A pectin gel may be regarded as a system in which the polymer is in a state between fully dissolved and precipitated. It is theorized that segments of the molecule chains are joined together by limited crystallization to form a three-dimensional network in which water, sugar and other solutes are held.

Formation of a gel, from a state where the polymer is fully dissolved, is caused by physical or chemical changes that tend to decrease the solubility of the pectin and this favors the formation of local crystallization. The most important factors which influence the solubility of pectin (tendency to gel) are:

1. Temperature
2. Molecular composition of the pectin (pectin type)
3. pH
4. Sugar and other solutes
5. Calcium ions

pectin03.gif (2759 bytes)
Simplified model of the molecular network of a pectin gel. Shaded areas represent local crystallization.

Temperature

When cooling a hot solution containing pectin, the thermal motions of the molecules are decreased and their tendency to combine into a gel network is increased. Any system containing pectin at potential gelling conditions has an upper temperature limit above which gelation will never occur. Below this critical temperature low ester pectins gel almost instantly while the gelation of high ester pectins is time dependent, the time taken being related to the temperature at which gelation occurs. In contrast to low ester pectin, high ester pectin gels are not temperature reversible.

HM-Pectin type	Degree of esterfica- tion	Gelling time, when cooled to and subsequently held at
HM-Pectin type	Degree of esterfica- tion	95°C	85°C	75°C	65°C
Rapid set	73.5	60 min.	10 min.	Pre-gel	Pre-gel
Medium set	69.5	No gel	40 min.	5 min.	Pre-gel
Slow set	64.5	No gel	No gel	No gel	30 min.
Gelation of HM-pectins with various DE (pH = 3.0, SS = 65%, pectin concentration = 0.43%)

Types of pectin

The overall distribution of hydrophilic and hydrophobic groups on the pectin molecule determines the solubility (tendency to gel) of a particular pectin.

The degree of esterification of a high ester pectin influences the gelling properties. The ester group is less hydrophilic than the acid group and consequently a high ester pectin with a high degree of esterification gels at higher temperatures than a high ester pectin with a lower degree of esterification. This difference is reflected in the terms rapid set, medium set and slow set, which is illustrated in the above table.

The solubility of the calcium salt of completely de-esterified pectin (polygalacturonic acid) is extremely low and a similar tendency to precipitate (form gels) in the presence of calcium ions is found with low ester pectin. The lower the degree of esterification - the more pronounced the similarity to polygalacturonic acid - and the greater the reactivity with calcium as reflected in the higher gelling temperatures observed.

Introduction of amide groups into the LM-pectin molecule tends to make the pectin less hydrophilic - increasing the tendency to form gels. In practice, amidated LM-pectins show a wider "working range" with regard to calcium content of the system and yield increasing gelling temperatures with increasing degree of amidation.

08pect01.GIF (9314 bytes)

pH

Pectin is an acid with a pK-value of approx. 3.5.

09pect01.GIF (7772 bytes)

The increasing ratio of dissociated acid groups to non-dissociated acid groups generally makes the pectin molecule more hydrophilic. The tendency to form gels is therefore strongly increased by decreasing the pH of the system. This is especially evident for high ester pectins which normally require a pH below 3.5 in order to gel.

Sugar and other solutes

Sugar and similar solutes generally tend to dehydrate the pectin molecules in solution. At higher solids there is less water available to act as a solvent for the pectin and the tendency to crystallize or gel is consequently favored.

Above 85% soluble solids, the dehydration effect is so strong that, in practice, gelation of any commercial pectin can hardly be controlled. High ester pectins form gels at soluble solids down to approx. 55%. For each soluble solids value above 55% there is a pH-value at which gelation is optimum for a particular high ester pectin, and a pH-range within which gelation can be obtained in practice.

Low ester pectins may gel at any soluble solids. For a particular pectin, the gelling temperature decreases with decreasing soluble solids.

Calcium ions

In contract to high ester pectin, low ester pectin forms gels in the presence of divalent cations such as calcium. As illustrated below, acid-demethylated low ester pectins require a fairly high amount of calcium within quite narrow limits to give optimum gel strength. Amidated low ester pectins show greater flexibility in this respect. For both pectin types increasing calcium concentration results in increasing gel strength - and increasing gelling temperature - to a point where pregelation. occurs. I.e. the gelling temperature close to the boiling point.

09pect02.GIF (8726 bytes)
Amidated LM-pectin (standardized)
Acid demethylated LM-pectin (standardized)