The role of pectin in gel formation

PECTIN - ROLE IN GEL FORMATION

Pectin is the term designated to those water-soluble pectinic acids of varying methyl ester content. They are in between cell walls in soft tissues of most plants acting as cementing substances.

In general it is the pulp and not the juice of fruits that contain pectin. Apples contain abundant pectin in their cores and skin. In the preparation of jams, the cores and skins are cooked with the pulp for pectin extraction. In citrus fruits, the pectin is chiefly in the white part of the rind. Other sources are sunflower seeds, guava and peels of mango and orange.

Heat is essential to extract the pectin. The usual way to extract the pectin from fruit is to heat the fruit in a small amount of water. Apples are cut into small pieces or ground with skin or core left intact and cooked to extract the maximum amount of pectin. Guavas are sliced thin and cooked with water to extract pectin.

Some points to be remembered for extracting pectin

·         The maximum quantity of pectin is extracted in an acid solute. If fruits are rich in pectin but low in acidity, acidifying the solution before cooking increases the viscosity of the extraction.

·         Cooked extractions contain more pectin than raw juices.

·         Short periods of cooking (usually 10-20 minutes) yield extractions of better jellying power than does long

boiling.

The role of pectin in gel formation

The formation of a firm jelly takes place only when pectin, acid and sugar and water are in definite proportions.

When sugar is added to the pectin solution, it acts as a dehydrating agent and destabilizes the pectin-water equilibrium and the pectin conglomerates forming a network of insoluble fibres. Large amounts of sugar solution can be held in this mesh-like structure.

The strength of the jelly depends on the structure of fibres, their continuity and rigidity. The continuity of the network depends upon the amount of pectin present in the system and the firmness depends on sugar concentration and acidity.

A soft jelly can be obtained by decreasing the amount of sugar. However, the rate of setting is modified by acidity. The fibrils of pectin become tough in the presence of an acid and thus able to hold the sugar solution in the interfibrillar spaces. If the amount of acid is large, the fibrils lose their elasticity and as a result jelly becomes syrupy.