Microwave-assisted hydrothermal processing of pine nut shells for oligosaccharide production

Pine nut shells, a biomass residue from the Mediterranean Pinus pinea pine nut industrial processing, were treated by microwave-assisted autohydrolysis to produce xylo-oligosaccharides. Microwave-assisted processes provide alternative heating that may reduce energy input and increase overall process efficiency. The autohydrolysis treatments were performed under isothermal and non-isothermal operations within a wide range of operational conditions (temperature/reaction times) covering several severity regimes (as measured by the log R₀ severity factor). The composition of the autohydrolysis liquors was determined in terms of oligo- and monosaccharides, aliphatic acids and degradation compounds. The process was highly selective towards hemicelluloses hydrolysis and liquid streams containing a mixture of oligomeric compounds (mainly xylo-oligosaccharides) could be obtained under relatively mild operation conditions (190 °C, 30 min) with a maximal oligosaccharides’ concentration of 18.48 g/L. The average polymerization degree of the obtained oligosaccharides was characterised by HPLC, showing that for the optimal conditions a mixture of oligomers with DPs from 2 to 6.