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Milk Processing


Fresh Milk Technology

Milk separation
Butter-making with fresh milk or cream
Cheese-making
Milk fermentations

Sour-milk technology

Making butter and cottage cheese from sour whole milk


In rural areas, milk may be processed fresh or sour. The choice depends on available equipment, product demand and on the quantities of milk available for processing. In Africa, smallholder milk-processing systems use mostly sour milk. Allowing milk to ferment prior to processing has a number of advantages and processing sour milk will continue to be important in this sector.

Where greater volumes of milk can be assembled, processing fresh milk gives more product options, allows greater throughput of milk and, in some instances, greater recovery of milk solids in product.

Because of differences between processing systems, each will be dealt with separately. The section on fresh-milk technology deals with techniques used for processing fresh milk in batches of up to 500 litres. Sour-milk technology is used for processing batches of up to 15 litres of accumulated sour milk. This will be described in the section on sour-milk technology.

Fresh Milk Technology

This section describes the manufacture of skim milk, cream, butter, butter oil, ghee, boiled-curd and pickled cheese varieties and fermented milks from fresh milk. The processing scale envisaged is 100 to 200 litres of milk per day. However, the processes described are suitable for batches of up to 500 litres per day. Most of the equipment described can be fabricated locally. Equipment not available locally, such as a milk separator, has a cost advantage and quickly gives a good financial return in terms of increased efficiency. Hand-operated milk separators are durable and have a long life when properly maintained. Importation of such equipment is, therefore, advantageous.

The procedures given here are very precise. In many rural dairy processing plants, however, monitoring equipment may not be available and, although yields may be maximised by adhering to the prescribed procedures, all these products can be successfully made by approximating temperature, time, pH etc to the best of one's ability. It is particularly important in cheese-making to proceed when the curd is in a suitable condition. Therefore, times given are only approximate and the processor will, with experience, adopt methods suitable to his/her own environment.

6.1.1 Milk separation

The fat fraction separates from the skim milk when milk is allowed to stand for 30 to 40 minutes. This is known a `creaming'. The creaming process can be used to remove fat from milk in a more concentrated form. A number of methods are employed to separate cream from milk. An understanding of the creaming process is necessary to maximise the efficiency of the separation process.

Gravity separation

Fat globules in milk are lighter than the plasma phase, and hence rise to form a cream layer. The rate of rise (V) of the individual fat globule can be estimated using Stokes' Law which defines the rate of settling of spherical particles in a liquid:

V = (r2 (d1   d2)g)/9η

where r = radius of fat globules

Particle r2: As temperature increases, fat expands and therefore r2 increases. Since the sedimentation velocity of the particle increases in proportion to the square of the particle diameter, a particle of radius 2 (r2 = 4) will settle four times as fast as a particle of radius 1 (r2 = 1). Thus, heating increases sedimentation velocity.

dl d2: Sedimentation rate increases as the difference between d1 and d2 increases. Between 20 and 50°C, milk fat expands faster than the liquid phase on heating. Therefore, the difference between dl and d2 increases with increasing temperature.

g: Acceleration due to gravity is constant. This will be considered when discussing centrifugal separation.

η: Serum viscosity decreases with increasing temperature. Calculation of the sedimentation velocity of a fat globule reveals that it rises very slowly, As shown in the equation, the velocity of rise is directly proportional to the square of the radius of the globule. Larger globules overtake smaller ones quickly. When a large globule comes into contact with a smaller globule the two join and rise together even faster, primarily because of their greater effective radius. As they rise they come in contact with other globules, forming clusters of considerable size. These clusters rise much faster than individual globules. However, they do not behave strictly in accordance with Stokes' Law because they have an irregular shape and contain some milk serum.

Factors affecting creaming: Cream layer volume is greatest in milk that has high fat content and relatively large fat globules, because such milk contains more large clusters. However, temperature and agitation affect creaming, irrespective of the fat content of the milk. Heating to above 60°C reduces creaming; milk that is heated to above 100°C retains very little creaming ability.

