In this article we will discuss about:- 1. Definition of Cream 2. Classification of Cream 3. Composition and Nutritive Value 4. Physico-Chemical Properties 5. Production 6. Factors Influencing Fat Percentage 7. Factors Affecting Fat Loss in Skim Milk during Separation 8. Yield 9. Quality 9. Collection.
Definition of Cream:
Cream has been known from time immemorial as the fatty layer that rises to the top of the milk when it stands undisturbed for some time. The production of cream in India in 1966 was estimated to be about 1.9 per cent of the total milk production and 3.4 per cent of the milk used for manufacture of dairy products.
Cream may be defined as:
(i) ‘That portion of milk which is rich in milk fat’,
(ii) ‘That portion of milk into which has been gathered and which contains a large portion of milk fat’,
(iii) ‘When milk fat is concentrated into a fraction of the original milk, that portion is known as cream’ or
According to the PFA Rules (1976), cream, excluding sterilized cream, is the product of cow or buffalo milk or a combination thereof which contains not less than 25 per cent milk fat.
Classification of Cream:
Cream is not a definite specific substance. It contains all the milk constituents but in varying proportions. The milk fat in cream may vary from 18 to 85 percent; the solids-not-fat constituents occur in lower proportions than in milk.
Cream may be classified broadly as:
(a) Market cream, which is used for direct consumption, and
(b) Manufacturing cream, which is used for the manufacture of dairy products.
The various types of cream are:
Composition and Nutritive Value of Cream:
The chemical composition of two specific creams has been given in Table 3.1.
Note:
It will be observed from the above that the higher the fat percentage in cream, the lower the solids-not-fat content.
The formula for determining the percentage of solids-not- fat in cream is:
Food and Nutritive Value:
Cream is rich in energy-giving fat and fat-soluble vitamins A, D, E and K, the contents of which depend on the fat level in cream.
Physico-Chemical Properties of Cream:
(a) Viscosity:
This may be defined as the resistance offered by a liquid to flow. It is an important property of cream from the commercial point of view, since the consumer judges the ‘richness’ of cream from its viscosity.
Viscosity of cream may be affected by the following factors:
(i) Fat Percentage:
The higher the fat percentage, the greater the viscosity, and vice versa.
(ii) Temperature:
The higher the temperature, the lower the viscosity, and vice versa.
(iii) Separation Conditions:
The higher the temperature of separation, the lower the viscosity, and vice versa. (A lower temperature of separation, however, leads to higher fat losses.)
(iv) Homogenization:
Single-stage homogenization increases viscosity in direct relation to pressure used; double-stage homogenization reduces viscosity.
(v) Cooling:
Slow cooling of cream increases viscosity.
(vi) Ageing:
Increases viscosity.
(vii) Clumping:
The greater the degree of clumping, the greater the viscosity.
Note:
Clumping refers to the tendency of fat globules to loosely adhere to one another to form clumps or clusters.
The degree of clumping largely depends on:
(i) Fat Globule Size:
Large globules clump more readily than smaller ones;
(ii) Temperature:
The maximum clumping takes place around 7°C (45°F); clumping decreases with rise in temperature;
(iii) Agitation:
At a favourable temperature, i.e., around 7°C (45°F), clumping increases by agitation; at an unfavourable temperature, i.e. around 60°C (140°F), clumping decreases and previously formed clumps are broken up;
(iv) Method of Separation:
Gravity-Separated cream clumps to a greater degree than centrifugally separated cream of the same fat content;
(v) Adhesives (such as Gelatin):
Help in clumping.
(b) Whipping Quality:
Whipping refers to the beating of cream to produce froth or foam (emulsion of gas/air in a liquid). Whipped cream is a special foam possessing remarkable stability and is used in cakes, ice creams, etc., for decorative purposes.
The main factors affecting whipping quality are:
(i) Fat Percentage:
The most satisfactory is 30-35 per cent.
(ii) Whipping Temperature:
Lowers progressively above 4°C (40°F).
