Question

How do divalent cations and pH affect gelling properties of pectins?

Answer 1

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Gel formation is caused by hydrogen bonding between free carboxyl groups on the pectin molecules and also between the hydroxyl groups of neighbouring molecules. In a neutral or only slightly acid dispersion of pectin molecules, most of the unesterified carboxyl groups are present as partially ionised salts. Ionisation produces a negative charge on the molecule, which together with the hydroxyl groups causes it to attract layers of water. The repulsive forces between these groups, due to their charge, can be sufficiently strong to prevent the formation of a pectin network. When acid is added, the carboxyl ions are converted to mostly unionised carboxylic acid groups. This decrease in the number of negative charges not only lowers the attraction between pectin and water molecules, but also lowers the repulsive forces between pectin molecules Thus as the pH is lowered, as a result of reduced ionisation, the polysaccharide molecules no longer repel each other over their entire length, and as a result, they can associate and form a gel.LM pectins can gel in the presence of divalent cations like calcium. the electrostatic interactions lead to preciptation and gellation. There are minimum levels of calcium needed to create a gel. Above that level, the gel strength will increase rapidly, until it reaches maximum saturation, after which point adding additional calcium will cause the gel strength to decline. A sequestrant can be used to control the availability of the calcium present; as sequestrant levels increase, the system will gel less easily and at lower temperatures.