Excessive agitation disrupts normal cluster formation, but creaming in cold milk may be increased by mild agitation since such treatment favours larger, loosely packed clusters.

Batch separation by gravity: Cream can be separated from milk by allowing the milk to stand in a setting pan in cool place. There are two main methods.

Shallow pan: Milk, preferably fresh from the cow, is poured into a shallow pan 40 to 60 cm in diameter and about 10 cm deep. The pan should be in a cool place. After 36 hours practically all of the fat capable of rising by this method will have come to the surface, and the cream is skimmed off with a spoon or ladle (Figure 12). The skim milk usually contains about 0.5 to 0.6% butterfat.

Figure 12. Batch separation of milk by gravity: (a) Shallow pan method, (b) deep-setting method

Deep-setting: Milk, preferably fresh from the cow, is poured into a deep can of small diameter. The can is placed in cold water and kept as cool as possible. After 24 hours the separation is usually as complete as it is possible to secure by this method. The skim milk is removed through a tap at the bottom of the can (Figure 12). Under optimum conditions, the fat content of the skim milk averages about 0.2 or 0.3 %.

The pans should be rinsed with water immediately after use, scrubbed with hot water and scalded with boiling water (see section on cleaning).

Centrifugal separation

Gravity separation is slow and inefficient. Centrifugal separation is quicker and more efficient, leaving less than 0.1% fat in the separated milk, compared with 0.5–0.6% after gravity separation.

The centrifugal separator was invented in 1897. By the turn of the century it had altered the dairy industry by making centralised dairy processing possible for the first time.

It also allowed removal of cream and recovery of the skim milk in a fresh state.

The separation of cream from milk in the centrifugal separator is based on the fact that when liquids of different specific gravities revolve around the same centre at the same distance with the same angular velocity, a greater centrifugal force is exerted on the heavier liquid than on the lighter one. Milk can be regarded as two liquids of different specific gravities, the serum and the fat.

Milk enters the rapidly revolving bowl at the top, the middle or the bottom of the bowl (Figure 13). When the bowl is revolving rapidly the force of gravity is overcome by the centrifugal force, which is 5000 to 10 000 times greater than gravitational force. Every particle in the rotating vessel is subjected to a force which is determined by the distance of the particle from the axis of rotation and its angular velocity.

Figure 13. Cutaway diagrams of (a) hand-operated milk separator and (b) the bowl showing the paths of milk and cream fractions.

If we substitute centrifugal acceleration expressed as r1ω2 (where r1 is the radial distance of the particle from the centre of rotation and ω2 is a measurement of the angular velocity) for acceleration due to gravity (g), we obtain:

V = (r2(d1 – d2) r1ω2)/9η

Thus, sedimentation rate is affected by r1ω2. In gravity separation, the acceleration due to gravity is constant. In centrifugal separation, the centrifugal force acting on the particle can be altered by altering the speed of rotation of the separator bowl.

In separation, milk is introduced into separation channels at the outer edge of the disc stack and flows inwards. On the way through the channels, solid impurities are separated from the milk and thrown back along the undersides of the discs to the periphery of the separator bowl, where they collect in the sediment space. As the milk passes along the full radial width of the discs, the time passage allows even small particles to be separated. The cream, i.e. fat globules, is less dense than the skim milk and therefore settles inwards in the channels towards the axis of rotation and passes to an axial outlet. The skim milk moves outwards to the space outside the disc stack and then through a channel between the top of the disc stack and the conical hood of the separator bowl.

Efficiency of separation is influenced by four factors: the speed of the bowl, residence time in the bowl, the density differential between the fat and liquid phase and the size of the fat globules.

Speed of the separator. Reducing the speed of the separator to 12 rpm less than the recommended speed results in high fat losses, with up to 12% of the fat present remaining in the skim milk.