(iii) Separation Temperature:
32°C (90°F) more satisfactory than 38°C (100°F).
(iv) Ageing:
For 24 hours at 4°C (40°F) is optimum.
(v) Homogenization:
Above 300 psi is detrimental.
(vi) Acidity:
Reduces progressively.
(vii) Addition of Stabilizers:
Reduces.
(c) Specific Gravity:
This depends on its fat percentage, as is evident from Table 3.2.
(d) Acidity:
The per cent titratable acidity of all fresh cream should be consistent with the fat percentage of the cream. There exists an inverse relationship between the per cent fat and per cent titratable acidity. The per cent titratable acidity of freshly separated cream is always lower than that of the milk from which it was separated, and can be calculated by the formula-
Production of Cream:
Principle:
The basic principle of cream separation, whether by gravity or centrifugal methods, is based on the fact that milk fat is lighter than the skim milk portion. At 16°C (60°F), the average density of milk fat is 0.93 and skim milk 1.036. Hence when milk, which may be considered to be a mixture of fat (as cream) and skim milk, is subjected to either gravity or a centrifugal force, the two components, viz., cream and skim milk, by virtue of their differing densities, stratify or separate from one another.
Methods:
Cream is obtained from milk by either gravity or centrifugal methods:
I. Gravity Methods:
When milk is allowed to stand for some time, there is a tendency for the fat to rise. The velocity, or rate at which the fat globules rise, is given by the following equation, which is known as Stocke’s Law-
From Stoke’s Law it will be observed that, theoretically, velocity is increased by:
(i) Increase in radius of fat globule;
(ii) Increase in difference in densities of skim milk and fat;
(iii) Decrease in viscosity of skim milk.
However in practice the important factors affecting the rate of rise of cream in gravity methods are:
(i) Size of Fat Globules:
As the size of fat globules increases, the rate at which cream rises also increases. (Thus in buffalo milk. gravity creaming occurs faster due to the larger size of fat globules than those in cow milk.)
(ii) Temperature:
As temperature increases, viscosity decreases and hence velocity increases.
(iii) 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 in Table 3.3.
(iv) Addition of Adhesives:
Ultimately helps in increasing the rate at which fat globules rise.
Note:
Gravity methods, being very slow, are no longer used commercially for cream separation.
II. Centrifugal Methods (Used Commercially):
When milk enters the rapidly revolving bowl of the cream separator, it is immediately subjected to a tremendous centrifugal force, which is 3000 to 6000 times greater than gravitational force. While both the fat and skim milk are subjected to the centrifugal force, the difference in density affects the heavier portion (i.e. skim milk) more intensely than the lighter portion (i.e., cream).
Thereby the skim milk is forced to the periphery while the fat portion moves towards the centre. The skim milk and cream both form vertical walls within the bowl and are separated by being led through separate outlets. (The cream outlet is at a higher level than the skim milk outlet, both being near the axis of rotation.)
The important parts of the centrifugal cream separator (open-bowl type) are as follows:
Supply can; Faucet; Regulating chamber (Float); Cream or skim milk screw; Bowl shell; Milk distributor; Cream spout; Skim milk spout; Top disc; Discs; Bowl nut; Rubber ring; Spindle; Set of gears; Crank handle, etc.
The various types of cream separators are as follows:
(i) Warm milk separator
(ii) Cold milk separator
(iii) Power driven separator (Factory)
(iv) Hand driven separator (Farm)
(v) Open bowl or Gravity fed separator, and
(vi) Semi enclosed separator.
The merits and demerits of the different types of cream separators are given in Table 3.4.
Note:
The drawback of frequent stoppage of the usual type of centrifugal cream separators for cleaning, especially with high-sediment milk (such as obtained in this country), has been overcome today by what is known as the self-cleaning separator. Of slightly different design, its characteristic feature is that the bowl can be opened for discharging the accumulated slime while the machine is running.
