Structure 8.11 XI Partially methylated chain of poly(galacturonic acid) of pectin gelation in this case. In the absence of inorganic cations, however, a high degree of esterification aids gelation, suggesting that hydrophobic interactions cause the chains to associate. The properties of the formed gels also depend on the degree of esterification; the rigidity of the 40-60% ester pectin gels is higher than that of 70-80% ester jellies. This suggests that rigidity is due to hydrogen bonding between the hydroxyl groups and the free carboxyls.

8.4.4 Chitosan

Chitosan is a polymer obtained by the deacetylation of chitin, one of the most abundant polysaccharides. Chitosan, or poly[a-(1, 4)-2-amino-2-deoxy-D-glucopyranose] has the structure (XII). As might be expected the degree of deacetylation has a significant effect on the solubility and rheological properties of the polymer. At low pH, the polymer is soluble, with the sol-gel transition occurring at approximate pH 7. Chitosan also has filmforming abilities and its gel- and matrix-forming abilities make it useful for solid dosage forms, such as granules or micro-particles. 5 The molecular weight, crystallinity and degree of deacetylation are all factors that can be varied to control the release rates from chitosan-based granules. When the positively charged chitosan is mixed in solution with polyanions such as gelatin, alginic acid and hyaluronic acid, interesting new matrix materials are formed. 6

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