Isolation and characterisation of a coffee melanoidin fraction

BACKGROUND: Melanoidins contribute to the colour and flavour of coffee brews and are also reported to be physiologically active components. However, structural information about coffee melanoidins is rare and current models are supported by few data. RESULTS: Using hydrophobic interaction chromatography (HIC), melanoidin fractions were isolated from different high-molecular-weight fractions of coffee brews obtained by ultrafiltration. Generally, melanoidin fractions isolated by HIC showed three- to fourfold higher antioxidant activities than the remaining high-molecular-weight material in the ultrafiltration fractions. Melanoidin fractions showed an intense brown colour: aqueous solutions with a concentration as low as 40 mu g mL(-1) were distinguishable from pure water. The melanoidin fractions contained less than 6% each of releasable carbohydrates and amino acids. The molecular masses of the melanoidins were estimated to be between 3 and 22 kDa, irrespective of the ultrafiltration fraction from which they were obtained. Two-dimensional nuclear magnetic resonance studies revealed an enrichment of phenolic/aromatic/olefinic structural units and did not support the idea of intact ferulate or caffeate moieties integrated into the melanoidin polymer. CONCLUSION: These studies support melanoidin models describing them as high-molecular-weight compounds themselves as opposed to models describing them as low-molecular-weight chromophores bound to polysaccharides or proteins. Phenolic constituents are more likely integrated into melanoidins as condensed phenolics than as intact hydroxycinnamates from chlorogenic acids. (C) 2008 Society of Chemical Industry.