Residence time in the separator: Overloading the separator reduces the time that the milk spends in the separator and consequently reduces skimming efficiency. However, operating the separator below capacity gives no special advantage—it does not increase the skimming efficiency appreciably but increases the time needed to separate a given quantity of milk.

Effect of temperature: Freshly drawn, uncooled milk is ideal for exhaustive skimming. Such milk is relatively fluid and the fat is still in the form of liquid butterfat. If the temperature of the milk falls below 22°C skimming efficiency is seriously reduced. Milk must therefore be heated to liquify the fat. Heating milk to 50°C gives the optimum skimming efficiency.

Effect of the position of the cream screw: The cream screw regulates the ratio of skim milk to cream. Most separators permit a rather wide range of fat content of cream (18–50%) without adversely affecting skimming efficiency. However, production of cream containing less than 18% or more than 50% fat results in less efficient separation.

Other factors that affect the skimming efficiency are:

When separation is complete the separator must be dismantled and cleaned thoroughly.

Hand separator

In order to understand how centrifugal separation works, we shall follow the course of milk through a separator bowl. As milk flows into a rapidly revolving bowl it is acted upon by both gravity and the centrifugal force generated by rotation. The centrifugal force is 5000 to 10 000 times that of gravity, and the effect of gravity thus becomes negligible. Therefore, milk entering the bowl is thrown to the outer wall of the bowl rather than falling to the bottom.

Milk serum has a higher specific gravity than fat and is thrown to the outer part of the bowl while the cream is forced towards the centre of the bowl.

Assembling the bowl

  1. Fit the milk distributor to the central feed shaft.
  2. Fit the discs on top of each other on the central shaft.
  3. Fit the cream screw disc.
  4. Next, fit the rubber ring to the base of the bowl.
  5. Put on the bowl shell, ensuring that it fits to the inside of the base.
  6. Finally, screw the bowl nut on top.

Now the bowl is assembled and ready for use. The rest of the separator is essentially a set of gears so arranged as to permit the spindle, on which the bowl is carried, to be turned at high speed. The gears are normally enclosed in an oil-filled case. The bowl is usually supported from the bottom and has two bearings; one to support its weight and the second to hold it upright. The upper bearing is usually fitted inside a steel spring so that it can keep the bowl upright even if the frame of the machine is not exactly level.

The assembled bowl is lowered into the receptacle, making sure that the head of the spindle fits correctly into the hollow of the central feed shaft.

Operation

  1. When the bowl is set, fit the skim milk spout and the cream spout.
  2. Fit the regulating chamber on top of the bowl.
  3. Put the float in the regulating chamber.
  4. Put the supply can in position, making sure that the tap is directly above and at the centre of the float.
  5. Pour warm (body temperature) water into the supply can.
  6. Turn the crank handle, increasing speed slowly until the operating speed is reached: This will be indicated on the handle or in the manufacturer's manual of operation. The bell on the crank handle will stop ringing when the correct speed is reached.
  7. Open the tap and allow the warm water to flow into the bowl. This rinses and heats the bowl and allows a smooth flow of milk and increases separation efficiency.
  8. Next, put warm milk (37 – 40°C) into the supply can. Repeat steps 6 and 7 above and collect the skim milk and cream separately.
  9. When all the milk is used up and the flow of cream stops, pour about 3 litres of the separated milk into the supply can to recover residual cream trapped between the discs.
  10. Continue turning the crank handle and flush the separator with warm water.

Cleaning the separator: Many of the impurities in the milk collect as slime on the wall of the separator bowl. This slime contains remnants of milk, skim milk and cream, all of which will decompose and ferment unless removed promptly.

If not washed and freed from all impurities the separator bowl becomes a source of microbial contamination. Skimming efficiency is also reduced when the separator bowl and discs are dirty. Milk deposits on the separator can cause corrosion.

