Principle of Separation

The fat is present in milk as fat-in-water type emulsion and is lighter than the skim milk. The mean density of milk fat is 0.93 as compared to 1.036 of skim milk at 16°C. Separation of cream is based on the principle that milk fat,because of its lower density is lighter than the skim milk portion. Hence it tends to rise to the surface and separates from the serum (skim milk). This principle is applicable to both gravity method and mechanical method of cream separation.

 

i. Gravity Method

In gravity method when milk is allowed to stand undisturbed for sometime the fat being lighter will rise and float on the surface under the influence of gravitational force. The fat floating on the surface will form a layer and can easily be removed or separated. It is common method of fat separation, which is governed by this principle and adopted at household level. However the rate at which the fat separates is governed by the Stoke’s law as given by the following equation:

v= (2G (ds – df)r2)/9n

Where

V = Rate at which fat globule rises
G = Acceleration due to gravity
ds = Density of skim milk
df = Density of fat
r = Radius of fat globule
η = Viscosity of skim milk

Thus, it can be inferred from the above equation that rate of separation of fat from milk increases with the following factors:

a) Increase in radius of the milk fat globule (clumping)

b) Increased difference in the densities of milk and fat (dilution)

c) Decrease in viscosity of milk

The gravity method of separation is a slow and simple process. It is followed at household and cottage level. It separates fat from milk in the form of cream. This is the easiest way to separate cream. Fresh or boiled and cooled milk is left to stand in a vessel and after a while the fat globules cluster together or aggregate and rise to the surface forming a layer of fat rich portion (cream). The layer can be removed by hand or ladle.

If the milk is not properly heat-treated it may undergo undesirable fermentation,sometimes leading to curdling of milk. The separation of fat will occur but the resulting cream and the skim milk will be of very poor quality. The fat separating on the surface of milk in the form of skin is scooped and the process is repeated at least two times to extract fat. By this method it is possible to extract about 50% of fat present in milk. The fat thus obtained is preserved with lactic culture to avoid putrefaction. The rate of separation of fat by gravity method is affected by the following factors:

i) Size of fat globules: If the size of fat globules is bigger, the separation will be faster. This is the reason separation is faster in buffalo milk as compared to cow milk.

ii) Temperature: It affects the viscosity, which in turn, influences the flow or velocity with which the fat will separate. Hence separation at higher temperature is faster because of the reduced viscosity of milk.

iii) Clumping: Clumping of fat globules together increases the size and thus increases the velocity and rate of separation.

ii. Mechanical Method

In the mechanically operated cream separator, which is used commercially, the separation of fat is achieved by application of the centrifugal force. The centrifugal force acting on the system is about 3000- 6000 times greater than the gravitational force. Hence the separation of fat, which is governed by the Stoke’s law, is faster than the gravity method of separation. Fat globules of smaller size separate 6500 times faster in a centrifuge rotating at 5400 rpm than they do by gravity. The Stoke’s law as applied to centrifugal separation process is expressed by the following equation:
 
v=(r2 (ds – df)K.R.N2))/9η

Where,

V = Velocity of movement of fat globule
r = Radius of fat globule
ds = Density of skim milk
df = Density of fat
N = Speed of bowl (rpm)
R = Distance of fat globule from the axis of rotation
K = Constant
= Viscosity of skim milk

The mechanized process is also known as centrifugal method of separation. It is a faster process. It is adopted as an industrial method of cream production. In this case it is very essential to fix the mechanized cream separator firmly to avoid vibration during the course of its operation. The large capacity separators (250 litres milk / hr or above) are installed on the ground while the smaller ones or laboratory models on wooden stool or tables. To run the separator the electrical connection is switched on. The separator is allowed to acquire the requisite speed.Generally the milk is preheated to 37-50oC before separation for optimum results.This makes the process easier and more efficient as the warm milk is less viscous than the cold milk. Milk is fed down as an inlet tube into the separator bowl where it rotates and moves up through a series of aligned holes in the disc stack. In-flow of milk, is regulated by adjusting the milk in-let valve to the separator. As the disk stack revolves the cream moves towards the center of the bowl and the skim milk is directed outwards by the centrifugal force. Thus milk is separated in two streams,a highly concentrated milk fat stream termed as cream and a nonfat stream of skim milk. Under normal conditions it produces skim milk and cream in the ratio of 90:10