PHYSICAL CHEMISTRY OF MILK

Lesson 5. GELS

Module 2. Colloidal chemistry

Lesson 5
GELS

5.1 Introduction

Gels play a very important role not only in the food system but also in the industrial application. The cross linking of the molecules give the typical three dimensional structure to these gels. This has increased their usefulness in various fields of application.

5.2 Definition

Gels are defined as a substantially dilute cross linked system, which exhibits no flow in steady-state. Gels are mostly liquid, yet they behave like solids due to a three-dimensional cross linked network within the liquid. It is the cross links within the fluid that gives structure to a gel (hardness) and contribute to stickiness (adhesiveness). In other words, a gel is formed when particles aggregate to such an extent forming a continuous network throughout the liquid.

A gel has an elastic behavior similar to a solid and is deformed when stress is applied to it but recovers to its original shape after removal of the stress as deformation has left the bonds between the particles intact. A gel also has viscous properties because part of the deformation is not recovered after the stress is removed and the gel flows since the bonds are broken and new bonds are formed as in a liquid when stress is applied.

If the stress is applied for a very short duration the elastic deformation will be predominant i.e. recoverable. If the stress lasts long the viscous deformation is most conspicuous i.e. flow.

A gel thus shows the visco -elastic behavior which is characterized bu the two rheological parameters.

Gels exhibit an elastic or storage module (G’) and the viscous or loss module (G”). If the G’ is greater than the G” the elastic properties prevail, and if the G’ is less than the G” the viscous properties prevail. The overall resistance to deformation is expressed in the combined modulus G^*. The moduli give the ratio of the stress to the relative deformation and thus have the dimension of stress e.g N.M^-2 or Pa. They often strongly depend on the timescale of deformation.

The above reasoning holds when the deformation is about proportional to the stress (so called linear behaviour). Gels unlike solids often can be deformed considerably (e.g.by 10%) and still show linear behaviour. At larger deformation this relationship breaks down and as the bonds in the net work are broken that do not reform within the time scale of the experiment. A still larger deformation soon causes yielding or breaking of the gel and the net work is locally destroyed. The stress needed to do this may be called ‘yield stress' It is not a well defined quantity as it depends much on conditions like geometry of the measuring instrument and time scale. The rheological properties will be altered when the deformation of the gel results in permanently breaking of the bonds.

5.3 Types of Gels

There are two types of gels

1. Elastic gels and

2. Non elastic gels

5.3.1 Elastic gels

Gradual removal of water from elastic gel becomes elastic solids. By adding water they can again be transformed into a gel, eg. gum used for pasting. The linkages between the particles of a gel are due to electrical attraction and they are not rigid or strong.

5.3.2 Non elastic gels

These non elastic gels are rigid and upon dehydration they set in to become glossy powder and thus lose their elasticity. The glossy powder cannot be converted back to gel by addition of water. These are prepared by suitable chemical action such as addition of concentrated hydrochloric acid to sodium silicate solution of correct concentration makes silica gel. Initially silicic acid is formed which gradually polymerizes to silica gel. This has a strong rigid covalent structure.

5.4 Food Gels

A food gel consists of a continuous phase of inter connected particles and/ or macro molecules intermingled with a continuous liquid phase such as water. Gels possess various degrees of rigidity, elasticity and brittleness depending on the type and concentration of the gelling agent, salt content and pH of the aqueous phase and temperature. Gelling agents present at levels of 10% of less may be polysaccharides, proteins or colloidal complex particles such as casein micelles. Firm gels can be prepared by few types of gums, pectins, and gelatin at levels of 1% or lower. Gels prepared with colloidal particles are generally not very rigid even when the solids content is considerably higher than 1%. Some of the gels can be melted (liquefied) and reset with the addition or removal of thermal energy and these have been designated as thermo reversible. Gels with covalent bonds between the molecules or complex particles however are generally thermo irreversible.

Conditions for the transformations of a sol into a gel are:

• Temperature change
• Chemical alteration of gelling agent
• Reduction in number of charged groups by adjustment of pH or addition of salt and addition of a water competitive compound such as sugar.

During the sol gel transformation a three dimensional network is formed involving interaction of groups of polymer chains or particles to form cross linkages at the site of junction zones. The aqueous phase is entrapped in the interstitial areas of the structure. In some gelling agents, the junction zones consist of microcrystallites involving specific chain units arranged in a crystal like fashion. Bonds in the junction zones are electrostatic, hydrophobic, covalent and hydrogen bonds. Thermo reversible gels have preponderance of intermolecular hydrogen bonds whereas in a protein gel a few disulfide linkages per polymer chain may be sufficient to render them thermo irreversible

5.5 Properties of Gels

5.5.1 Hydration: Elastic gels after complete dehydration can be regenerated by adding water while non elastic gels cannot be hydrated.

5.5.2 Swelling

Partially dehydrated gels when dipped in water absorb water resulting in increase in the volume of gel and this process is called swelling.

5.5.3 Synersis

Most of the inorganic gels on standing shrink and exude water and discharge water in the form of tear. This is known as synersis. During storage of a gel ,the system may shift to a more stable state with changes in the junction zones and solvent solute relationships. This process leads to synersis which involves spontaneous release of water and contraction of gel volume.

5.5.4 Thixotrophy

Some gels liquefy on shaking and reset when allowed to stand. This reversible sol gel transformation is known as thixotrophy.