Washing the separator: After flushing the separator with warm skim milk, the bowl should be flushed with clean water until the discharge from the skim milk spout is clean. This removes any residual milk solids and makes subsequent cleaning of the bowl easier. The bowl should then be dismantled. Wash all. parts of the bowl, bowl cover, discharge spouts, float supply tank and buckets with a brush, hot water and detergent. Rinse with scalding water. Allow the parts to drain in a clean place protected from dust and flies. This process should be followed after each separation.

Cream screw adjustment

The cream screw should be adjusted so that the fat content of the cream is about 33%. Producing excessively thin cream reduces the amount of separated milk available for other uses and increases the volume of cream to be handled. Low-fat cream is also more difficult to churn efficiently.

Cream containing more than 45% fat clogs the separator and causes excessive loss of fat in skim milk. Cream of abnormally high fat content also gives butter a greasy body due to lack of milk SNF. When adjusting the cream screw it is important to remember that it is very sensitive; a quarter turn of the screw is sufficient to change the percentage fat in the cream appreciably.

The fat content of whole milk influences the fat content of cream and this must be considered when adjusting the cream screw. For example, if the cream screw is set to separate milk at a ratio of 85 parts of separated milk to 15 parts of cream then, with all other conditions constant and assuming efficient separation, milk of 3% fat produces cream of 20% fat whereas milk of 4.5% fat produces cream of 30% fat.

The fat content of the cream can be calculated using the following equation:

Wm = weight of milk          Fm = fat content of milk

Wc = weight of cream       Fc = fat content of cream

In the first example, Fc = (100 × 3)/15 = 20

In the second example,  Fc = (100 × 4.5)/15 = 30

Therefore the setting of the cream screw depends on the fat content of the milk being separated. The milk should be mixed thoroughly prior to separation to ensure even distribution of cream in the milk.

Separator maintenance

Calculations

Once milk passes through a separator it is recovered in two fractions, the high-fat cream fraction and the low-fat skim milk.

Assuming negligible loss of fat in the separator, the amount of fat entering the separator with the whole milk will be collected at the other side of the separator in either the cream or the skim milk. Therefore, if we separate 200 kg of milk containing 4.5% butterfat, what weight of cream containing 30% butterfat can we expect?

Let Wm = weight of milk

Assuming that all of the fat present in the milk is recovered in the cream, then:

Wm × Fm = Wc × Fc

and Wm – Wc = Ws

and Wm – Ws = Wc

Since Wm × Fm = Wc × Fc

(Wm × Fm)/Fc = Wc

Therefore Ws = Wm – (Wm × Fm)/Fc = Wc

In this case: Ws = 200 – (200 × 4.5)/30

= 200–30

= 170 kg

Since Wc = Wm – Ws

Wc = 200 – 170 = 30 kg

Percentage yield of skim milk:

=Ws × 100)/Wm = (170 × 100)/200 = 85%

Percentage cream (%Wc)

= %Wm – %Ws

= 100 – 85 = 15%

If in practice we obtain only 28 kg of cream containing 30% butterfat, then (2 × 0.30) kg or 0.6 kg of butterfat has not been recovered in the cream. Since it is assumed that there are no significant losses of fat in the cream separator, the fat not recovered in the cream is lost in the skim milk.

Since 28 kg of cream was produced, and

Ws = Wm – Wc

then Ws = 200 – 28 = 172 kg

Thus there is 0.6 kg of fat in 172 kg of skim milk. The fat percentage of the skim milk is therefore:

(0.6 × 100)/172= 0.35%*

* The skim milk contains 0.35% fat, which may be incorporated in cottage cheese. If the skim milk is consumed, no nutritional loss occurs, but a financial loss is incurred since the fat is more valuable if sold as butter than as cottage cheese or if it is consumed directly.

The percentage of fat in milk and in cream influences Wc and Ws where the fat is recovered in the cream.

