Colloids Introduction

Food is mainly composed of macronutrients and macronutrients apart from water. Along with these there are organic acids, pigments, enzymes, flavouring substances and other organic constituents present in food in varying amounts in different foods.

Food is mixture of two or more different types of substances.

These substances are present as particles of various sizes. Depending on the particle size or size of the molecule in the mixture, these substances maybe classified as:-

1. True solution.
2. Colloidal solution.
3. Coarse suspension.

TRUE SOLUTION– homogenous mixture of two or more different substances.

• It is composed of two parts:-
  • Solute.
  • Solvent.

Solute: – which is dissolved substance.

Solvent: – which is the substance in which the solute is dissolved in a liquid.

• In a true solution, ions or molecules smaller than one mill micron are dissolved in a liquid.

SUSPENSION: –

Suspensions are dispersions of coarse particles in the liquid. The particles are large & require continuous agitation to keep them dispersed.

• When agitation ceases, these coarse suspended particles settle down because of the force of gravity.
• When the mixture is stirred, the suspension is formed again.
• In a suspension, the particle size is larger than one micrometre or micron e.g. Starch & cold water paste.
• Many suspensions in food contain substances which are larger than one-micron size.

COLLOIDAL SYSTEMS: –

• Between the particle sizes of the solutions and those of suspensions lies the area of the colloidal system.
• The particles are large enough to impart to the system some properties different from those found in true solutions but small enough so that they do not separate out on standing.
• Colloidal systems deal with dispersions of a definite size, since it is the size of the particles in the colloidal range that impact the specific and characteristic properties to the system.
• Colloidal dispersions are characterized by particles ranging between one millimicron (0.001 µm) and 100 millimicron (0.01 µm) with maximum size of up to one micrometre (µm) in diameter.
• All colloidal dispersions or colloidal systems have 2 phases:-
  • Continuous phase.
  • Discontinuous or dispersed phase.
• The continuous phase extends throughout the system and surrounds the dispersed phase completely.
• Proteins, carbohydrates and fats exist in food as particles of colloidal dimensions.
• The system is a colloidal system as long as the particle size of the dispersed phase is between colloidal dimensions.

In food the following colloidal systems are of importance:-

• SOL – Colloidal dispersion of a SOLID dispersed in LIQUID.
• GEL – Colloidal dispersion of a LIQUID dispersed in SOLID.
• EMULSION – Colloidal dispersion of LIQUID dispersed in LIQUID.
• FOAM – Colloidal dispersion of GAS dispersed in LIQUID.

• In a simple dispersion a colloid may consist of a solid dispersed in a liquid e.g. When gelatine is dissolved in warm water, a simple dispersion called a sol is formed.
• Complex dispersion:-
  • Mayonnaise is an example of complex dispersion since it is an emulsion, a sol and foam combined in one.
  • Milk is another example of a complex dispersion i.e. more than one phase is dispersed in liquid. Milk is a solution of lactose in water, an emulsion of fat in water and a sol as milk protein is dispersed in water.
• Colloidal particles have different characteristics. Some are attracted to water are called hydrophilic or they are water-loving. They get hydrated easily. Those which repel water are hydrophobic or water-hating. This difference is because of the difference in chemical composition of the compounds.
• Those parts or functional groups that are attracted to water are POLAR GROUPS.
• NON-POLAR groups are hydrophobic.
• Stability of colloidal system depends on two factors:-
  • Charge on the colloidal particle.
  • A layer of water that is tightly bound to the molecule.