Pectins also carry nonsugar subsituents, essentially methanol, acetic acid, phenolic acids and occasionally amide groups. The esterification of galacturonic acid residues with methanol or acetic acid is a very important structural characteristic of pectic substances. The degree of methylation (DM) is defined as the percentage of carbonyl groups esterified with methanol. If more than 50% of the carboxyl groups are methylated the pectins are called high-methoxy pectins (HM), and less than that degree of methylation are called low methoxy (LM) pectins. This same principal applies to acetylation although the degree of acetylation (DAc) can be larger than 100% as galacturonosyl residues can be acetylated with more than one group per monosaccharide. Acetyl groups are generally present in the 'hairy' rhamnogalacturonan regions and only present in very low amount in homogalacturonan from apple and citrus. They can be present in much higher amounts in homogalacturonan from sugarbeet and potato.
Methyl esterification is common in native pectins but acetylation is rarer in natural extracts. Overall the degree of substitution is known as the degree of esterification (DE). For native apple pectins a random distribution of the metheyl esters groups over the galacturonan backbone was found. For commercially extracted pectins with tailored DM the distribution depends on the raw material and the extraction and de-esterification conditions. The overall polysaccharide is therefore ionic but with large neutral regions giving interesting functionalities.
Commercial pectins can also be amidated. The amidation improves the gelling ability of low methoxy pectins in that they need less calcium to gel and are less prone to precipitate at high calcium levels.
Table 1: Composition of a number of pectin sources
Rhamnose and Fucose