Chapter 8 :- Colloids

 (A.) Definition:-

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.
(B.) Application of colloidal systems in food preparation:-

Colloid Systems which are most common in Food are:-

  • Sol
  • Gel
  • Foam
  • Emulsion

Let’s have a look at their application in food preparation.

Sol

  • In this system, solids of colloidal dimensions are dispersed throughout a liquid.
  • Solids form the dispersed phase and liquids form the continuous phase.
  • The viscosity of sols may range from liquid e.g. Skimmed milk to extremely viscous e.g. Tomato ketchup which barely flows.
  • The viscosity of the sol will depend on the concentration of solid and the temperature of the sol.
  • The higher the concentration of solid in a sol, the more viscous the sol.
  • The viscosity of a sol can be adjusted by adding more liquid.
  • Irrespective of the viscosity, in a sol the solid is always distributed throughout the sol and does not settle at the bottom.
  • Protein in milk remains dispersed because of the like electrical charges on the surface of the protein molecule, which repels each other.
  • When the charge on the dispersed protein molecules in neutralized by addition of acid, protein flocculates & separates out as soon while preparing paneer.
  • All sols have flow properties. They flow more readily at a higher temperature than at a lower one.
  • Sometimes a sol may change into a gel when the system is vicious and there is a drop in energy level e.g. during cooling.
  • Milk, cream soups, pouring custard and gravy are commonly used sols in the kitchen.

 

Gel

  • A gel is a colloidal system in which liquid forms the dispersed phase and solid forms the continuous phase.
  • It is called a reverse sol.
  • A gel does not flow.
  • Some of the liquid is absorbed on the surface of the solid molecules and is called bound water. Because of this bound liquid, the gel has structure. The remaining liquid is trapped in the solid three-dimensional network of the gel.
  • A food gel consists of a continuous phase of interconnected particles or macromolecules in which liquid is dispersed.
  • The rigidity, elasticity and brittleness of the gel depend on the type & concentration of the solid or gelling agent, the pH, salt content & temperature e.g. pectin does not form gel unless the pH is acidic.
  • The gelling agent may be a polysaccharide like corn flour in blancmange, a protein like albumin in caramel custard or complex colloidal particles like calcium caseinate in curds.
  • Sols and gels are reverse colloidal systems & can be changed from one type to another.
  • Many gels are first sols which on cooling form gels provide the concentration of solids is adequate.

Dissolved gelatin jelly crystals —–Cool—-> Jelly

Sol<—–heat—-  Gel

  • When sol is converted into a gel, the energy level falls. This is seen during the cooling process.

Foam

  • A foam is a dispersion of gas bubbles in a liquid or semisolid phase.
  • In this colloidal dispersion, gas forms the dispersed phase and liquid is the continuous phase.
  • Foams are of two types:
    • Gas in liquid.
    • Gas in solid.
  • In food systems, the continuous phase is usually a liquid with added solids or changed to a solid by heating.
    • E.g. beaten egg white and sugar foam is a gas in the liquid dispersion. When it is baked it becomes a gas in the solid dispersion, e.g. meringue.
  • To form a foam, the energy is required to overcome the surface tension of the liquid and stretch it into thin films, which surrounds bubbles of gas.
  • The presence of solid matter increases the stability of foam.
  • When egg white, cream or gelatine is whipped into a foam, the protein collect at the air-water interface gets denatured or coagulated by the energy used for whipping and helps in making the foam stable.
  • Foams used in cookery include egg white, egg yolk, gelatin and cream.
  • They contribute towards lightness, volume and texture of the product.

Emulsion

  • An emulsion is a colloidal dispersion of tiny droplets of one liquid suspended in another.
  • In this colloidal system, liquids form the dispersed as well as the continuous phase.
  • For an emulsion to form, agitation or shaking the two liquids is necessary till they are well mixed.
  • Emulsions form only when the two liquids are immiscible in each other e.g. oil & water.
  • The liquid with the higher surface tension forms small droplets or the dispersed phase.
  • Food emulsions are of two types: –
    • Oil in water emulsions or O/W emulsions in which the droplets of oil are dispersed in water for e.g. mayonnaise & milk.
    • Water in oil or W/O emulsions in which the droplets of water are dispersed in oil for e.g. margarine & butter.
  • Emulsions are also classified on the basis of stability: –
    • Temporary emulsions – e.g. French dressing.
    • Semi-permanent emulsions – e.g. Milk.
    • Permanent emulsions – e.g. Homogenised milk.
  • In a temporary emulsion, the droplets that form the dispersed phase tend to coalesce as they bump into one another and form layer droplets till the emulsion breaks or separates into oil and water.


Characteristic of Emulsion –

  • It should not damage the fiber.
  • The emulsion should be colorless.
  • It should be stable for a certain period of time.
  • Its softening quality should be high.
  • The emulsion droplets should be very small.

Types of Colloidal System in Food

SystemMinor phaseMajor phaseProducts
SolSolidLiquidRaw custard, unset jelly
GelLiquidSolidJelly and jam
EmulsionLiquidLiquidMayonnaise, milk
Solid emulsionLiquidSolidButter, margarine
FoamGasLiquidWhipped cream, whisked egg white
Solid foamGasSolidMeringue, bread, cake, ice cream

Key Terms:-
  • Colloidal dispersion:- A two-phase system which has particles of 0.001 micrometre and 1 micrometre dispersed in the continuous phase. 
  • Emulsifying agent:- A compound which stabilizes an emulsion by orienting itself at the interface of the two phases of an emulsion. An emulsifying agent contains polar and non-polar groups in its molecule. 
  • Gums:- Complex carbohydrates of plant origin made up of galactose and other sugars or sugar derivatives. 
  • Homogenization:- A mechanical process in which milk is forced through tiny apertures under pressure so that fat globules are reduced in size and do not separate out as cream. 
  • Immiscible:- That which cannot be mixed together. 
  • Lecithin:- Phospholipid present in egg yolk which is an effective emulsifying agent and is commercially obtained from soya bean. 
  • pH:- The measure of the acidity or alkalinity of a substance. 
  • Surface Tension:- Attraction between molecules at the surface of a liquid. 



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