Low methoxy pectin (LM)
LM pectins can gel in the presence of divalent cations, usually calcium. In these systems gelation is due to the formation of intermolecular junction zones between homogalacturonic smooth regions of different chains. The structure of such a junction zone is generally ascribed to the so called 'egg box' binding process. Initial strong association of two polymers into a dimer is followed by the formation of weak interdimer aggregation, mainly governed by electrostatic interations.
The gel forming ability of LM pectins increases with decreasing degree of methylation.
LM pectins with a blockwise distribution of free carboxyl groups are very sensitive to low calcium levels. The presence of acetyl groups prevents gel formation with calcium ions but gives the pectin emulsion stabilising properties.
High methoxy pectin (HM)
HM pectins have the ability to form gels with sugar and acid, so-called low water activity gels or sugar-acid-pectin gels. Such a gel is considered a 2-dimensional network of pectin molecules in which the solvent (water) with the co-solutes sugar and acid are immobilised. This results in a system resisting deformation and showing a stress-strain relationship for small deformation. The build up of the 3-d network is based on the formation of junction zones in which there are chain associations stabilised by hydrogen bonding between undissociated carboxyl and secondary alcohol groups and by hydrophobic interaction between methyl esters.
The gelation mechanism of pectins is mainly governed by their degree of esterification (DE). For the low methoxyl pectins, denoted LMP (DE < 50%), gelation results from specific non-covalent ionic interactions between blocks of galacturonic acid residues of the pectin backbone and with divalent ions such as calcium. The affinity of pectin chains towards calcium is known to increase with decreasing degree of esterification or ionic strength, and with increasing polymer concentration. Besides the influence of the charge density of the polygalacturonate chain, the distribution pattern of free and esterified carboxyl groups has an important effect on the strength of calcium binding.
Molecular weight of pectins can be expected to vary with plant source, raw material and extraction conditions but molecular weight determination is a challenge because of the extra problems of heterogeneity and aggregation which can obscure data gathering.