This in turn means that the machine can be operated continuously for any length of time while still maintaining the high efficiency of a clean bowl. It can be cleaned by circulation of detergent (CIP), thereby making time and labour-consuming dismantling and manual cleaning unnecessary.
The bowl is opened after a certain period of time just before it is estimated that slime will begin to interfere with the separating efficiency. Opening is effected by hydraulic pressure and the impulse of opening can be supplied manually or, preferably, by an automatic timer. The functioning of these machines is in other respects similar to that of a conventional separator. The fat percentage in cream can be controlled from outside and foam-free cream and skim milk are discharged at about 70 psi.
Stoke’s Law applied to centrifugal separation is as follows:
It will be seen from the above that the speed (rate) of cream separation is increased by:
(i) Greater radius of fat globule
(ii) Greater difference in density between skim milk and fat
(iii) Greater speed of bowl (r.p.m.)
(iv) Greater size of bowl
(v) Lower viscosity of skim milk.
(f) Gravity and centrifugal cream separation have been characterized in Table 3.5.
Factors Influencing Fat Percentage of Cream:
The important factors influencing the fat percentage of cream by centrifugal separation are:
(a) Position of the cream screw (or skim milk screw);
(b) Fat percentage in milk;
(c) Speed of the bowl;
(d) Rate of inflow of milk;
(e) Temperature of milk;
(f) Amount of water or skim milk added to flush the bowl.
(a) Position of the Cream Screw (or Skim Milk Screw):
(Cream screw IN or Skim milk screw OUT, higher fat percentage in cream and vice versa.) The cream screw/outlet consists of a small, threaded, hollow screw pierced by a circular orifice through which the cream emerges. This screw can be driven IN or OUT, thus bringing it nearer to, or away from, the centre of rotation.
Similarly, the skim milk screw/outlet is for the removal of skim milk. Once the cream screw or skim milk screw has been adjusted, the cream separator delivers, under normal conditions, a definite ratio of skim milk and cream, which is usually 90: 10 (or 85: 15) by volume.
Basically, any change in the separation procedures which alters the relative quantities of skim milk and cream will influence the fat test of the cream. By altering the position of the cream screw (or skim milk screw), the ratio of skim milk to cream changes.
Thus, when the cream screw is moved IN towards the axis of rotation, a higher fat percentage in cream is obtained, and vice versa; this is because the force tending to discharge cream through the orifice is decreased (‘R’ in the formula F = KWRN2 is decreased), while that tending to discharge skim milk remains unaltered.
A smaller proportion of cream is therefore discharged, which, containing the same quantity of fat, shows a higher fat percentage. Screwing OUT the cream screw produces thinner cream. Similarly, the skim milk screw OUT results in richer cream, and vice versa.
(b) Fat Percentage in Milk:
(The higher the fat percentage in milk, the higher the per cent fat in cream, and vice versa.) Since practically all the fat in milk is contained in the cream, the cream from the separation of high-fat milk has a higher fat content than that from low-fat milk; a greater fat content in cream, the amount of which remains unaltered in the two cases, will obviously show a higher fat percentage in it, and vice versa.
(c) Speed of Bowl:
(The higher the speed of the bowl, the higher the fat percentage in cream, and vice-versa.) The higher the speed of the bowl, the greater will be the centrifugal force, and the more rapidly will the skim milk leave the bowl. An increase in bowl speed, therefore, increases the capacity of skim milk discharge. This means less cream is discharged and, consequently, with the same fat content, a higher fat percentage in cream will obtain.
(d) Rate of Milk Inflow:
(The higher the rate of milk inflow, the lower the fat percentage in cream, and vice versa.) When the rate of inflow increases, the discharge from the cream outlet increases, as the skim milk discharge remains constant (with constant centrifugal force); more cream containing the same amount of fat results in a lower test, and vice versa.
