LPT 311: Milk and Milk Products Technology (1+1)

HOMOGENIZATION

• Homogenised milk is produced by mechanically forcing milk through a small passage at high velocity. This breaks down the fat globules in milk into much smaller ones and creates a stabile fat emulsion. Homogenisation diminishes the tendency of the fat globules to clump together and coalesce into cream.
• Homogenised milk has many advantages
  • Uniform distribution of fat, no cream layer
  • Full-bodied flavour
  • Whiter, more appetising colour
  • Faster coagulation in the manufacture of rennet cheese
• Milk is an oil-in-water emulsion, with the fat globules dispersed in a continuous skimmilk phase. If raw milk were left to stand, however, the fat would rise and form a cream layer. Homogenization is a mechanical treatment of the fat globules in milk brought about by passing milk under high pressure through a tiny orifice, which results in a decrease in the average diameter and an increase in number and surface area, of the fat globules. The net result, from a practical view, is a much reduced tendency for creaming of fat globules. Three factors contribute to this enhanced stability of homogenized milk: a decrease in the mean diameter of the fat globules (a factor in Stokes Law), a decrease in the size distribution of the fat globules (causing the speed of rise to be similar for the majority of globules such that they don't tend to cluster during creaming), and an increase in density of the globules (bringing them closer to the continuous phase) oweing to the adsorption of a protein membrane. In addition, heat pasteurization breaks down the cryo-globulin complex, which tends to cluster fat globules causing them to rise.

The homogenizer consists of a high-pressure pump fitted with a minute orifice having an adjustable opening through which fluids are forced at high pressure, thereby causing a marked change in the physical properties of the product treated and producing a very intimate mixture of the ingredients of the fluid.

The effect of homogenization upon milk may bee described as follows.

• The fat globules in normal milk are usually in sizes varying from 1 to 15 microns, depending upon the breed of cow and various other factors. By means of homogenization, the fat globules are broken up into numerous smaller ones and all the fat globules are under 2 microns in size.
• The fat globules no longer rise to the top to form a cream layer, as a normal milk, for they are so small that few of them have the power to rise against the pull of gravity.
• The curd tension of the homogenized milk is reduced.
• An increase in viscosity of the milk, and an apparent increase in creaminess and richness.

Homogenization

The process of making a stable emulsion of milk fat and milk serum by mechanical treatment is termed as “ homogenization. The machines used for this purpose is called homogenizer. The size of the fat globules in milk is between 1 and 20 microns, the average being 4-6 microns. When milk or cream is passed through the homogenizer, there is desired reduction in the size of the fat globules. Most of them are reduced to 2 microns or less. No cream line is formed on milk after homogenization or butter granules cannot be produced from the homogenized cream by churning. Furthermore, there is an increase in the viscosity of milk like cream, which means that the smaller fat globules encounter greater resistance. The increase in viscosity is attributed to the fact that a larger proportion of the milk protein will be adsorbed on the surfaces of the greater number of the fat lobules. When milk is homogenized its clotting properties are altered, so that the curd produced when rennet or pepsin is added is said to be softer. Homogenized milk or cream may become rancid more quickly than untreated products owing to the hydrolysis of fat by lipolytic enzymes adsorbed on the additional surfaces furnished by the more numerous small fat globules. To avoid these changes the milk or cream must be pasteurized immediately before or after homogenization.

The milk is usually homogenized following primary heating before it is pasteurized. Heated milk is forced through a small valve against hard surfaces and is subjected to a pressure of 2500 psi. and in a second stage to 500 psi. The aperture through which the liquid is forced is extremely minute, having a diameter of 1/10000 inch.

The following types of homogenizes are at present in use.

• High pressure type
• Low-pressure rotary type
• Sonic vibrator or oscillator.

In the manufacture of evaporated milk it is important that the fat globules be reduced in size to avoid churning upon agitation. In the preparation of ice cream mix, the entire mixture is passed thorough the homogenizer immediately after pasteurization. The result is a smoother texture in the finished ice cream.