Separation of Milk

We have studied that milk contains fat and non-fat constituents, also called solids-not-fat (SNF). Fat is present as globules whereas the SNF form an ionic solution(e.g. certain salts), true solution (e.g. lactose and whey proteins), or a colloidal solution (e.g. casein micelles) in the water part of milk. Thus, milk represents an emulsion in which the relatively large fat globules are dispersed in the continuous aqueous phase (serum). Since fat globules are lighter as compared to other solids,they tend to readily separate out from the serum (or skim milk), as can be seen in the formation of a ‘cream’ layer on the top of milk held undisturbed in a container for a few hours. Cream is that portion of milk, which is rich in milk fat,but poorer in SNF. This suggests that much of the fat can be easily separated in the form of cream from milk, leaving behind the skim milk containing very little fat.Cream separation enables the processor to manufacture a variety of fat-rich dairy products such as cream of various types, butter, ghee, etc. Cream separation also makes it possible to adjust the composition of milk with respect to its fat and SNF contents. Such compositional modification (vide Sec) may be desired for products manufacture as also for meeting the legal requirements of different types of fluid milk

 

i. Methods of Separation

Two methods of separation of cream from milk are commonly used: (i) gravity separation and (ii) centrifugal separation. Both these methods rely on the basic principle of separation of two immiscible liquids having different densities, under the influence of gravitational or centrifugal force.

Gravity Separation: As mentioned above, when milk is allowed to stand undisturbed for some time, a layer of cream (or ‘malai’) forms on the top due to rising of the fat globules which are initially dispersed throughout the bulk of milk. The upward movement of the lighter fat globules (density, 0.93 g/cc at 20oC) in the heavier serum (density, 1.035 g/cc) takes place owing to gravity. Creaming may become evident in as short time as half an hour.

The rate of cream separation is directly proportional to the difference between the densities of fat and serum and to the square of fat globule diameter, and inversely proportional to the viscosity of serum. Thus, for a given sample of milk, the creaming rate will be maximum when the density difference is maximum and viscosity is minimum. Both these factors are, in turn, affected by temperature of milk. As the temperature rises, the ratio of the density difference and the serum viscosity increases favouring the separation process. This increase is particularly prominent between 10o and 30oC and much less above 50oC.Cream separation by gravity is, however, a very slow and inefficient process. It is of little practical value for commercial purposes. Hence, mechanized cream separation employing a centrifugal machine is most commonly used in the dairy industry. Even for a very small scale separation involving, say 10-20 litres of milk, a centrifugal separator is used, be it hand-driven or motor-driven.

Centrifugal Separation: In principle, this method of cream separation is similar to gravity separation but gravity as the driving force is replaced by the centrifugal force for which a rotational machine is used. Since the latter force is much larger than the gravitational force, separation is greatly accelerated. The centrifugal separator is similar to the clarifier discussed in the earlier section, but milk entering through the bottom of the separator bowl holding a stack of conical discs rises up through holes located somewhere in the middle of the inner and outer edges of the discs.The milk between discs is subjected to a centrifugal force in the rotating bowl and thereby tends to fly out from the centre. The skim milk fraction, being heavier,moves away and forms a layer on the outer edge of the discs, whereas the fat globules gather on the inside edge. The incoming un-separated milk forces the separating layers further and upward out at the top of the bowl. Thus, there are two outlets in a cream separator, one for skim milk and the other for cream, the cream outlet being nearer to the centre.

The rate of cream separation in ease of a centrifugal separator is influenced by the same factors affecting gravity separation, but the speed of the separator bowl and the disc diameter are also very important here. The higher the speed of the bowl or larger the diameter of discs, the greater will be the separation rate.

 

ii. Factors affecting Skimming Efficiency

Since fat removal from milk is the principal function of a cream separator, the efficiency of the process, also called skimming efficiency, is determined by the effectiveness with which the fat content of the out-coming skim milk is reduced.The residual fat content of skim milk is usually in the range of 0.01 – 0.05% in the modern machines. A fat content higher than 0.06% represents poorer separation efficiency. The skim milk fat content is inversely related to fat recovery in the cream. Hence, the skimming efficiency is often defined as the percentage of total fat in whole milk recovered in the cream separated from it. For a given fat content of whole milk, the higher the fat content of skim milk, the lower the skimming efficiency. The factors that affect the skimming efficiency are related to either the milk being separated or the separator.

Intense agitation of milk prior to separation, air incorporation (or foaming) and high acidity of milk adversely affect the separation efficiency. Further, if the proportion of smaller fat globules (especially below 2 mm in diameter) is greater, the skimming efficiency will go down. It should, therefore, be obvious that homogenized milk with its very small globules (please see Unit 3) cannot be separated. Gravity or centrifugal separation of fat globules from skim milk is faster at a higher temperature. Thus skimming efficiency increases with increasing temperature of milk up to about 80oC, beyond which increasing viscosity of milk tends to make the separation process less efficient. Depending upon location of the cream separator in the milk processing line (particularly with respect to HTST pasteurization), the separation temperature may range usually from 35-75oC, optimum being 50-55oC (‘warm milk’ separation). However, cold milk separators’ may operate at 5-10oC giving an advantage of less foaming, but partial churning of fat, bowl clogging and reduced flow rate (separator capacity) are the associated disadvantages.

Adjustment of the ‘cream screw’ for high-fat cream (above 55%), or excessive flow rates of feed milk reduce the skimming efficiency. However, feeding rates below the normal separator capacity does not enhance the skimming performance,but it may lead to undesirable air incorporation. A higher bowl speed gives higher skimming efficiency but, since increased speed requires greater energy input, normal range of 4000-7000 rpm (sometimes as low as about 2000 rpm) giving efficient separation is normally not exceeded in the separator design. Poor disc condition(e.g. deshaped, dirty or scratched one), vibrations of the separators, and defective gaskets in the cream section could cause unacceptable skimming efficiency.Excessive slime getting collected in the sludge space of the bowl would also have an adverse impact on the separator performance.

 

iii. Factors affecting Yield and Fat Content of Cream

The yield of cream and skim milk can be given by the following formulae:

i) Yield of cream (% of feed milk) =((fm - fs)/(fc-fs))*100......  (Eq.1)

ii) Yield of skim milk (% of feed milk) =((fc - fm)/(fc - fs))*100.....(Eq.2)

where,

fm = fat in milk, %

f s = fat in skim milk, %

fc = fat in cream, %.

All those factors which affect the skimming efficiency can be expected to influence the cream yield too. Conditions leading to a higher skimming efficiency would give a better yield. However, the fat content of cream is obviously the major factor influencing the yield of cream. Accordingly, the adjustment of the cream screw or skim-milk screw is critical with regard to cream yield.

The position of the cream screw i.e. a valve provided in the cream outlet controls the flow rate of the cream being discharged. Turning the screw ‘inward’ reduces the cream discharge rate thereby increasing the fat content of cream. Adjusting the valve by ‘outward’ movement has the opposite effect. Similarly, manipulation of the skim milk screw so as to decrease the flow rate of the exiting skim milk will decrease the fat concentration of cream, and vice-versa. Thus, changing the position of the cream screw or skim-milk screw alters the ratio of cream to skim milk; an increased ratio decreases the fat content of cream and a decreased one raises it.

Further, a lower separation temperature and a higher fat content of milk lead to an increased fat content of cream, whereas an increased feed rate causes a decreased richness of cream, and vice-versa.