Pectin is an essentially linear polysaccharide containing from a few hundred to about 1000 saccharide units in a chain-like configuration; this corresponds to average molecular weights from about 50,000 to 150,000.

D-galacturonic acid is the principal constituent of the pectin molecule, but some neutral sugars are also commonly present in pectin. The D-galacturonic acid units are linked together by alpha-1.4 glycosidic linkages.

The polygalacturonic acid is partly esterified with methyl groups and the free acid groups may be partly or fully neutralized with sodium, potassium or ammonium ions. The ratio of esterified galacturonic acid groups to total galacturonic acid groups - termed the degree of esterification (DE) - has vital influence on the properties of pectin, especially the solubility and the gel forming characteristics. The highest DE that can be achieved by extraction of natural raw material is approx. 75%. Pectins with DE from 20-70% are produced by controlled de-esterification in the manufacturing process.

The DE of 50% divides commercial pectins into high ester (HM) and low ester (LM) pectin. These two groups of pectin gel by different mechanisms.

HM-pectin require a minimum amount of soluble solids and a pH within a pretty narrow range, around 3.0, in order to form gels. LM-pectins require the presence of a controlled amount of calcium or other divalent cations for gelation and do not require sugar and/or acid.

Degree of esterification of HM-pectins controls their relative speed of gelation as reflected by the designations ‘slow set' and ‘rapid set' high ester pectin. Degree of esterification of LM-pectins controls their calcium reactivity. Some types of LM-pectins also contain amide groups, which strongly affects the calcium reactivity.