Effect of roasting on degradation and structural features of polysaccharides in Arabica coffee beans

The degree and nature of polysaccharide degradation at different roasting levels was determined for three Arabica (Coffea arabica) bean varieties. Between 12 and 40% of the bean polysaccharides were degraded depending on the roasting conditions. The thermal stability of the arabinogalactans, (galacto)mannans and cellulose was markedly different. The arabinogalactans and mannans were degraded up to 60 and 36%, respectively, after a dark roast, while cellulose showed negligible evidence of degradation. Roasting led to increased solubility of both the arabinogalactans and (galacto)mannans from the bean but the structural modifications, which accompanied this change in solubility, were different for each polysaccharide. Despite the moderate degradation of the (galacto)mannans, those remaining in the bean after roasting showed no evidence of change to their molecular weight even after a dark roast. In contrast, arabinogalactans were depolymerised after a light roast both by fission of the galactan backbone and loss of arabinose from the sidechains. The recently discovered covalent link between the coffee bean arabinogalactans and protein survived roasting. The glucuronic acid component of the AG was degraded markedly after a dark roast, but approximately 30% of the original content remained as part of the AG polymer. The results show that polysaccharide degradation during roasting is more marked than previously documented, and points to roasting induced changes to the polysaccharides as major factors in the changing physicochemical profile of the coffee bean during processing.