The fractionation of grape seeds as a model biomass was studied using a combination of two processes: solvothermal extraction using ethanol and hydrothermal fractionation-autohydrolysis process in a semicontinuous reactor using subcritial water. Around 70% of the polyphenols were extracted in the first step and close to 80% of the sugars in the second step.
Autohydrolysis is a hydrothermal depolymerisation of biomass using only water under high pressures and temperatures. Due to the degradation of the acetyl group (deacetylation) and uronic acids (e.g. liberation of glucoric acid) the pH in the media decreases from 5.5-6.0 down to 3.5-4.0. The protons catalyse the depolymerisation. A simplified mechanism is presented next.
First, grape seeds were subjected to an extraction process with ethanol/water (70/30% wt.) at 90ºC during 60 min obtaining ca. 13.0% wt. of oil and extractable components with 4.46% wt. of polyphenols (66% of the maximum). Afterwards, the solvent was water and the biomass was treated in steps at different temperatures (150ºC to 340ºC). During the hydrolysis the pH decreased from 5.5 down to 3.0 due to acetyl group liberation. The total quantity of recovered sugars varied around 20.0 to 23.1% wt. The best experimental condition for obtaining the maximum amount of pentoses + hexoses + oligosaccharides was 180ºC (45min) + 250 to 265ºC (45 min) + 330 to 340ºC (45 min).