PHYSICAL CHEMISTRY OF MILK

Module 2. Colloidal chemistry

Lesson 3
MILK AS A COLLOIDAL SYSTEM

3.1 Introduction

Milk, when secreted by the epithelial cells of the mammary glands is a mixture of various substances which exist in different states. It is necessary to maintain similar conditions continuously to maintain the stability of substances in native state. Accordingly the constituents like casein micelles and minerals exist in the form of a colloidal suspension and exists in that condition as long as favorable conditions prevail. It is necessary to study the conditions that influence colloidal stability of milk which could be used to obtain different milk products by suitably modifying the conditions.

3.2 Characteristic Features of a Colloidal System

Particles larger but definitely too small to be visible are said to exist in colloidal state. The size range is approximately 1 to 10⁵ nm. A colloidal solution will have a continuous phase and a discontinuous phase. Depending on the affinity of the particles towards the dispersion phase colloids are divided (grouped) as lyophyllic or hydrophilic and lyophobic or hydrophobic colloids. The reversible colloids such as macromolecules like proteins or association of colloids like micelles are thermodynamically stable. Irreversible colloids constitute a true phase in aqueous environment such as oil in water emulsions.

3.3 Milk as Colloidal System

It is a well known fact that milk is not a homogeneous fluid. At the same time milk is also not a pure colloidal system. It could be even observed that in some milk products, the fat globules exist as hydrophobic colloids along with air bubbles, and crystals of fat and lactose. The hydrophobic colloids are thermodynamically unstable.

3.3.1 Stability of the colloidal system in milk

The stability of a colloidal system is the capability of the system to remain as it. Stability is hindered by aggregation and by sedimentation phenomena that determine phase separation. A phenomenon in solid solutions or liquid sols which describes the change of an inhomogeneous structure over time during which small crystals or sol particles dissolve, and redeposit onto larger crystals or sol particles is known as Ostwald’s ripening. This happens due to the difference in solubility larger particles which tend to grow at the expense of smaller ones and the latter disappears. Generally the radii should be below 0.1µm to exhibit this effect for solids or liquids or below 100 µm for gases. Further, it is necessary that the substance in the particles be at least slightly soluble in the surrounding medium. In milk the hydrophobic colloids are subjected to this Ostwald’s ripening particularly when they are very small. In the normal conditions the hydrophobic colloidal particles sediment or cream. Brownian motion prevails when the particles are so smaller than < 1 µm. The smaller particles show a tendency to flocculate which could effectively be prevented by the steric repulsion caused by their surface material.

• If the hydrophobic colloid particles are fluid (fat globules at normal temp and air bubbles) which may coalesce with each other if the thin film of continuous phase between closely approaching globules or bubbles is ruptured
• Two colloidal particles of equal composition always attract each other because of van der Waals forces. If the attraction energy is not too strong the Brownian motion will be sufficient to keep the particle apart and maintain the colloidal stability
• There will also be repulsion due to hydration on close contact of individual molecular chains when the protruding chains are hydrophilic. This is important for the stability of fat globules

3.3.2 Properties of colloidal system

The colloidal system will have two phases one is a dispersed phase and a dispersion medium. The state of aggregation of the dispersed phase and the intensity of its interaction with the dispersion medium determines many fundamental characteristics of the colloid system.

• Lyophillic colloids have a strong attraction for the molecules of dispersion medium and thus binding large number of them into so called solvent shells
• In lyophobic colloids the particles do not interact so strongly with the molecules of the surrounding medium
• Lyophobic colloids are heterogeneous and are highly disperse colloidal systems
• Lyophillic colloids are in fact true solutions which mean that molecular species disperse homogenous in the system and not colloids.
  
• The great difference in the size of the solvent and solute particles and to the structure of these particles However to the large size of the molecules solutions of such substances are in many respects similar to colloid system

3.4 Sedimentation and Coagulation of Colloidal System

Colloidal systems widely differ with respect to the stability.

• Disintegration of the colloid system could be by two processes namely sedimentation and coagulation
• The dispersed phase may either settle out or raise to the surface of the system depending upon the difference in densities between the dispersed particles and the dispersion medium and is known as sedimentation
• The dispersed particles adhere to one another or coalesce increase in size. This process is called coagulation. Under definite conditions coagulation can proceed spontaneously since it is accompanied by decrease in surface
• Coagulated particles can be redispersed by adding certain substances
• The determination of kinetic stability and aggregate stability will help in characterizing the stability of colloidal system with respect to sedimentation process and to change in particle size (coagulation)