Lignin-Rich PHWE Hemicellulose Extracts Responsible for Extended Emulsion Stabilization

Abstract

Wood hemicelluloses have an excellent capacity to form and stabilize oil-in-water emulsions. Galactoglucomannans (GGM) from spruce and glucuronoxylans (GX) from birch provide multifunctional protection against physical breakdown and lipid oxidation in emulsions. Phenolic residues, coextracted with hemicelluloses using the pressurized hot water (PHWE) process, seem to further enhance emulsion stability. According to hypothesis, phenolic residues associated with hemicelluloses deliver and anchor hemicelluloses at the emulsion interface. This study is the first to characterize the structure of the phenolic residues in both GGM-and GX-rich wood extracts and their role in the stabilization of emulsions. PHWE GGM and GX were fractionated by centrifugation to obtain concentrated phenolic residues as one fraction (GGM-phe and GX-phe) and partially purified hemicelluloses as the other fraction (GGM-pur and GX-pur). To evaluate the role of each fraction in terms of physical and oxidative stabilization, rapeseed oil-in-water emulsions were prepared using GGM, GX, GGM-pur, and GX-pur as stabilizers. Changes in droplet-size distribution and peroxide values were measured during a 3-month accelerated storage test. The results for fresh emulsions indicated that the phenolic-rich fractions in hemicelluloses take part in the formation of emulsions. Furthermore, results from the accelerated storage test indicated that phenolic structures improve the long-term physical stability of emulsions. According to measured peroxide values, all hemicelluloses examined inhibited lipid oxidation in emulsions, GX being the most effective. This indicates that phenolic residues associated with hemicelluloses act as antioxidants in emulsions. According to chemical characterization using complementary methods, the phenolic fractions, GGM-phe and GX-phe, were composed mainly of lignin. Furthermore, the total carbohydrate content of the phenolic fractions was clearly lower compared to the starting hemicelluloses GGM and GX, and the purified fractions GGM-pur and GX-pur. Apparently, the phenolic structures were enriched in the GGM-phe and GX-phe fractions, which was confirmed by NMR spectroscopy as well as by other characterization methods. The frequency of the main bonding pattern in lignins, the β-O-4 structure, was clearly very high, suggesting that extracted lignin remains in native form. Furthermore, the lignin carbohydrate complex of γ-ester type was found, which could explain the excellent stabilizing properties of PHWE hemicelluloses in emulsions.

Strategies for structuring diverse emulsion systems by using wood lignocellulose-derived stabilizers

“…94 In addition to surface activity and the physical capacity to stabilize emulsions, phenolic compounds such as lignin fragments introduce antioxidant properties to hemicelluloses. 60,61,91,95 This was shown by Wrigstedt et al, who synthesized ferulic acid and sinapic acid esters on birch GX and testing the antioxidant and emulsification properties. 96 In a different study, spruce GGM containing phenolic residues efficiently inhibited lipid oxidation of rapeseed oil o/w emulsions, as indicated by the resulting peroxide values, volatile oxidation products, and analysis of polymerized lipids.…”

“…12 However, the composition of isolated GX and GGM extracts is more complex than that described above, as the extracts often contain co-components, such as lignin fragments or other wood-derived extractives. [59][60][61] Pressurized hot waterextracted hemicelluloses are characterized by up to 70 mg gallic acid equivalent per g hemicelluloses, 14 indicating presence of lignin. 61 Covalent bonding may occur between the polysaccharide chains and lignin residues, creating lignincarbohydrate complexes (LCC).…”

“…[59][60][61] Pressurized hot waterextracted hemicelluloses are characterized by up to 70 mg gallic acid equivalent per g hemicelluloses, 14 indicating presence of lignin. 61 Covalent bonding may occur between the polysaccharide chains and lignin residues, creating lignincarbohydrate complexes (LCC). [61][62][63] These may either originate from native structures in wood or form through reactions that occur during the extraction process.…”

“…61 Covalent bonding may occur between the polysaccharide chains and lignin residues, creating lignincarbohydrate complexes (LCC). [61][62][63] These may either originate from native structures in wood or form through reactions that occur during the extraction process.…”

“…12 For example, native beech GX isolates that contained residual lignin showed the capacity to stabilize emulsions. 91 In addition, spruce GGM extracts often contain varying amounts of lignin residues, 61 and they have been shown to stabilize colloidal wood resin emulsions 92 as well as orange o/w emulsions. 93 Furthermore, GGM adsorbed at the rapeseed o/w emulsion droplet interface and stabilized it.…”