Homogenization

Milk is an oil-in-water emulsion. Fat globules in the milk are dispersed in a continuous water phase (skim milk) and normally vary in sizes ranging from 1 mm to 22 mm,with a mean size of approximately 3-4 mm. As the density of milk fat is less than that of skim milk, the fat globules tend to rise to the surface during storage and form a cream layer. The rise of fat globules follows Stoke’s law where the velocity of rising fat globules is expressed as:

V a(d2 (rs - rf))/18h

Where, d = diameter of the fat globule, rs = density of the serum phase, rf = density of milk fat and n = viscosity of milk serum.Homogenization 

Very small fat globules (<1 mm) remain suspended in the serum phase due to brownian motion and adversely affect the creaming phenomenon. The presence of cryoglobulins in the raw milk causes agglomeration of fat globules, which subsequently have increased tendency to rise to the surface.

Homogenization is a mechanical process in which milk is forced through a homogenization valve under very high pressure. The milk is thus deflected at right angles through a narrow opening of about 0.1 nm (100mm). As the milk comes out of this valve opening, there is sudden drop in pressure and the milk is subjected to impact against an impact ring. This complete process results in disruption of fat globules leading to decrease in the average diameter (typically from 0.2 to 2 mm) and an increase in the number and surface area of fat globules.

Homogenization with reference to milk/ dairy applications thus refers to a mechanical process that is used to reduce the size of fat globules such that milk fat does not rise to form a cream layer during storage of milk. Although homogenization renders fat globules uniformly distributed in the body of the milk, upon prolonged storage it does not remain completely dispersed.