Chemical Composition And Structure

Mildly extracted pectic substance is primarily a helical block copolymer of Z)-galacturonic acid and its methyl ester

(7). These comonomers are (l->4) O-linked as poly( a-D-galactopyranosyluronic acid) and its methyl ester. Blocks are interrupted by (l->2) O-linked a-L-rhamnopyranosyl inserts. Some of these features are incorporated in Figure 1. Neutral sugars associated with pectin other than rham-nose include arabinose, galactose, glucose, xylose, and mannose. At least three of these neutral sugars, arabinose, galactose, and xylose, have been found in pectin as short or oligomeric side chains that themselves may be branched

(8). Some portions of the pectin backbone have a high density of side chains and have been designated as hairy regions, whereas other portions of the pectin backbone are completely devoid of side chains and have been labeled as smooth regions. The hairy regions contain most of the neutral sugars found in pectin, namely rhamnose, arabinose, galactose, and xylose. A few sources of pectin (eg, sugar beets) contain acetyl and feruloyl esters. Acetyl esters are linked to ring hydroxyls in the glacturonate backbone, whereas feruloyl esters appear to be linked to neutral sugar side chains. Approximately 80 to 90% (by weight) of total sugars in commercial citrus pectins are residues of galacturonic acid and its methyl ester with the remaining sugars being neutral. Degree of methyl esterification (DM) in pectic substances will vary with plant source and method of extraction. Commercial citrus and apple pectins with DM ranging from 80 to 18% are available. X-ray diffraction patterns of sodium polygalacturonate indicate that water molecules are bound to each galacturonate residue in the backbone (9). Thus, bound water may account for about 8% of pectin samples by weight.

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