cream of milk

                                                         CREAM

i. Definition

Cream is defined as the fat rich portion of milk obtained by gravity or mechanical method of separation. It is the light weight portion of milk which still contains all the main constituents of milk but in different proportions. The fat content of cream varies widely in the range of 20- 80 % depending upon the method of separation

ii. Classification

Cream on the basis of fat content has been categorized into following categories:

(i) Low fat cream- The cream in this group contains fat in the range of 25-39%,e.g., table cream, light cream, whipping cream, etc.

(ii) Medium fat Cream- Cream containing 40-59 % fat is grouped in this category,e.g., heavy cream, plastic cream.

(iii) High fat Cream - Cream containing above 60% fat are high fat cream, e.g.,plastic cream, heavy cream.

(iv) Coffee cream is a popular variety of cream in which fat content ranges from 10-20 %. However, this type of cream does not meet the minimum legal standards prescribed for cream by the Prevention of Food Adulteration Act.

Hence, it cannot be designated as cream.

Types of Cream :

1.Sterilized Cream

2. Plastic Cream
3. Frozen Cream
4. Sour Cream
5. Whipped Cream

Methods of Cream 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

Types of Cream Separators

Common types of cream separators are listed below:

a) Cold milk separator: It produces skim milk and viscous cream of high quality with less foam. But it may cause partial churning of milk during separation and has low capacity.

b) Warm milk separator: It facilitates close skimming, and has high capacity but produces low viscosity in cream and also foam.
Definition, Composition,Standards and Processing of Cream

c) Hand driven cream separator: It is a low capacity machine suitable for farm scale operation and is economical.

d) Power driven cream separator: It is high capacity machine suitable for dairy plants but are expensive.

e) Open bowl cream separator: It has low capacity and so suitable for small dairy plants. But it produces foam during separation.

f) Hermetically sealed/Air-tight/Foamless tri-process separator:

As the name indicates it does not produce foam and performs three operations. It can clarify,separate and standardize the milk or cream. It can deliver cream or skim milk to respective tanks without any additional pumps, produces cream of high viscosity and of desired fat content without stopping the machine. Since it is airtight there is no contact of cream or skim milk with outside atmosphere and hence the quality is better. But it is expensive and regular maintenance is required.

g) Domestic cream separator:

Domestic food processors or mixers are usually provided with a speed variation from 1400 to 18000 rpm. However they lack a rigid foundation and sufficient mechanical strength of the driving unit. Therefore an attachment has been developed at the National Dairy Research Institute,Karnal. The attachment consists of raw milk, cream and skim milk pans of the matching size of the mixer. The lowest pan has built-in- power transmission assembly and is fixed with the mixer to give a fairly rigid base to the bowl. The bearings are so designed that they take care of the vibrations and overheating of the mixer. It has 8-9 discs and operates at 3250 rpm. It can separate about 4 litres of milk in 10 min and produces cream containing 40% fat.

Factors Affecting Fat Losses in Skim Milk :

There are several factors, which influence the efficiency of cream separation process. Consequently the entire fat present in milk is not recovered during centrifugal separation. The percent of total fat recovered in cream from milk is referred to as skimming efficiency. The skimming efficiency is calculated by the equation given below

Skimming Efficiency (%) = (Total fat in cream)/(Total fat in milk) x 100

Factors affecting the skimming efficiency are listed here:
i) Temperature of milk: The temperature of milk at the time of separation should be around 37-50°C. Separation at lower temperatures results in higher fat loss in skim milk and may lead to partial clogging of the bowl due to the increased viscosity of cream.

ii) Speed of bowl: The fat loss in skim milk will be higher at lower speed of the bowl. This loss is ascribed to the insufficient centrifugal force generation.Hence, milk should not be fed in to the cream separators unless the cream separator attains its full speed.

iii) Rate of milk inflow: The flow of milk to the cream separator should be at optimum level. If the flow is at higher rate it will result in greater loss of fat in skim milk.

iv) Size of fat globules: Smaller the size of fat globules in milk higher will be the fat content in skim milk. Due to this reason, it is observed that generally cow milk and goat milk have lower separation efficiency in comparison to buffalo milk.

v) Presence of air: Greater the amount of air in milk higher will be the fat loss in skim milk. The entrapped air reduces the efficiency of hermetically sealed separators more than that of the normal cream separators.

vi) Acidity of milk: Higher acidity of milk reduces the efficiency of separators.This is mainly due to the partial coagulation of milk, which in turn, increases the sludge formation in the bowl affecting the efficiency of separation.

vii) Mechanical condition of separator: Vibration in the machine, use of deformed/dirty/scratched/rough discs and accumulation of separator slime causes increased losses of fat in skim milk.

viii) Fat percentage in cream: Production of cream containing more than 50-60% fat causes more losses of fat in skim milk.

ix) Degree of agitation and temperature of milk: Higher temperature of milk and more agitation cause higher losses of fat in skim milk.

x) Position of cream screw: As high fat in cream causes more losses the cream screw should be appropriately adjusted.

Factors Influencing Fat Percentage of Cream

The following factors influence the fat percentage in cream
a) Position of cream or skim milk screw: Any of the screws can be moved IN or OUT. Thus it comes nearer to or moves away from the center of rotation. Adjusting the cream screw towards IN position or Skim milk screw towards OUT position yields high fat percentage in cream or vice-versa.
b) Fat percentage of milk: Milk containing high fat yields rich cream.
c) Bowl speed: The higher and recommended speed of the bowl yields cream of higher fat content.
d) Rate of milk in-flow: Faster rate of inflow of milk to the separator produces cream with lower fat percentage.
e) Temperature of milk: Lower temperature of milk during separation yields cream with higher fat percentage.
f) Quantity of water: If the amount of water used to flush the bowl is more the fat percentage in cream will be low.