Statistical modeling of pressurized hot-water batch extraction (PHWE) to produce hemicelluloses with desired properties

The pressurized hot-water extraction (PHWE) process of Norway spruce sawdust has been optimized aiming at the production of a hemicellulose-rich fraction consisting mainly of galactoglucomannans (GGM). The independent process parameters temperature, reaction time, and liquid-to-wood (L/W) ratio were in focus of the statistical modeling. The main target product properties were the average molecular mass (Mw) and the GGM content of the dissolved solids in the extracts and the yield of polymeric hemicelluloses with Mw larger than 4 kDa in the ethanol-water precipitate. According to the model, the highest Mw (>30 kDa) of the total dissolved solids in the extract can be obtained at a low extraction temperature (ET), a short extraction time (Et), and a low L/W ratio. The best result was 37 kDa, corresponding to a degree of polymerization (DP) about 230. The highest GGM content of the extract (>11% of the sawdust, which is about 70% of the GGM in sawdust) can be obtained with a high ET, a long Et and a high L/W ratio. According to the model, the PHWE process gives rise to the largest possible amount of polymeric hemicelluloses at 170°C, 11 min reaction time, and at L/W 5. Provided that a large-scale extraction apparatus works under these conditions with the same efficiency, it should be possible to produce around 60 g polymeric hemicelluloses (mainly GGM) with a Mw around 15 kDa from 1 kg spruce sawdust, which is roughly 25% of the original hemicelluloses in the sawdust.