Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
FIGURE 14.3 Solubility versus pH curves of chitosans. : F A = 0.01, : F A = 0.17, ˙: F A
= 0.37, : F A = 0.60. (From Vårum, K.M., Ottøy, M.H., and Smidsrød, Carbohydr. Polym.,
25, 65, 1994. With permission.)
made it possible to extrapolate beyond the measured points and determine the pK a
values. All three chitosans had the same pK a values of 6.7, only slightly higher than
those derived from the ELS study.
14.4 CHEMICAL STABILITY
For many applications of chitosans, including food applications, it is important to
know the factors that determine and limit the stability of chitosans both in the solid
state and in solution, and the chemical reactions responsible for the degradation.
The glycosidic linkages are susceptible to both acid and alkaline degradation and
oxidation by free radicals. However, because most applications of commercial chitosans
are limited by its solubility at acidic pH, mostly the stability at low pH has
been studied.
The acid-catalyzed degradation rates of chitosans have been found to be dependent
on F A (Figure 14.5), and the initial degradation rate constant was found to
increase in direct proportion to F A. 25 Acid hydrolysis was found to be highly specific
to cleavage of A–A and A–D glycosidic linkages, which were found to be hydrolyzed
three orders of magnitude higher than D–D and D–A linkages. This is probably due