Determination of the degree of esterification of pectinates with decyl and benzyl ester groups by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and curve-fitting deconvolution method

Pectinates with benzyl and decyl ester groups were prepared by alkylation of the tetrabutylammonium salt of pectic acid with benzyl and decyl bromides, respectively. The degree of esterification (DE) of the pectin derivatives was determined by diffuse reflectance infrared Fourier transform spectroscopy and the curve-fitting deconvolution method. A linear relationship between DE and the ratio of the peak area at 1745 cm⁻¹ to the sum of the peak areas at 1745 and 1608 cm⁻¹ was established with a high correlation coefficient 0.98. The deconvolution analysis using the curve-fitting method allowed the elimination of spectral interferences from pectin components and their degradation products. The limits of the method are given by DE 6 and 93%. The method was compared with chemical analysis and found to be equivalent in view of accuracy and repeatability (t-test, F-test). The method is applicable in analysis of natural or synthetic mixtures and/or crude pectin substances.