The carboxyl group of each galacturonic acid unit can be in its free form or can be esterified by methanol. In nature, more than 80% of these units are esterified. This pectin is called protopectin and is insoluble. The proportion of esterification is reduced more or less during pectin extraction.
The ratio of esterified to non-esterified galacturonic acid determines the behaviour of pectin in food applications. Pectins are classified as either high methyl ester (HM) or low methyl ester (LM) pectin;
this depends on whether more than 50% or less than 50% of the galacturonic acid units are esterified. The exact percentage of methyl ester groups is indicated as Degree of Esterification (DE). If a de-esterification process takes place in the presence of ammonia, some of the galacturonic acid groups are converted to carboxylic acid amide and amidated pectins are then attained.
The Degree of Amidation (DA) indicates the number of amidated carboxylic groups per 100 galacturonic acid residues.