Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
values, showing a very pronounced increase in the rate with increasing F A. NMR
studies of enzymatic degradation of chitosans 32,33 give more direct information on
the specificities with respect to cleavage of the four differerent glycosidic linkages
(A–A, A–D, D–A, and D–D) as determined by the identity of the new reducing and
nonreducing ends, and in some cases also the variation in the identity of the nearest
neighbours to the new reducing and nonreducing ends. From both viscometric and
enzymatic studies of lysozyme (using both hen egg white and human lysozyme)
degradation of chitosans, it was concluded that minimum four A-units had to be
contained in the active site of lysozyme in order to obtain maximum degradation
rates, explaining the dependence of lysozyme degradation rates of chitosans on F As
given in Table 14.2. It was also found that chitosans with a low degree of Nacetylation
may bind specifically to lysozyme 34,35 without cleaving the polysaccharide,
which was recently demonstrated by using immobilized lysozyme to fractionate
chitosan chains containing A-units from fully de-N-acetylated chains. 36
14.6 TECHNICAL PROPERTIES
14.6.1 FILM-FORMING PROPERTIES
Chitin and especially chitosan have been extensively studied for applications as films
or membranes. The use of such films to improve the quality of foods has been
examined 37 and is especially interesting due to the antimicrobial action of chitosan. 38
The preparation of a composite film of cellulose and an almost fully de-N-acetylated
and highly viscous chitosan has been reported. 39 A Schiff's base is formed between
the carbonyl groups on the cellulose and the amino groups on chitosan, resulting in
a film in which the chitosan does not dissolve in water and has good wet tensile
strengths. Uragami has reported the preparation and characteristics of different
chitosan membranes. 40
14.6.2 GELLING PROPERTIES
The preparation of chitin gels has been reported. 41–43 The preparation and characterization
of a few hydrogels of chitosan have also been reported, such as thermoreversible
chitosan-oxalate gels. 44,45 So far, no simple ionic and nontoxic cross-linking
agent has been found that gives reproducible chitosan gels at low concentrations,
such as calcium ions for gelling of alginates. However, aqueous chitosan gels crosslinked
with molybdate polyoxy-anions have been reported, resulting in transparent,
thermoirreversible gels that are able to swell several times their original size in
aqueous solutions, depending on the ionic strength. 46
Different chitosan gels made with covalent cross-linking have been reported,
with cross-linking with glutaraldehyde being the most widely applied. 47–49 Recently,
an enzymatic gelling system with chitosan has been reported. 50,51
14.6.3 ANTIMICROBIAL PROPERTIES
Although the mechanism(s) involved in the antimicrobial activity of chitosans has
not been explained, a number of studies support that the polycation chitosan does