If      Fm = 3 %

        Fc = 30%

        Wm = 100

Then Wc = Wm × Fm/Fc

Wc = 100 × 3/30 = 10 kg

Ws = Wm – Wc

= 100 – 10

= 90 kg

whereas if     Fm = 4%

                     Fc = 30

                    Wc = 100

Then Wc =100 × 4/30 = 13.3 kg

Ws = 100 – 13.3

=86.6 kg

Standardisation of milk and cream

If fine adjustment of the fat content of cream is required, or if the fat content of whole milk must be reduced to a given level, skim milk must be added. This process is known as standardisation.

The usual method of making standardisation calculations is the Pearson's Square technique. To make this calculation, draw a square and write the desired fat percentage in the standardised product at its centre and write the fat percentage of the materials to be mixed on the upper and lower left-hand corners. Subtract diagonally across the square the smaller from the larger figure and place the remainders on the diagonally opposite corners. The figures on the right-hand corners indicate the ratio in which the materials should be mixed to obtain the desired fat percentage.

The value on the top right-hand corner relates to the material on the top left-hand corner and the figure on the bottom right relates to the material at the bottom left corner.

Example 1

In this example, the fat content of whole milk is to be reduced to 3.0%, using skim milk produced from some of the whole milk. Using Pearson's Square, it can be seen that for every 2.9 litres of whole milk, 0.6 litres of skim milk must be added.

Example 2

How much skim milk containing 0.1 % fat is needed to reduce the percentage fat in 200 kg of cream from 34% to 30%?

If 29.9 parts of cream require 4 parts of skim milk, 200 parts of cream require x parts of skim milk.

Weight of skim milk needed = x = (200 × 4)/29.9 = 26.75 kg

Example 3

The fat content of 300 kg of whole milk must be reduced from 4.2% to 3% using skim milk containing 0.2% fat.

Every 4.0 kg of the mixture will contain 2.8 kg of whole milk and 1.2 kg of skim milk.

If 2.8 kg of whole milk requires 1.2 kg skim milk, 300 kg of whole milk requires

(1.2 × 300)/2.8 = 128.6 kg of skim milk

Thus, 128.6 kg of skim milk (0.2% fat) must be added to 300 kg of whole milk (4.2% fat) to give 428.6 kg of milk containing 3% fat.

Example 4

The fat content of milk must be reduced from 4.5 to 3% prior to sale as liquid milk but skim milk for standardisation is not available.

In this case, we must calculate (a) what proportion of the milk must be separated to provide enough skim milk to standardise the remaining whole milk and (b) the expected yield of cream.

Assume that the fat content of 100 kg of milk containing 4.5% milk fat must be reduced to 3%. The amount of cream to be removed can be calculated as follows:

Let M = weight of milk to be standardised—in

this example, 100 kg. Therefore M = 100

Fm = fat content of the original milk = 4.5

C = weight of cream

Fc = fat content of the cream = 35

SM = weight of standardised milk

Fsm = fat content of the standardised milk = 3.0

Since the milk is separated into cream and standardised milk

SM + C = M

(1) or SM + C = 100

There are no fat losses; therefore the weight of fat in the original milk will be equal to the weight of fat in the standardised milk and cream.

(Weight of fat in a product is the weight of product × % fat/100)

Therefore (SM × Fsm)/100 + (C × Fc)/100 = (M × Fm)/100

or (3 × SM)/100 + (35 × C)/100 = (100 × 4.5)/100

(2) or 0.03SM + 0.35C = 4.5

Equations (1) and (2) give two equations with two unknowns, so they can be solved as follows:

(1) SM + C = 100

(3) or 0.03SM + 0.03C = 3

Subtracting (3) from (2)

0.32 C = 1.5

The weight of cream is thus 4.7 kg.

Therefore, the weight of standardised milk is 95.3 kg.

Answer check

The original milk contained 4.5 kg of fat.

The cream contains (4.7 × 35)/100 =1.645 kg of fat

Therefore 4.5 – 1.645 = 2.855 kg of fat in the standardised milk.

