Structural features of an arabinogalactan-protein isolated from instant coffee powder of Coffea arabica beans

An arabinogalactan-protein (AGP) composed of d-galactose and l-arabinose in 9:1 mol proportion containing about 1.6% of protein has been isolated from the instant coffee powder of roasted Coffea arabica beans by water extraction followed by barium hydroxide and ethanol precipitations. AGP was recovered in 3.2% yield of instant coffee powder and its average molecular mass was significantly decreased due to severe manufacturing conditions to 5400. Chemical and spectroscopic studies on the instant coffee AGP revealed a β-(1,3)-linked Galp backbone branched at C6 by side 6-linked Galp residues terminated by Araf and/or Galp and confirming thus an α-arabino-3,6-β-galactan in the instant coffee powder composite. It has been found that about 74% of branched 3,6-linked Galp residues have their origin in the backbone while 26% is located in side chains (6-linked Gal units substituted at C3 by Araf or terminal Galp units). Moreover, about 11% of Galp residues involved in linear 1,6-linkages of AGP side chains survived a severe industrial process.