Effects of pH control with phthalate buffers on hot-water extraction of hemicelluloses from spruce wood

► We extract high-molar-mass GGMs with pressurised hot water from spruce in high yield. ► Extraction with and without phthalate buffer addition are compared. ► Such extraction is strongly dependent on the pH in a narrow region around pH 4. ► Control of pH to around 4 gives similar or higher yield of GGM extraction. ► Control of pH to around 4 also gives higher average molar mass of GGM extracts. Ground spruce wood was extracted with water at 170°C at four different pH levels (3.8, 4.0, 4.2 and 4.4) achieved by using phthalate buffers. Static batch extractions were carried out in an accelerated solvent extractor (ASE-300). The extracted non-cellulosic carbohydrates, predominantly galactoglucomannans (GGMs), were characterised mainly by sugar unit analysis and molar mass determination. Compared to extraction with plain water, extractions with phthalate buffer solutions gave similar yields of non-cellulosic carbohydrates, but gave up to 70% less monosaccharides, and consequently higher molar masses of extracted GGMs. Moreover, at these pH levels, the hydrolysis of acetyl groups were decreased by 40% compared to extraction with plain water, thus maintaining the water solubility of GGMs. It is concluded that hot water extraction of hemicelluloses in high-molar-mass form (average Mw about 10kDa) from wood in good yields (8% of wood) demands appropriate control of pH, to a level of about 4.