A strategic laccase mediated lignin degradation of lignocellulosic feedstocks for ethanol production

• Correlation between type of lignin and laccase action determined from FTIR studies.
• Optimization of proportions of lignocellulosic components in the mixed feedstock.
• Effect of delignification on percent crystallinity and reducing sugar yield.
• Structural, morphological, and calorific value analyses of pretreated biomass.
• GC–MS analyses were conducted on lignin degradation compounds.

Lignin degradation using laccase is a cleaner, biocatalytic, and substrate specific alternative to improve holocellulose recovery from lignocellulosic feedstocks for bioethanol production. The degree of laccase mediated lignin degradation varied with the biochemical composition of the biomass. In the present article, Fourier-Transform Infra-Red (FTIR) spectroscopy and simplex centroid mixture design algorithm based studies were conducted to probe the correlation between biomass composition and laccase mediated lignin degradation. FTIR peak intensity analyses of the mixtures i.e., combination 20, 21 and 31 from 57 mixed biomass sets studied suggested that G type lignin has synergistic effect on laccase mediated delignification while presence of S type lignin showed antagonistic relationship with laccase adsorption and delignification. Simplex centroid based optimization of the selected mixture (i.e., combination 31) resulted in a concoction composed of Saccharum spontaneum (0.2031), Saccharum officinarum tops (0.1968), Ricinus communis (0.6000) with maximum delignification 80.13%. Enzymatic delignification of this optimized mixture was supported by FTIR studies where peak intensity at 1514 cm−1 and 1595 cm−1 corresponding to the aromatic skeletal vibrations of lignin decreased in magnitude after delignification. X-Ray Diffraction study deciphered 3.66% increase in crystallinity of the delignified sample corresponding to cellulose while Scanning Electron Microscopy revealed structural distortion in the sample. Besides, reduction in the calorific value of the biomass (2.75 kJ/g) further substantiated the delignification process. Lignin degradation products analyzed through gas chromatography mass spectroscopy unveiled the presence of various carboxylic acids, phenolic acids, heterocyclic aromatic compounds and N-heterocyclic compounds manifesting laccase action on lignin. The study on the effect of pretreatment on saccharification process showed that 80% delignification was sufficient for maximum reducing sugar yield (587 mg/g).

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