CHAPTER 19: Polysaccharides and Other Carbohydrates

Polysaccharides are the predominant carbohydrates present in green coffee beans, accounting for about 50% of their dry weight. Galactomannans, type II arabinogalactans, and cellulose are the most abundant ones. Coffee galactomannans are composed of a main backbone of (β1 → 4)-linked d-mannose residues, some of them substituted at O-6 by single residues of α-d-galactose or l-arabinose, and at O-2 and/or O-3 by acetyl groups. Also, (β1 → 4)-linked d-glucose residues are components of the mannan backbone. Coffee arabinogalactans are composed of a main backbone of (β1 → 3)-linked d-galactose residues, some of them substituted at O-6 with short chains of (β1 → 6)-linked d-galactose residues. These short chains are substituted with various combinations of α-l-arabinose, α-l-rhamnose, and β-d-glucuronic acid residues. The structure of coffee galactomannans and arabinogalactans is changed during the roasting process, namely by the occurrence of depolymerization, debranching, dehydration, isomerization, and polymerization (via non-enzymatic transglycosylation) reactions. The occurrence of non-enzymatic transglycosylation reactions involving galactomannans and arabinogalactans results in arabinogalactan–galactomannan hybrid polysaccharides. During coffee roasting, galactomannans and arabinogalactans, as well as sucrose, are also involved in the formation of high molecular weight brown compounds known as melanoidins, compounds that have been associated with health benefits of coffee consumption.