Arabinosyl and glucosyl residues as structural features of acetylated galactomannans from green and roasted coffee infusions

Abstract

A good yield mild fractionation procedure was developed for the purification of mannans from green and roasted coffee infusions that included anion-exchange chromatography and phenylboronic acid immobilized Sepharose chromatography of the dialyzed and ethanol precipitated material. Enzymatic hydrolysis with endo-β-mannanase and ESIMS allowed finding that the mannans from roasted coffee infusions, as well as those from green coffee, are acetylated (8 mol % and 11 mol %, respectively). Fragmentation pattern obtained by ESIMS/MS analysis of the acetylated oligosaccharide ions indicates that the acetylation also occurs at O-2 of the mannose residues. Doubly acetylated and contiguously acetylated hexose residues were also found. Arabinose residues, as side chains, were also found as structural features of hot water soluble green (2%) and roasted (<0.9 mol) coffee galactomannans. Methylation analysis, hydrolysis with specific glycosidases and GC–EIMS analysis of the reduced and methylated oligosaccharides allowed to conclude that β-(1→4)-linked glucose residues are also structural features of green and roasted coffee galactomannans (6 and 1 mol %, respectively). In hot water soluble green coffee mannans, glucose residues are a constituent of the mannan backbone, and in the roasted coffee they were detected only at the reducing end of the mannan backbone.