Gel formation properties of pectin
The most important use of pectin is based on its ability to form
gels. HM-pectin forms gels with sugar and acid. This can be seen as
a partial dehydration of the pectin molecule to a degree where it is in
a state between fully dissolved and precipitated. The particular
structure of pectin imposes some specific constraints. HM-pectin,
unlike LM-pectin, does not contain sufficient acid groups to gel or
precipitate with calcium ions, although other ions such as aluminium
or copper cause precipitation under certain conditions. It has been
suggested by Oakenfull (1991) that hydrogen bonding and
hydrophobic interactions are important forces in the aggregation of
pectin molecules. 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.
Those that are ionised produce 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