Characterisation of Norway spruce hemicelluloses extracted by pressurised hot-water extraction (ASE) in the presence of sodium bicarbonate

Ground spruce sapwood was extracted with pressurised hot water with addition of different amounts of sodium bicarbonate (NaHCO3) to find optimal pH conditions for extraction of hemicelluloses, especially of galactoglucomannans (GGMs). The extractions were carried out in an accelerated solvent extractor (ASE) at 170 degrees C. When extraction was carried out with 2.5 mM NaHCO3 during 60-100 min extraction time (end-pH 3.9), more than 60% of the total extracted substances were hemicellulose-derived carbohydrates. Under these conditions, approximately 80% of the extracted carbohydrates were from GGMs (13% b.o. wood). At higher concentrations of NaHCO3, the yield of carbohydrates, especially those derived from GGMs, decreased. Hydrolytic depolymerisation of hemicelluloses occurring during extraction with plain water was largely inhibited in the presence of 2.5-5 mM NaHCO3. The hydrolytic deacetylation of GGM was diminished at low NaHCO3 concentrations but increased dramatically at higher NaHCO3 concentrations. It can be concluded that 2.5-5 mM NaHCO3 giving an end-pH value of 4 is better for GGM isolation than plain water.