Viscosity in milk
If increase of viscosity increases the stability of an emulsion, why is milk more stable and show less settling even though the viscosity is less?
RAKESH SUNDARAN
Coimbatore, Tamil Nadu
An emulsion is a dispersion of one liquid in another immiscible one (for example, oil and water). One phase is called continuous and other is called disperse phase.
Oil droplets are dispersed in water by fragmenting it in presence of a surface-active species such as surfactants or polymers, which provide stability.
The diameter of the disperse phase varies from a few nanometres to several microns. Milk is a natural emulsion where each fat globule is stabilized by a membrane of phospholipids and proteins.
Milk contains 88 per cent water, 3.3 per cent protein (casein 82 per cent ), 3.3 per cent fat , 4.7 per cent carbohydrate, 0.7 per cent ash. The percentage of disperse phase in milk is very low compared to some of the industrial products.
Now coming to the answer to the question, what happens when the viscosity of the continuous phase (water) goes up? The settling velocity decreases due to viscous resistance and hence the settling time increases.
Therefore, we may be right to say that the stability of an emulsion goes up with viscosity. However, the answer is not fully correct.
The density mismatch between the two phases, the diameter of the dispersed droplets and the electrical charges of the surfactant molecules also play a crucial role in the stability of emulsions.
Dr. JOHN PHILIP
Senior Scientist
IGCAR, Kalpakkam
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