Determination of the degree of methylesterification of pectic polysaccharides by FT-IR using an outer product PLS1 regression

The combination of the absorbance spectra in two infrared regions (1800-1500 and 1200-850 cm(-1)) was used to build a calibration model for the determination of the degree of methylesterification (DM) of pectic polysaccharides. The wavenumbers in the region 1800-1500 cm(-1) are related to the carbonyl esters and carboxylates and those in the region 1200-850 cm(-1) are related to the sugars composition. The model was done by means of a matrix of the outer product of the two regions and PLS1 regression, using cell wall pectic polysaccharide extracts from olive and pear pulps. The application of the Durbin-Watson criterion to the regression vector of OP-PLS1 gave an 'optimal' model with nine latent variables, with a RMSEP of 7.7% and a coefficient of determination (R-2) of 0.95. The model explained 95% of the total variability present in the y vector. The results allowed the prediction of the DM of commercial standard pectic polysaccharides and of impure pectic polysaccharide extracts. A prediction error of 22% was found, which contrasted with the prediction error of 72% when only the region 1800-1500 cm(-1) region was used. (C) 2002 Elsevier Science Ltd. All rights reserved.