Polysaccharides from hot water extracts of roasted Coffea arabica beans: isolation and characterization

A polysaccharide fraction was obtained from the hot water extract of defatted ground dark roasted coffee (pure Coffea arabica blend for espresso brewing technique) by means of classical precipitation methods. This high molecular weight (〈Mw〉=10 900 Da) product was shown to be composed of mannose, galactose, arabinose and traces of proteinaceous material. Further fractionation yielded two different carbohydrate polymers, which were structurally characterized. One polysaccharide was identified as a β-d-(1-4) mannan containing small amounts of galactose and arabinose. The second polysaccharide, obtained in low yield after removal of the mannose-containing polymeric material, was identified as an arabinogalactan. The starting fraction was found to be composed of about 80% (mol basis) of mannan and of about 20% of arabinogalactan. 13C-NMR spectra revealed that the arabinogalactan has a backbone chain of β-(1-3)-d-galactopyranose units. Some of these units were substituted with either terminal β-d-galactose or terminal α-l-arabinofuranose side chains mainly in C-6 position. Both the mannan and the arabinogalactan isolated are structurally related to the polysaccharides originally present in the green coffee beans.