Hydrocolloids in Cooking

Pectin is a  structural polysaccharide that is integral for the stability of plant cell walls. The biological function of pectin is to cross-link cellulose and hemicellulose fibers, providing rigidity to the cell wall. Pectin is also a major component of the middle lamella, where it helps to bind cells together.

Below is a fragment of the Pectin backbone with an α-(1->4)-linked D-galacturonic acid
pectin molecule fragment (ball and stick)

Molecular Structure of Pectin

Galacturonic acid is a sugar acid, an oxidized form of D-galactose (a C-4 epimer of glucose). It is the main component of pectin, in which it exists as the polymer polygalacturonic acid

chemical structure of galacturonic acid

Pectin contains a polysaccharide backbone with an α-(1->4)-linked D-galacturonic acid. The acid groups along the chain are largely esterifed with methoxy groups in the natural product. There can also be acetyl groups present on the free hydroxy groups.

molecule of pectin

Gelling Properties of Pectin

A pectin-gel can be formed when the ingredients are heated, thereby dissolving the pectin. Upon cooling below gelling temperature, a gel starts to form. If gel formation is too strong, syneresis --the extraction of the liquid component-- occurs possibly producing a granular texture. Weak gelling leads to excessively soft gels. Pectins gel according to specific parameters, such as sugar, pH and bivalent salts (especially Ca2+).

In high-ester pectins at soluble solids content above 60% and a pH-value between 2.8 and 3.6, hydrogen bonds and hydrophobic interactions bind the individual pectin chains together. These bonds form as water is bound by sugar and forces pectin strands to stick together. These form a 3-dimensional molecular net that creates a gel.

In low-ester pectins, ionic bridges are formed between calcium ions and the ionised carboxyl groups of the galacturonic acid. Low-ester pectins need calcium to form a gel, but can do so at lower soluble solids and higher pH-values than high-ester pectins.

In the kitchen HM (high methoxyl) pectin produces a theroirreversible gel that will not melt when subject to high temperature. The optimal concentrations for gel formation are:

  • 1-3% of HM pectin
  • 50-60% sugar
  • 0.5-1% citric acid

Pectin Hydration

The functional properties of pectins in food systems arise because of their ability to form crosslinked polymer networks that are highly hydrated.

Readings and References

Chemistry and uses of pectin-- a review

Water Structure and Science -- Pectin

An Introduction to Pectin - Structure

Pectin Basics - Gelling properties and applications

Effects of calcium, pH, and blockiness on kinetic rheological behavior and microstructure of HM pectin gels.

The Hydration Behavior of Pectin