2.8 Purification of galactoglucomannan from steam pretreated softwood for the development of novel biobased materials
Galactoglucomannan (GGM) has been identified as a valuable resource and can be used in fields such as oxygen barrier films, food coatings or hydrogels. This hemicellulose compound can be extracted from softwood using steam explosion (STEX) at mild conditions. However, this GGM-solution also contains impurities, such as lignin, lignin-carbohydrate complexes, organic acids and salts, which requires purification before further use. Two different STEX-severities were evaluated: (1) STEX pre-treatment at 190˚C for 5 minutes and (2) 200˚C for 2 minutes. Even though the results showed only minor differences between the two severities, the option using 200˚C for 2 minutes was slightly superior with a mannan yield of 17.6 % and a purity of 27.1% and was thus chosen for the subsequent purification. For the purification, four different ultrafiltration polymeric flat-sheet membranes were investigated in a recirculating batch setup. The best membrane was the GR90PP (Alfa Laval, 5kDa MWCO) which gave a flux of 11 L/m2h at 3 bar transmembrane pressure and a GGM retention of 96%. This membrane was used to concentrate the GGM solution to a concentration factor of 2. Three different purification processes were evaluated on the retentate: (1) Diafiltration with 4 feed volumes of water using the GR90PP membrane, (2) precipitation of hemicellulose with anti-solvents and (3) adsorption of the impurities. The diafiltration showed promising ability to remove low-molecular weight impurities and thereby increasing the mannan purity to 38 %. The precipitation and adsorption processes demonstrated somewhat lower and varying efficiencies regarding the separation of GGM and lignin.