The fat percentage of the standardised milk is

(2.855 × 100)/95.3 = 3%

The calculation can also be made using Pearson's Square. This is essentially a reverse standardisation, i.e. "how much cream containing 35% fat and milk containing 3% fat should be mixed to get milk containing 4.5% fat?" is mathematically the same as "how much cream containing 35% fat must be removed from milk containing 4.5% fat to standardise the milk to 3% fat content?"

1. Place the fat content of whole milk in the centre.

2. Place the fat content of cream on the top left-hand corner.

3. Place the desired fat content of the standardised milk on the bottom left-hand corner.

4. For every 32 parts of whole milk, there are 1.5 parts of cream to be removed and 30.5 parts of standardised milk.

Therefore Wc = (1.5)/32 × 100 = 4.6875 = 4.7

Wsm = Wm – Wc = 95.3

The Wsm and fat to be removed can be calculated in a number of ways. Whatever method is used to calculate the amount of cream to be removed, it is then necessary to calculate the amount of milk to be separated to achieve the desired reduction in fat content.

Wm × Fm = Wc × Fc

Therefore Wm × 4.5 = 4.7 × 35

and Wm = (4.7 × 35)/4.5 = 36.5

Therefore, 36.5 kg of milk are separated and the skim milk is then combined with the remaining whole milk.

Standardisation such as this can be used to increase income from milk production as follows:

Assume liquid milk price of 70 cents/kg

Assume butter price of EB* 10/kg

Income from 100 kg of milk = EB 70

Income from 95.3 kg of milk = 66.71

Fat removed = Wc × Fc = 4.7 × 0.35 = 1.645

Expected butter yield = 1.9 kg

Income from butter = EB 19

Total income = EB 85.76

Margin = EB 15.76/ 100 kg of milk

*EB = Ethiopian birr (US$ 1 = EB 2.07)

Butter-making with fresh milk or cream

Butterfat can be recovered from milk and converted to a number of products, the most common of which is butter. Butter is an emulsion of water in oil and has the following approximate composition:

Fat                 80%
Moisture        16%
Salt                2%
Milk SNF        2%

In good butter the moisture is evenly dispersed throughout the butter in tiny droplets. In most dairying countries legislation defines the composition of butter; and butter makers conform to these standards insofar as is possible.

Butter can be made from either whole milk or cream. However, it is more efficient to make butter from cream than from whole milk.

Butter-making theory

To make butter, milk or cream is agitated vigorously at a temperature at which the milk fat is partly sold and partly liquid. Churning efficiency is measured in terms of the time required to produce butter granules and by the loss of fat in the buttermilk. Efficiency is influenced markedly by churning temperature and by the acidity of the milk or cream.

In churning, cream is agitated in a partly filled chamber. This incorporates a large amount of air into the cream as bubbles. The resultant whipped cream occupies a larger volume than the original cream. As agitation continues the whipped cream becomes coarser. Eventually the fat forms semi-solid butter granules, which rapidly increase in size and separate sharply from the liquid buttermilk. The remainder of the butter-making process consists of removing the buttermilk, kneading the butter granules into a coherent mass and adjusting the water and salt contents to the levels desired.

Theory of the mechanism of churning

In considering the mechanism of churning the following factors must be taken into account:

Air is thought to be necessary for the process, but some workers have demonstrated that milk or cream can be churned in the absence of air, although it takes longer.

About one half of the stabilising material is liberated into the buttermilk during churning. It is thought that during churning the fat globule membrane substance spreads out over the surface of the air bubbles, partly denuding the globules of their protective layer, and that a liquid portion of the fat exudes from the globule and partly or entirely covers the globule, rendering it hydrophobic. In this condition the globules tend to stick to the air bubbles. Free fat destabilises the foam, causing it to collapse. The partly destabilised globules clinging to the air bubbles thus collect in clusters cemented together by free fat. These clusters appear as butter grains.

