Non-covalent interaction between procyanidins and apple cell wall material. Part III: Study on model polysaccharides

The adsorption of condensed tannins (procyanidins) of varying degrees of polymerisation and percentage of galloylation on solid polysaccharides substrates was quantified using the Langmuir isotherms formulation. Pectins and xyloglucans, which are soluble polysaccharides, were first cross-linked by, respectively, dibromopropane and epichlorohydrin to obtain insoluble covalent gels. Cellulose and starch, being insoluble in the buffer solution at room temperature, were used as bought. Apparent affinity constants obtained for the pure polysaccharides were as follows : pectin ≫ xyloglucan > starch > cellulose. The apparent affinity constants increased with the molecular weight of the procyanidins, except with cellulose. Higher affinities were obtained with pectin, a polysaccharide having the ability to develop a gel-like network, forming hydrophobic pockets able to encapsulate procyanidins. Filamentous and globular polysaccharides, like cellulose and xyloglucan, bound procyanidins weakly. Higher apparent saturation levels were obtained for cellulose and xyloglucans, the arrangement of which would favour cooperativity and stacking. Pectin had lower saturation levels probably due to a steric hindrance effect.