(e) Temperature of Milk:
(The lower the temperature of milk during separation, the higher the fat percentage of the cream, and vice versa.) Lowering of temperature increases viscosity of both cream and skim milk, but that of cream increases (proportionately) more than skim milk. Hence the quantity of cream discharged is reduced (due to clogging of the bowl), thereby resulting in a higher fat test.
(f) Amount of Water or Skim Milk Added to Flush the Bowl:
(The greater the quantity of water or skim milk added to flush the bowl, the lower the fat percentage in cream, and vice versa.) The addition of more water or skim milk will cause an increase in the amount of cream produced, which, with the same fat content, will show a lower fat test.
Factors Affecting Fat Loss in Skim Milk during Separation:
(a) The ‘skimming efficiency’ (SE) of a cream separator refers to the ‘percentage total fat from milk recovered in the cream. The higher the fat percentage in milk and/or the greater the fat loss in skim milk, the lower is the skimming efficiency, and vice versa. (The best index for SE is the fat test of skim milk, which should be checked regularly.)
Examples:
1. Given- 100 kg milk testing 7.5% fat; cream produced 14.1 kg testing 52.5% fat.
2. Given- 100 kg milk testing 4.8% fat; cream produced 10.3 kg testing 45.5% fat.
Note:
In the above examples, high-test milk shows a higher SE than low-test milk.
(b) The factors affecting fat loss in skim milk are:
(i) Temperature of milk;
(ii) Speed of bowl;
(iii) Rate of milk inflow;
(iv) Position of cream screw;
(v) Mechanical condition of the machine:
1. Vibration of the separator
2. Condition of the discs
3. Amount of separator slime in the bowl;
(vi) Size of fat globules;
(vii) Degree and temperature of agitation given to milk before separation;
(viii) Presence of air in milk;
(ix) Acidity of milk.
(i) Temperature of Milk:
(The lower the temperature, the higher the fat loss in skim milk, and vice versa.) For efficient separation, the temperature of milk should be above the melting point of fat, so that the milk fat in the fat globules is entirely in liquid form. A satisfactory temperature for separation is around 40°C (104°F).
The higher the temperature, the more efficient the separation. There is no marked increase in efficiency after 43-49°C (110-120°F). On the other hand, separation at low temperatures (in warm-milk separators) may lead to partial clogging of the bowl due to the high viscosity of cream at these temperatures, resulting in a greater fat loss in skim milk.
Note:
The milk is invariably heated before separation (in warm- milk separators) to 35-40°C, in plate or tubular heaters, for efficient separation. This is known as ‘pre-heating/ forewarming’ of milk.
(ii) Speed of Bowl:
(The lower the speed, the higher the fat loss in skim milk, and vice-versa.) At below-rated speed there will be more fat loss in skim milk because insufficient centrifugal force is generated for efficient cream separation. However, at above- rated speeds, the skimming efficiency will not increase greatly.
(iii) Rate of Milk Inflow:
(The higher the rate of inflow, the higher the fat loss in skim milk, and vice versa.) If the rate of inflow is increased above the designed capacity of the separator, the milk passes through the bowl too rapidly to allow for complete separation, thereby resulting in a higher fat loss in skim milk. On the other hand, underfeeding the separator does not greatly increase the efficiency of the separation.
(iv) Position of the Cream-Screw:
If up to 50 per cent cream is present, there is little effect on the fat test of skim milk; where there is 50 to 60 per cent cream, there is greater fat loss in skim milk; if above 60 per cent of cream is obtained, still higher fat losses in skim milk at low-temperature separation result.
A good separator is designed to give efficient skimming within a fairly wide range of positions of the cream screw, so that the fat test of the cream can be varied without influencing the efficiency of skimming.
With most separators, the position of the cream screw has little effect on the fat test of skim milk until the cream test is above 45 to 50 per cent. From this point up to a 60 per cent fat test in cream, the fat content of the skim milk increases. Separation of very thick cream at low temperatures may lead to higher losses through clogging of the bowl with viscous cream.
