Biomacromolecules 2003, 4, 337-343
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Thermoreversible Gelation of Aqueous Mixtures of Pectin and Chitosan. Rheology Marianne H. Nordby,† Anna-Lena Kjøniksen,‡ Bo Nystro¨m,*,‡ and Jaan Roots‡ Department of Pharmaceutics, School of Pharmacy, University of Oslo, P.O. Box 1068, Blindern, N-0316, Oslo, Norway, and Department of Chemistry, University of Oslo, P.O. Box 1033, Blindern, N-0315, Norway Received September 23, 2002; Revised Manuscript Received December 5, 2002
The synergistic interaction between pectin and chitosan in aqueous acid solution and in the gel phase has been studied by oscillatory shear measurements. Mixtures of pectin and chitosan form thermoreversible gels over a broad composition range by lowering the temperature. The value of the gelation temperature depends on the composition of the mixture, with low values for mixtures with low pectin contents. For incipient gels, a power law can describe the frequency dependence of the complex viscosity, with power law exponents close to -1. The gel evolution of pectin-chitosan mixtures upon a temperature quench below the gel point has been studied. Evidence is provided for a relation between gelation and phase separation in the process of temperature-induced gelation of pectin-chitosan mixtures. A simple model is proposed to rationalize the gelation process in these systems. Introduction Aqueous systems of mixed polysaccharides are used in a wide range of applications1-8 because of their ability to interact synergistically and provide materials with controlled properties. The interaction between two different polysaccharides, such as chitosan and pectin, can cause the formation of a physical hydrogel. Interpolymer complexes of pectin and chitosan have been considered as oral drug delivery systems5,6,9,10 and may be valuable both in restricting drug release in the upper gastrointestinal tract and allowing rapid release in the colon. Chitosan, a linear cationic polysaccharide prepared by N-deacetylation of chitin11 found in the shells of shrimp, lobster, and crabs, is poly[β-(1 f 4)-linked 2-amino-2-deoxy-β-D-glucopyranose] (there may be some residual N-acetylation). Pectins are anionic polysaccharides, extracted from cell walls in most plants, that consist primarily of (1 f 4)-linked R-D-galacturonyl units occasionally interrupted by (1 f 2)-linked R-L-rhamnopyranosyl residues (the latter may carry oligosaccharides).12 Pectins are usually classified according to their degree of methoxylation (DM) into two groups: high-methoxyl pectin (methoxyl content >50%) and low-methoxyl pectin (methoxyl content
