Low methoxy pectin gels
Calcium induced gelation is predominant in low methoxy pectin gels. Gelation is due to the formation of intermolecular junction zones between the ‘smooth’ HG regions of separate polymers. The nature of the interaction, although known to be electrostatic to some extent, is still debated. Gel forming ability decreases with degree of methoxylation and some blockwise distributions of carboxyl groups are very sensitive to calcium presence. The effect of calcium is decreased by the acetylation of the pectin. Amidation, conversely, improves the gelling ability of LM pectins and are less prone to precipitation by high calcium levels.
The modification of the hydrogen bonding nature of the polymer by the addition of amide or acetyl moieties indicates that the ‘egg-box’ mechanism of gelation may not apply for all situations of calcium induced low methoxy pectin gelation.
High methoxy pectin gels
Jams and preserves are of course the main use of industrially extracted pectins. High dissolved sugar and acid conditions ensure that chain-chain interactions dominate over chain-solvent interactions. Most chain-chain interactions in these systems are not based on electrostatic interactions and so the other hydrophobic and hydrogen bonding effects exert most influence. High sugar conditions create low water activities which can be mimicked by other solutes with the same resulting gels. This is reflected in the subdivision of HM pectins into rapid-set pectins of DM~77 to slow-set with DM~60. The larger hydrophobic element in HM pectins allows for rapid arrest of the systems.
|HM Pectin||Ultra Rapid Set||Rapid Set||Medium Set||Slow Set|
|Setting time (min)||1 – 3||3 – 7||15 – 25||30-120|
|pH||3.1 – 3.4||3.0 – 3.3||2.8 – 3.1||2.6 – 2.9|
|Application||Jams with whole fruits||‘Classical Jams’||Acid jams and jellies||Acid to very acid and jellies|
Many factors influence the conditions of gel formation and the gel strength achieved. The major role is played by the pectin molecules, their chain length, and the chemical nature of the junction zones. Under equal conditions gel strengths increase with the molecular weight of the pectin used, and any treatment depolymerising the pectin chains is reflected in weaker gels. Rupture strength depends on the number of junction zones per single long chain molecule whereas for rigidity the number of junction zones per unit gel volume plays a greater role. Acetyl groups prevent gelation and the DM within the group of high methoxyl pectins determines the setting temperature of a gel.