosans with Different F As
FA Relative Rate of
Degradation
0.04 0.033
0.12 1
0.17 2
0.27 12
0.42 44
0.47 53
0.51 125
0.53 169
0.59 280