Effects of pH and temperature of ultrafiltration on the composition and physicochemical properties of hot-water-extracted softwood galactoglucomannans

The recovery of softwood galactoglucomannans (GGM) by pressurized hot water extraction and further concentration by membrane filtration followed by spray drying yield biopolymers suitable as raw materials for renewable products. GGM are often characterized as having low viscosity in water and excellent emulsion stabilizing capacity, enhanced by lignin structures co-extracted with GGM. To reduce membrane fouling during filtration and subsequently to increase product yield, the pH and temperature of GGM liquor can be increased, but effects of such conditions on properties of recovered GGM have not been well understood. Herein, we systematically varied the ultrafiltration pH (6−10) and temperature (30–60 °C) and characterized the composition and physicochemical properties of spray-dried GGM powders in comparison with freeze-dried (fGGM) and ethanol precipitated GGM (eGGM). The GGM samples ultrafiltrated at 60 °C and pH 10 (GGM-10/60) showed lower molar mass (2200 Da), degree of acetylation (0.09) and absolute ζ-potential (13 mV) than the other ultrafiltrated samples at pH (6−10) and temperature (30–45 °C) (3200–3700 Da, 0.11–0.15 and 23–32 mV, respectively). These differences could explain the unique gel formation capacity of GGM-10/60 after ultrasonication, which opens new prospects in GGM applications such as thickening agents or in 3D printing. The present results allow the design of biorefinery processes to obtain GGM with desirable properties for specific applications.