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

Principles applied in preparation of cream

PRINCIPLES APPLED IN PREPARATION OF CREAM

• Cream may be defined as that portion of milk in to which milk fat has been gathered or which is rich in fat.  According to PFA rules (1976) cream, excluding sterilised cream is the product of cow or buffalo milk or combination there of which contains not less than 25 per cent milk fat.  Cream is rich in fat soluble vitamins A, D, E & K. Milk can be converted into cream and skim milk by using cream separator.
• Cream can be broadly classified as Market cream (which is used for direct consumption) or Manufacturing cream (which is used for the manufacture of dairy products).
• Based on the fat content it may be
  • Light/coffee cream (20-25 per cent),
  • Whipping cream (30-40 per cent) and
  • Heavy/plastic cream (65-85 per cent).

Traditional method (Gravity: method)

• Stokes law states that the when difference in density exists the velocity of particle (fat globule) is directly proportional to the difference in densities of the media and gravity. It is inversely proportional to the viscosity of skim milk.
• Difference in density of fat and serum results in the upward movement of fat globules and the velocity at which fat globules rise is given by stokes law

                                        V = 2G(ds-df)r² / 9 u

where ,

G - Acceleration due to gravity  

ds – Density of skim milk 

df – density of fat 

r- radius of fat globule  

u - voscosity of skim milk

Using cream separators by application of Centrifugal force

• When milk is allowed to stand for sometime, there is a tendency for the fat to rise. This method being very slow, can not be used commercially for cream separation.
• In a cream separator milk enters the rapidly revolving bowl of the cream separator, it is subjected immediately to tremendous centrifugal force, which is 3000 to 6000 times greater than the gravitational force. 
• The difference in density affects the heavier portion (skim milk) more intensely than the lighter portion (cream).  Due to this skim milk is forced to the periphery, while the fat portion moves towards the centre to form two vertical columns in the bowl.
• When Stokes law is applied to centrifugal separation, the velocity is changed as follows. The angular velocity “w” =KwRN² and hence                     

V = KR (ds-df)r²N²  / u                                                                                                                                                                

where

V: velocity of fat globule

G: Acceleration due to gravity  

K: Constant

N: RPM of bowl

R: Distance of the fat globule from the axis of rotation

ds: Density of skim milk

df: density of fat ( is the viscosity of milk)

r: radius of fat globule.

• Clarifier also works on the same principle. It is used to remove impurities along with maximum bacterial load.