Isolation and chemical characterization of lignin from brewer's spent grain

Comunicación presentada en el Proceedings of the 18th International Symposium on Wood, Fibre and Pulping Chemistry, 09 - 11 September 2015 / Austria, Vienna

Brewer's spent grain (BSG) is the solid waste residue produced from barley (Hordeum vulgare L.) during the brewing process. This low cost by-product represents up to 30% of the starting malted grain (w/w) and, although it is produced in important amounts from brewing companies worldwide, most of these residues remain largely underexploited. BSG is mainly composed of cellulose (17-25%), non-cellulosic carbohydrates (25-35%), protein (15-24%), lignin (8-28%), and lipids (10%). Recent studies regarding the valorization and potential uses of BSG indicate that it could be considered as an important feedstock for the production of new products, including the production of second-generation bioethanol. However, the lignin content and composition greatly influences digestibility and delignification reactions and, therefore, the structural characterization of the lignin from BSG is an essential step to develop appropriate methods to design effective lignin depolymerization strategies. Moreover, the detailed knowledge of the lignin polymer in BSG will help developing lignin-based high-value added products from this waste material. In this study, the structure of the lignin from BSG has been studied in detail. Three different lignin preparations, the so-called ¿milled-wood¿ lignin (MWL), dioxane lignin (DL), and cellulolytic enzyme lignin (CEL), were isolated from BSG and then thoroughly characterized by pyrolysis GC/MS, 2D-NMR, and derivatization followed by reductive cleavage (DFRC). The data indicated that BSG lignin presents a predominance of guaiacyl units (syringyl/guaiacyl ratio of 0.4¿0.5) with significant amounts of associated p-coumarates, ferulates, and tricin. The main substructures present are ß-O-4¿ alkyl-aryl ethers (77¿79%) followed by ß-5¿ phenylcoumarans (11¿13%) and lower amounts of ß-ß¿ resinols (5¿6%) and 5-5¿ dibenzodioxocins (3¿5%). The results from 2D-NMR and DFRC indicated that p-coumarates are acylating the ¿-carbon of lignin side chains and are mostly involved in condensed structures. DFRC analyses also indicated a minor degree of ¿-acylation with acetate groups, which takes place preferentially on S lignin (6% of S units are acetylated) over G lignin (only 1% of G units are acetylated). The comprehensive understanding of the composition and structure of BSG lignin will help development of lignin-based high added-value products from this agro-industrial waste material and design of effective deconstruction strategies for biorefinery purposes.

This study has been funded by the EU-projects LIGNODECO (KBBE-244362) and INDOX (KBBE- 2013-7-613549), and the Spanish MICINN projects (co-financed by FEDER funds) LIGNOCELL (AGL2011-25379), BIORENZYMERY (AGL-2014-53730-R) and LIGNIN (CTQ2014-60764-JIN). We thank Prof. Craig B. Faulds (INRA, Marseille, France) for providing the BSG sample analyzed in this study. We also thank Dr. Manuel Angulo (CITIUS, University of Seville) for performing the NMR

Peer reviewed