(v) Mechanical Condition of the Machine:
Unsatisfactory mechanical condition of the cream separator causes greater fat loss in skim milk.
These include:
1. Vibration of the Separator:
This reduces the efficiency of separation by disturbing the counter-currents of cream and skim milk. (Vibration is caused by installation on an insufficiently firm foundation, the bowl being out of balance, bearings being worn out, the axis of rotation not exactly vertical, etc.)
2. Condition of Discs:
Discs in an unsatisfactory condition suffer a loss of skimming efficiency due to the uneven flow of the counter- current streams of cream and skim milk between them. (An unsatisfactory disc is one which is out of shape, dirty, scratched or rough.)
3. Amount of Separator Slime in Bowl:
If too much slime accumulates, the fat loss in skim milk increases; this is caused not only by a disturbance in the even flow of the counter-currents of cream and skim milk, but by reduction in the centrifugal force (because of decrease in the ‘effective’ diameter of the bowl).
Note:
Separator slime (which is usually considered identical with clarifier slime) consists of the slimy mass which accumulates inside the bowl shell of the cream separator. It is made up of foreign matter, milk proteins, leucocytes, fragments of the secreting cells from the udder, fat, calcium-phosphate and other minerals, bacteria and, occasionally, red blood corpuscles.
The average composition of separator slime is given in Table 3.6.
(vi) Size of Fat Globules:
(The greater the number of fat globules of less than 2 micron size, the higher the fat loss in skim milk and vice versa.) Fat globules of less than 2 micron size usually enter the skim milk, as they are not subject to sufficient centrifugal force to be recovered in the cream. Hence, the greater the number of less- than-2 micron size globules, the greater is the fat loss in skim milk.
(vii) Degree and Temperature at Which Milk is Agitated before Separation:
(The higher the degree and temperature of agitation, the greater the loss in skim milk and vice versa.) Agitation of hot milk causes the disintegration of the normal fat globules into smaller ones which escape the effect of centrifugal force, thereby leading to more fat loss in skim milk.
(viii) Presence of Air in Milk:
(The greater the amount of air, the higher the fat loss in skim milk). If the milk delivered to the separator contains entrapped air bubbles, separation of air as a consequence of centrifugal force disturbs the counter-current streams of cream and skim milk between the discs, and lowers the efficiency of separation. The effect of air in the milk is greater with hermetic than with non-hermetic separators.
(ix) Acidity of Milk:
(The higher the acidity, the lower the efficiency of separation.) The higher the acidity, the lower the stability of casein particles, which in turn get precipitated and clog the bowl, thereby lowering the efficiency of separation.
Yield of Cream:
This can be calculated by the formula:
Yield of Skim Milk:
This can be calculated by the following formula:
Fat Recovery in Cream:
This can be calculated by the formula:
Fat Lost in Skim Milk:
This can be calculated by the formula:
where fc, fm and fs are as above.
Note:
In formula II, weights are not required.
Quality of Cream:
‘Cream is no better than the milk from which it is made.’ It is not possible to obtain good (bacteriological) cream from low-grade milk.
In order to produce high quality cream, the following steps are needed:
(i) Clean milk production;
(ii) Cooling of milk soon after production (preferably within 3 hours);
(iii) Separating milk under hygienic conditions (cream separator should be thoroughly cleaned and sanitized before use);
(iv) Prompt cooling of cream and its storage at a low temperature;
(v) Transport of cream to the dairy under low temperature.
Collection of Cream (for Butter-Making):
Factory butter is made either from cream separated on the farms or from cream separated from milk delivered to the factory. The cream used for commercial butter-making is collected mainly by motor truck/lorry. Each vehicle is allocated one (or more) chosen routes, which is identified (preferably) by a letter, while the suppliers on each route are numbered.
The collection is done either by the creamery-factory itself or by private agencies who may themselves operate small cream buying stations/depots; in the depots, cream is received in producers’ cans, weighed, sampled, tested and bulked in creamery-owned cans for shipment to the factory.