FOOD HYDROCOLLOIDS Food Hydrocolloids 21 (2007) 256–266 Quantification of the degree of blockiness in pectins using 1 H NMR spectroscopy and chemometrics Hanne Winning a,à , Nanna Viereck a , Lars Nørgaard a , Jan Larsen b , Søren B. Engelsen a a Centre for Advance Food Studies, Quality and Technology, The Royal Veterinary and Agricultural University, Rolighedsvej 30, 1958 Frederiksberg C, Denmark b CP Kelco, Ved Banen 16, 4623 Lille Skensved, Denmark Received 26 September 2005; accepted 29 March 2006 Abstract The gelling properties of pectins are known not only to be closely related to the degree of esterification (DE), but also to the distribution of the ester groups. In this study we have examined an experimentally designed series of pectins originating from the same mother pectin and deesterified using combinations of two different enzymatic mechanisms. The DE and distribution patterns of methyl ester groups have been analyzed using high-resolution (HR) 1 H nuclear magnetic resonance (NMR) spectroscopy on pectin solutions. Quantitative calibration models using partial least squares (PLS) regression were developed with the ability to predict DE as well as the specific enzyme treatment, expressed as amount of ester groups removed with random and block enzyme, respectively. NMR spectroscopy was able to distinguish between enzyme treatments in simple classification by principal component analysis (PCA). This was due to the spatial structure of pectin together with the methyl ester distribution. Nuclear Overhauser effect spectroscopy (NOESY) experiments confirmed all the general assignments with the expected nuclear Overhauser effect (NOE) correlations. Degree of random deesterification (R) was better predicted than the degree of block deesterification (B). The calibration models for prediction of R obtained on extended inverted signal correction (EISC) processed data gave a root mean square error (RMSE) of cross-validation (CV) of 2%p with 4 PLS components (latent variables, LV) and a correlation coefficient (r) of 0.98. Spectral variable selection using interval PLS (iPLS) was shown to be valuable, as all the calibration models were improved. r 2006 Elsevier Ltd. All rights reserved. Keywords: Pectin; Noesy; NMR; Chemometrics; Esterification; Blockiness; Block-enzyme; Random-enzyme 1. Introduction Pectic molecules include primary homogalacturonan (HGA) and rhamnogalacturonan I (RG I). HGA is composed of unbranched a-1,4-linked galacturonic acid (GalA) residues, whereas RG I is composed of a backbone of repeating a-1,2-L-rhamnose(Rhap)-a-1,4-D-GalA disac- charide units. Side-chains, mainly consisting of arabinan and/or galactan, are attached to the RG I backbone at the C-4 position of the Rhap residues. These side-chains are called ‘hairy regions’ (Carpita & Gibeaut, 1993; Ridley, O’Neill, & Mohnen, 2001). The GalA residues in the HGA and RG I backbone may be methyl esterified and/or O- acetylated (Carpita & Gibeaut, 1993). The structure of pectin is important for the plant cell-wall strength and flexibility. Because of its excellent gelling, thickening, and ARTICLE IN PRESS www.elsevier.com/locate/foodhyd 0268-005X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodhyd.2006.03.017 Abbreviations: GalA, galacturonic acid; HGA, homogalacturonan; DE, degree of esterification; DP, degree of polymerization; RG-I, Rhamno- galacturonan-I; Rhap, ramnopyranosyl unit; PME, pectin methyl esterase; B, block deesterification; R, random deesterification; NMR, nuclear magnetic resonance; HR, high resolution; NOE, nuclear Overhauser enhancement; FID, free induction decay; FT, Fourier transformed; TSP, perdeuterated 3-(trimethylsilyl) propionate sodium salt; PC, principal component; PCA, principal component analysis; CV, cross validation; LV, latent variables; RMSECV, root mean square error of cross validation; PLS, partial least squares; iPLS, interval partial least squares; MSC, Multiplicative scatter correction; EISC, Extended inverted signal correction; %p, percent points; ppm, parts per million; A ˚ ,A ˚ ngstrøm; Nd, not determined; IPA, Isopropanol à Corresponding author. E-mail address: hie@kvl.dk (H. Winning).