Pectic substances from sugar beet pulp : structural features, enzymatic modification, and gel formation

Abstract

Pectic substances are present in high proportions in sugar beet pulp. This by-product is therefore a potential raw material for the pectin industry. However, sugar beet pectin has poor physico-chemical properties compared with pectins from other sources. In order to improve these properties, pectins obtained from beet pulp by autoclaving and acid extraction were chemically characterized and subjected to enzymatic modification and oxidative cross-linking.The autoclave extracts contained two populations of feruloylated rhamnogalacturonans with highly branched arabinose side-chains having apparent molecular weights of 1300 and 120 kDa, respectively. Furthermore, the presence of homogalacturonans (21 kDa) and of feruloylated arabinans (10-15 kDa) was shown. The acid extracted beet pectin consisted mainly of galacturonic acid with galactose as predominant neutral sugar. The homogalacturonans and rhamnogalacturonans were still linked in this extract.Cross-linking of the extracts obtained by autoclaving or acid treatment with hydrogen peroxide/peroxidase increased the viscosity and eventually led to the formation of a gel. Treatment of the autoclave extracts with ammonium persulfate decreased the viscosity. Cross-linking of the acid extracted pectin with ammonium persulfate slowly increased the viscosity. The ratio of the different ferulic acid dehydrodimers formed was similar for both oxidants. The total amount of dehydrodimers was lower for the ammonium persulfate cross-linked material. Cross-linking of purified rhamnogalacturonans with hydrogen peroxide/peroxidase led to an increase in viscosity or in gel formation at concentrations as low as 0.75 %. Oxidative cross-linking of the arabinans also led to gel formation, at a concentration of 4.0 %.Treatment of acid extracted beet pectin with rhamnogalacturonan acetyl esterase plus rhamnogalacturonase showed that the rhamnogalacturonan backbone has little effect on the intrinsic viscosity of this pectin. Removal of the arabinans from purifiedrhamnogalacturonans showed that the arabinan side-chains contribute little to the intrinsic viscosity of the rhamnogalacturonans. Addition of pectin acetyl esterase plus pectin methyl esterase to a mixture of acid extracted beet pectin and Ca 2+increased the release of acetyl groups and of methyl esters as compared with addition of PE or PAE alone, and gave rise to a stiffer gel