MARKET MILK

Module 6. Common dairy operations

Lesson 19
SEPARATION OF MILK - THEORY AND METHODS

19.1 Introduction

Cream is a fatty product of milk, and creams of different fat contents can be prepared by the separation of milk fat from non fat solids portion of milk. Market creams for retail sale are made to different fat contents according to intended use. Cream is a richly flavored product, which makes it desirable for use in applications such as desserts, cakes and some chocolate confectionery. It is also used in some beverages like coffee and cream liqueurs.

In dairy industry, the process of separating milk into cream and skim milk is known as separation. Cream comprises of fat concentrate in milk. Milk fat can be removed in the form of cream and the remaining portion is serum referred to as skim milk. The skim milk contains predominantly SNF and is having very little fat.

19.2 Principles of Cream Separation

Separation of cream can be done either by gravity (malai) or by applying the centrifugal force. Separation of milk is possible because of difference in density between the fat (0.93) and the skim milk (1.036). When the milk fat in the form of globules rises to the surface of the milk, the globules maintain their identity at the temperature below their melting point, thereby forming fat concentrate referred to as ‘Malai’.

19.2.1 Separation of cream by gravity

Separation is brought about by the force of gravity and the rate of separation is determined by Stoke's law.

A solid particle or liquid droplet moving through a viscous fluid medium under the influence of gravity will eventually attain a constant velocity. This is called sedimentation velocity and is denoted as Vg (g = force of gravity). The value of sedimentation velocity is calculated by the equation.

Where,
d = particle diameter
Pp = particle density
P1 = density of liquid
n = viscosity of fluid
g = gravitational force

19.2.2 Separation of cream by centrifugal force

When a mass is made to revolve in a circular path around its axis, a kind of force is generated, which is called centrifugal force. This force throws the heavier portion away from the centre. Simultaneously, there is another force called centripetal force, which acts on the lighter portion and attracts it towards the centre. Separation is completed by leading these two portions from the bowl through different outlets. The centrifugal acceleration created in the centrifuge is specified as r?2,
where, r = radius and ? = angular velocity

Substituting the value of centrifugal acceleration, we derive the sedimentation velocity (V) of every particle in centrifuge

Where,
V = velocity of movement of a single fat globule
r = radius of fat globule
ds = density of skim milk
df = density of fat
N = speed of bowl (r.p.m)
R = distance of fat globule from the axis of rotation
K = constant
n = viscosity of skim milk

19.3 Method of Separation

Separation of milk can be carried out by the following methods

• By gravity

• By centrifugal force

19.3.1 Separation by gravity

Earlier, this technique was used in dairies to separate fat from milk. Milk was left in a vessel where, after some time (hours), the fat globules aggregate and float on the surface forming a layer called ‘malai’ on top of the milk. There are two types of gravity separation as discussed below:

(a) Shallow pan method

The milk is poured into the pans, immediately after milking. The pans, which are four inches deep, are placed preferably in a cool place. Skimming is done at the end of 24 h, and by this time, the milk below the cream is coagulated. Skim milk from the shallow pan system contains 0.5-1.5% fat.

(b) Deep setting method

In this method, milk is set in 20 inches deep cans which are 8"-15" in diameter, maintained at 8-10°C. Glass strips are inserted in the wall of the can, one near the bottom and other near the top, to absorb cream. Due to low temperature better quality product results. After 12-14 h of storage, the fat layer from the top is skimmed off leaving skim milk in the container.

19.4 Advantages of Centrifugal Separation over Gravity Separation

• Speed of separation is greater (instantaneous) for centrifugal separation.

• Bacteriological quality of cream and skim milk is superior in centrifugal separation than gravity separation.

• Greater fat percentage of cream is possible using centrifugal separation (25-80%) vs. gravity separation (10-25%).

• Fat recovery in cream is 99-99.5% for centrifugal separation. Such value for gravity separation is about 75% or so.