ORIGINAL RESEARCH PAPER Lignin analysis using microwave digestion Fatma Gassara • C. M. Ajila • Satinder K. Brar • R. D. Tyagi • M. Verma • J. R. Valero Received: 17 January 2012 / Accepted: 19 June 2012 / Published online: 13 July 2012 Ó Springer Science+Business Media B.V. 2012 Abstract Lignin quantification in apple pomace residues was carried out using a microwave oven to replace traditional refluxing during the mild acidolysis step to augment the selectivity of this step towards cleavage of lignin–carbohydrate bonds and to reduce the time needed to quantify lignin. The pressure, temperature and time were optimized by response surface methodology and the results were compared to the Klason lignin methodology. Temperature and pressure had a significant positive effect (p \ 0.05) on the determination of lignin. However, the time was also significant (p \ 0.05) on lignin quantification. The optimal conditions of digestion were: 30 bar, 170 °C for 15 min. The digestion using microwave (lignin content = 33 % w/w) was more accurate (p \ 0.05) than the the traditional refluxing (lignin content = 27 % w/w). Keywords Apple pomace  Klason lignin  Lignin quantification  Microwave digestion Introduction Lignin is a complex biomacromolecule composed of methoxylated phenol units linked by ether and carbon bonds. It is the most abundant renewable source of aromatic polymers in nature. This complex is present in all vascular plants, fruits and vegetables. Vegetable and fruit processing around the world produce large quantities of agro-industrial solid wastes that are rich in lignin. Apple pomace is one such solid residue containing lignin which is generated in large quantities around the world. The presence of lignin in apple pomace favors its utilization in the production of many value products, such as organic acids, protein-enriched feeds, edible mushrooms, ethanol, aroma compounds, and enzymes, among others (Vendruscolo et al. 2008). Hence, the quantification of lignin in this waste and other lignocellulosic wastes is very important. There are several methods that can quantify lignin in lignocellulosic materials (Van Soest 1967; Alves et al. 2009). However, estimates of lignin concentrations vary widely between methods. Meanwhile, Klason lignin is the standard method of analysis for wood and lignocel- lulosic materials (Lai and Sarkanen 1971; Kirk and Obst 1988) and, as long as lignin is insoluble in mineral acids, it can be analyzed gravimetrically after hydrolyzing the cellulose and hemicellulose fractions with sulphuric acid. In an effort to further improve the yield and purity of the Klason protocol, to cleave more severely the lignin–carbohydrate bonds and reduce the time of digestion, a microwave was used with the aim of F. Gassara  C. M. Ajila  S. K. Brar (&)  R. D. Tyagi  J. R. Valero INRS-ETE, Universite ´ du Que ´bec, 490, Rue de la Couronne, Quebec, QC G1K 9A9, Canada e-mail: satinder.brar@ete.inrs.ca M. Verma Institut de recherche et de de ´veloppement en agroenvironnement (IRDA), 2700 rue Einstein, Quebec, QC G1P 3W8, Canada 123 Biotechnol Lett (2012) 34:1811–1815 DOI 10.1007/s10529-012-0991-7