Comprehensive Structural Characterization of Wheat Bran Lignin

Abstract

Wheat bran is a large volume sidestream of wheat flour production and is used in food and feed applications. Despite great advances in the characterization of the wheat bran hemicellulose component, thus far, only limited attention has been paid to wheat bran lignin. Here, we describe the comprehensive structural characterization of wheat bran lignin, facilitated by sequential enzymatic starch and protein removal, followed by mild γ-valerolactone organosolv extraction and extensive purification. Quantitative ¹³C-IS pyrolysis-GC-MS and HSQC NMR revealed that wheat bran lignin is enriched in syringyl subunits (S/G ∼ 0.9), as compared to wheat straw lignin (typical S/G ∼ 0.5), but surprisingly poor in p-coumarate incorporated (<1 per 100 aromatic rings), entirely free of tricin, and accordingly composed of typical β-O-4 aryl ether (84%), β-5 phenylcoumaran (7%), and β–β resinol (9%) interunit linkages. Moreover, and in line with the interunit linkage abundance (46 per 100 aromatic rings), alkaline SEC and ³¹P NMR, respectively, confirmed a macromolecular nature (M_w 6900 g/mol, Đ 5.9) and low phenolic hydroxyl content (1.6 mmol/g of lignin) of the wheat bran lignin structure. Our extensive characterization efforts contribute to the dedicated valorization of wheat bran lignin and support understanding potential physiological effects when incorporated into human and animal diets.