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 (R2) 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.