Lahtinen Maarit H, Valoppi Fabio, Juntti Venla, Heikkinen Sami, Kilpeläinen Petri O, Maina Ndegwa H, Mikkonen Kirsi S
Department of Food and Nutrition, University of Helsinki, Helsinki, Finland.
Faculty of Agriculture and Forestry, Helsinki Institute of Sustainability Science, University of Helsinki, Helsinki, Finland.
Front Chem. 2019 Dec 17;7:871. doi: 10.3389/fchem.2019.00871. eCollection 2019.
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.
木材半纤维素具有形成和稳定水包油乳液的出色能力。云杉中的半乳甘露聚糖（GGM）和桦木中的葡糖醛酸木聚糖（GX）为乳液提供了针对物理分解和脂质氧化的多功能保护。使用加压热水（PHWE）工艺与半纤维素共提取的酚类残留物似乎进一步增强了乳液稳定性。根据假设，与半纤维素相关的酚类残留物在乳液界面传递并锚定半纤维素。本研究首次表征了富含GGM和GX的木材提取物中酚类残留物的结构及其在乳液稳定中的作用。通过离心对PHWE GGM和GX进行分级分离，以获得浓缩的酚类残留物作为一个级分（GGM-phe和GX-phe），以及部分纯化的半纤维素作为另一个级分（GGM-pur和GX-pur）。为了评估每个级分在物理和氧化稳定方面的作用，使用GGM、GX、GGM-pur和GX-pur作为稳定剂制备了菜籽油水包油乳液。在为期3个月的加速储存试验期间测量了液滴尺寸分布和过氧化物值的变化。新鲜乳液的结果表明，半纤维素中富含酚类的级分参与了乳液的形成。此外，加速储存试验的结果表明，酚类结构改善了乳液的长期物理稳定性。根据测得的过氧化物值，所有检测的半纤维素均抑制了乳液中的脂质氧化，其中GX最为有效。这表明与半纤维素相关的酚类残留物在乳液中起抗氧化剂的作用。根据使用互补方法的化学表征，酚类级分GGM-phe和GX-phe主要由木质素组成。此外，与起始半纤维素GGM和GX以及纯化级分GGM-pur和GX-pur相比，酚类级分的总碳水化合物含量明显更低。显然，酚类结构在GGM-phe和GX-phe级分中富集，这通过核磁共振光谱以及其他表征方法得到了证实。木质素中主要键合模式β-O-4结构的频率显然非常高，表明提取的木质素保持天然形式。此外，还发现了γ-酯型的木质素碳水化合物复合物，这可以解释PHWE半纤维素在乳液中出色的稳定性能。
