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 neutr al sugar side-chains on the gelation properties of sugar beet pectin with C a2+. Sugar beet pectin was treated in the presence of Ca2+ with the enzymes pectin methyl esterase (PE), pectin acetyl esterase (PAE), rhamnogalacturo nan acetyl esterase (RGAE), arabinofuranosidase B (AF) and rhamnogalacturon ase (RGase) in various combinations. Addition of RGAE plus PE or PAE plus PE to the pectin-Ca2+ mixture signific antly increased the release of acetyl groups and methyl esters, in comparis on to the addition of only PE or PAE. This indicates that PE activity is hi ndered by the presence of acetyl groups both in the 'smooth' and in the 'ha iry' regions. Also the PAE activity is hindered by the presence of methyl g roups 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 alon e, while RGAE plus PE did not improve the gel forming properties. Addition of only PAE to the pectin-Ca2+ mixture did not result in gel formation. A l ower stiffness of the gel was found when RGase combined with RGAE and PE we re added to the pectin-Ca2+ mixture, in comparison to treatment with PE alo ne. Addition of AF plus PE to the pectin-Ca2+ mixture gave similar rheologi cal effects as treatment with only PE. A fraction representing the 'smooth' homogalacturonan regions, which was ob tained after treatment of the beet pectin with RGase and subsequent size-ex clusion chromatography, was also able to form a gel with Ca2+ and PE. Howev er, the gel formation was much slower, and the stiffness of the gel was low er than when the parental extract was used. Also with the modified pectin t he treatment with PAE plus PE gave an increased stiffness of the gel in com parison to PE alone. (C) 2000 Elsevier Science Ltd. All rights reserved.