Wood extractives in hemicellulose stabilized emulsions

Norway spruce (Picea abies) and silver birch (Betula pendula) are widely spread in Europe. They are important raw materials for sawmill, pulp mill and paper industry. This industry utilizes merely 30-40% of the wood in the form of cellulose. Hemicelluloses, mainly galactoglucomannans (GGM) in spruce and glucuronoxylan (GX) in birch, comprise around 30% of the mass of wood. By far, this part has not been utilized as raw material but burnt as a source of energy. GGM and GX can be recovered from sawmills, paper making and pulping process, according to current knowledge, utilized as emulsion stabilizers. This study aims at evaluating the role of wood extractives present in hemicellulose isolates for emulsion stabilization. This study investigated the characterisation of wood extractives in isolated hemicelluloses and evaluated the role of wood extractives in stabilization of hemicelluloses emulsions during lipid oxidation. In the experimental section, eight different spruce galactoglucomannan (GGM) isolates and two birch glucuronoxylan (GX) isolates were characterized for their wood extractive composition and their antioxidant capacities were evaluated. The isolates were collected from thermomechanical pulping (TMP), by pressurized hot-water extraction (PHWE) or by mild alkali pressurized hot-water extraction (BLN). The isolates were either used as such (Con), after spray-drying (Spdr) or after ethanol precipitation (EtOH). The content and composition of phenolic residues and triterpenes in hemicellulose isolates were determined and their effect on the emulsion stability were investigated. In addition, radical scavenging activities and metal binding capacity were investigated to further understand the role of wood extractives against lipid oxidation in emulsions. Lipid oxidation in emulsions were investigated by monitoring the formation of primary and secondary oxidation products. For all of the studied GGM and GX, the content of phenolic residues ranged from 18 µg/g to 4 mg/g. The quantities of phenolic compounds in PHWE GGM and GX were higher than in TMP and BLNGGM and GX. Con GGM and GX exhibited higher amount of phenolic residues than the corresponding EtOH ones. Triterpenes were not detected which indicated their negligible contribution to emulsion stabilization. Con_TMP exhibited high iron binding capacity whereas the others exhibited average capacities. TMP GGM showed higher hydroxyl radical scavenging capacity compared to other GGM. The content of phenolic residues correlated with DPPH radical scavenging capacity for both GGM and GX: Radical scavenging capacity was greatest in samples having high number of phenolic residues. The average droplet size D[3,2] of emulsions stabilized by TMP GGM increased at a higher rate than that of stabilized by EtOH_GX. EtOH_GX was more stable due to its higher phenolic content and hydroxyl radical scavenging capacity compared to TMP GGM. Peroxide values and volatile oxidation products indicated that Con_TMP stabilized emulsion oxidized at a higher rate compared to EtOH_GX stabilized emulsion. This study showed the potential benefits to have coextracted wood extractives in hemicellulose isolates instead of removing them. Wood extractives clearly affect the stability of emulsions.