Characterisation of structure-dependent functional properties of lignin with infrared spectroscopy

Fourier-transformed infrared spectroscopy (FT-IR) was evaluated as an analytical technique for the estimation of the chemical composition and functional properties of lignin. A sample set containing various non-wood, hardwood and softwood lignins isolated by different processing technologies was used. The lignin samples were characterised by both conventional chemical analysis and non-destructive methods, such as diffuse reflectance FT-IR. Principal component analysis (PCA) based on the IR-fingerprint spectral region allowed classification of lignins according to origin and processing conditions. The antioxidative properties of each lignin sample in both aqueous and micellar systems were determined. All lignin samples showed radical scavenging activity, with sisal and abaca lignin being the most effective radical scavengers. The radical scavenging efficiency of the most efficient lignin was about 20% of that of BHT and tocopherol (based on weight), compounds that are commonly used in food and cosmetic industries. Multivariate analysis was applied to correlate chemical composition and antioxidative properties of lignins with the FT-IR spectral data. Partial least squares (PLS) models were able to predict the major components' concentrations and radical scavenging activity at the 99% confidence level presenting r(2) values higher than 0.80 in most cases. (C) 2004 Elsevier B.V. All rights reserved.