Effect of enzymatic deacetylation on gelation of sugar beet pectin in the presence of calcium

This study deals with the effect of methyl esters, acetyl groups, and neutral sugar side-chains on the gelation properties of sugar beet pectin with Ca(2+). Sugar beet pectin was treated in the presence of Ca(2+) with the enzymes pectin methyl esterase (PE), pectin acetyl esterase (PAE), rhamnogalacturonan acetyl esterase (RGAE), arabinofuranosidase B (AF) and rhamnogalacturonase (RGase) in various combinations. Addition of RGAE plus PE or PAE plus PE to the pectin-Ca(2+) mixture significantly increased the release of acetyl groups and methyl esters, in comparison to the addition of only PE or PAE. This indicates that PE activity is hindered by the presence of acetyl groups both in the 'smooth' and in the 'hairy' regions. Also the PAE activity is hindered by the presence of methyl groups in the 'smooth' regions. Treatment with PAE plus PE led to a stiffer gel, as determined by the storage modulus (G'), than treatment with PE alone, while RGAE plus PE did not improve the gel forming properties. Addition of only PAE to the pectin-Ca(2+) mixture did not result in gel formation. A lower stiffness of the gel was found when RGase combined with RGAE and PE were added to the pectin-Ca(2+) mixture, in comparison to treatment with PE alone. Addition of AF plus PE to the pectin-Ca(2+) mixture gave similar rheological effects as treatment with only PE. A fraction representing the 'smooth' homogalacturonan regions, which was obtained after treatment of the beet pectin with RGase and subsequent size-exclusion chromatography, was also able to form a gel with Ca(2+) and PE. However, the gel formation was much slower, and the stiffness of the gel was lower than when the parental extract was used. Also with the modified pectin the treatment with PAE plus PE gave an increased stiffness of the gel in comparison to PE alone. (C) 2000 Elsevier Science Ltd. All rights reserved.