FAT RICH DAIRY PRODUCTS

Module 2. Production of cream

Lesson 4
PRINCIPLES AND METHODS OF CREAM SEPARATION

4.1. Introduction

When milk is allowed to stand un-disturbed for some time, an upward motion of the fat globules takes place, leading to the formation of a surface layer on milk in which the percentage of fat is considerably increased. This upward motion of fat based on the fact that milk fat is lighter than the skim milk portion. At 16°C, the average density of milk fat is 0.93 and of skim milk is 1.0404. Therefore when milk (a mixture of fat and skim milk) is subjected to either gravity or centrifugal force, the two components, cream (fat-rich portion of milk) and skim milk (reduced-fat portion of milk), by virtue of their differing densities, separate from each other.

Purpose of cream separation

1. To obtain a fat-reduced or fat-free milk

2. To concentrate milk fat for the production of high-fat products

3. To standardize the fat content of milk

4. To recover fat from milk

The cream separation process has significant economic importance, as it controls the efficiency of the fat separation. The key objective is to manufacture skim milk with the lowest possible fat content, which corresponds to good separation efficiency. Knowledge of the basics of the fat separation is important for an optimal de-creaming process. Cream separation is based on the facts that fat exists in poly-disperse system in an emulsified state and that the specific density difference between milk fat (p = 0.93 g/cm3) and skim milk (p = 1.035 g/cm3) is fairly large. Basically two processes for fat separation are possible, natural de-creaming and separation with machines. Natural creaming has no industrial significance.

4.2. Separation Processes

There are two methods of cream separation viz.,

· Gravity Method

· Centrifugal Method

4.2.1. Cream separation by gravity method

When milk is allowed to stand undisturbed for some time, there is a tendency of fat to rise. The velocity or rate at which the fat globules rise is given by the following equation, which is known as Stoke’s Law:

V = (2/9) * Gr2 * (ds - df) / N

Where,

V = rate of rise of fat globule in centimeter per seconds

r = radius of fat globule

G = Force of gravity (981 dynes)

ƞ= Viscosity of skim milk

ds = density of skim milk

df = density of fat globule

From, Stoke’s Law it is observed that theoretically velocity increases with:

a. Increasing radius of fat globule,

b. Increasing difference in densities of skim milk and fat

c. Decreasing viscosity of skim milk

However, in practice the factors affecting the rate of rise of fat in gravity method of separation are:

• Size of fat globules: As the size of fat globules increases, the rate at which fat rises also increases. Larger fat globules rise faster than smaller ones. Thus, in buffalo milk gravity creaming occurs faster due to the larger fat globules than those in cow milk.

• Temperature: As temperature increases, viscosity decreases.

• Clumping: A clump or cluster acts like a single globule in so far as movement through skim milk is concerned. Thereby the effective ‘r’ is increased, which in turn increases velocity, as shown below

Effect of size of fat globules on its rate of rise
Diameter of fat globule or cluster (µm)	Rate of rise (mm/h)
3.2 41.0	1.26 242

There are five various methods for separating the cream using gravity method:

i. Shallow Pan Method: Milk is allowed to stand in a pan of 10 cm depth and 45-60 cm diameter at 7°C for 24 h. During this time, cream rises to the surface.

ii. Deep Pan Method: Milk is allowed to stand in pan of 20” depth and 8 to 12” diameter at 10°C for 24 h. These tall cans have glass on one side of can and a faucet placed near the bottom. Skim milk is drawn through the faucet.

iii. Water Dilution Method: Milk is diluted with water and allows standing for 12 h at 37.7°C temperature. Water would make the milk less viscous, thus facilitating the rising of the fat globules.

iv. Scalding Method: Heating and cooling of milk slowly causes the formation of cream layer at surface of milk

v. Jersey Creamery Method: Milk is heated to 40°C using hot water in the jacketed vat and then cool to 10°C using chilled water in place of hot water in the jacket of Vat. The cream will be separated rapidly on cooling, immediately after heating the milk, by increasing the difference in densities of milk fat and serum.

Gravity method being very slow, it is no longer used commercially for cream separation.

4.2.2 Cream separation by centrifugal method

Milk is fed to machine through flow regulator. Milk comes to regulating chamber from milk basin by milk faucet. When milk enters the revolving bowl through milk regulator of machine, it is subjected to a gravity and centrifugal force. Centrifugal force is about 3000 to 6000 times more than gravitational force. Fat (0.9) and skim milk (1.037) are varying in their specific gravity. When fat and skim milk are subjected to centrifugal force, the difference in density affect the fat and skim milk i.e. (heavier Portion) affected more intensely than the fat (lighter portion). So skim milk is forced to the periphery and fat portion (cream) moves towards the centre. Cream and skim milk forms separated vertical walls within the bowl and goes out through separate outlets near the axis of rotation. The cream outlet is at higher level than skim milk outlet. The rate or movement of a fat globule in machine is estimated by following Stoke’s equation.

V = r2 *((as - df) / n) * N2 * R * K

Where,

V = rate of movement of a single fat globule

r = radius of fat globule

ds = density of skim milk

df = density of fat

N = Revolution per minute of bowl

R = Distance of fat globule from axis of rotation.

K = Constant

N = Viscosity of skim milk

It will be seen from the above that the speed (rate) of cream separation is increased by:

· greater radius of the fat globule

· greater difference in density between skim milk and fat

· greater speed of the bowl

· greater size of the bowl

· lower viscosity of skim milk

4.3 Characteristics of gravity and centrifugal methods

Gravity and centrifugal cream separation compare as shown below:

Particulars	Gravity Method	Centrifugal Method
Nature of force causing Separation	Gravitational force	Centrifugal force
Speed of separation	Extremely slow	Practically instantaneous
Direction of movement of fat and skim milk particles	Vertical	Horizontal
Bacteriological quality of cream or skim milk	Low	High
Fat % of cream	10-25% only	18-85 %
Skim milk	0.2 % above	0.1 or below
Scale of operation	Small	Large
Fat % recovered in cream	not more than 90	99-99.5

When the centrifuge bowl, filled with liquid, is put into rotation, the liquid surface level is lowered at first, and a rotation paraboloid is created. With an additional increase in revolutions, the surface level is lowered to the rotation axis and then rises in parallel. At corresponding high revolutions in the bowl a liquid ring is created. Imagine particles of skim milk and fat with the same particle diameter in a rotating liquid, the resulting force acts toward the inside. It corresponds to the force, according to motion theory, which is required to accelerate a body moving in a circle and maintain it in an orbit. This force is called the centripetal force (Fep); it is opposite to the centrifugal force Fef. In this example, the skim milk particles enter a phase in which the forces are in balance, as Fep = Fef. Because of their lower specific density, less centrifugal force is exerted on fat particles of equal size, resulting in Fep > Fef. The difference in force here is very small, so the fat particles migrate slowly toward the inside.