Churning cream

Cream prepared by gravitational or mechanical separation can be used. Good butter can be made in any type of churn provided it is clean and in good repair.

Churn preparation

The churn is prepared by rinsing with cold water, scrubbing with salt and rinsing again with cold water. Alternatively, it can be scalded with water at 80°C. After the butter has been removed, the churn should be washed well with warm water, scalded with boiling water and left to air. When not in use wooden churns should be soaked occasionally with water. A new churn should first be washed with tepid water, scrubbed with salt and then washed with hot water until the water comes away clear. A hot solution of salt should then be allowed to stand in the churn for a short time. After rinsing again with hot water the churn should be left to air for at least one day before being used.

Churning temperature

The temperature of the cream during churning is of great importance. If too cool, butter formation is delayed and the grain is small and difficult to handle. If the temperature is too high, the yield of butter will be low, because a large proportion of the fat will remain in the buttermilk, and the butter will be spongy and of poor quality. Cream should be churned at 10 –12°C in the hot season and at 14 –17°C in the cold season. The temperature may be raised by standing the vessel containing the cream in hot water, or may be lowered by standing the vessel in cold spring water for a few hours before the cream is churned. The churning temperature may also be adjusted by the water used to dilute the cream. In the hot season, the coldest water available should be used, preferably water that has been stored in a refrigerator.

The amount of cream to be churned should not exceed one half the volumetric capacity of the churn. An airtight churn should be ventilated frequently during the first 10 minutes of churning to release gases driven out of solution by the agitation. If butter is slow in forming, adding a little water which is warmer than the churning temperature, but never over 25°C, usually causes it to form more quickly. When the butter appears like wet maize meal, water (1 litre per 4 litres of cream) at 2°C below the churning temperature should be added. It may be necessary to add water a second time to maintain butter grains of the required size. Churning should cease when the butter grains are as large as small wheat grains.

Washing the butter

When the desired grain size is obtained, the buttermilk is drained off and the butter washed several times in the churn. Each washing is done by adding only as much water as is needed to float the butter and then turning the churn a few times. The water is then drained off: As a general rule two washings will suffice but in very hot weather three may be necessary before the water comes away clear. In the hot season the coldest water available should be used for washing, and in the cold season water about 2 to 3°C colder than the churning temperature should be used.

Salting, working and packing the butter

Equipment for working may consist of a butter worker or a tub or keeler. Good-quality spatulas are important, and a sieve and scoop facilitate the removal of butter from the churn. This equipment must be clean (refer to method of cleansing and preparing a churn). The butter is spread on the worker, which has been soaked previously with water of the same temperature as the washing water. If salted butter is required, the butter should be salted before working at a rate of 16 g salt/kg or according to taste. The salt used should be dry and evenly ground and of the best quality available.

The butter is then either rolled out 8 to 10 times or ridged with the spatulas to remove excess moisture. If the butter is to be heavily salted, it must be worked more in proportion to the amount of salt used, as uneven distribution of the salt causes uneven colour. The butter should be worked until it seems dry and solid, but it must not be worked too much or it will become greasy and streaky.

The butter is then weighed and packed for storage. It should be packed in polythene-lined wooden or cardboard cartons and stored in a cool, dry place. The butter should be firm and of uniform colour.

Washing the churn and butter-making equipment after use

The churn and butter-making equipment should be washed as soon as possible, preferably while the wood is still damp.

Churn: Wash the inside of the churn thoroughly with hot water. Invert the churn with the lid on in order to clean the ventilator; this should be pressed a few times with the back of a scrubbing brush to allow water to pass through. (N.B. The ventilator should be dismantled occasionally for complete cleansing.)

Remove the rubber band from the lid and scrub the groove. Scald the inside of the churn with boiling water. This step is very important. Invert and leave to air. Dry the outside and treat steel parts with vaseline to prevent rusting. The rubber band should not be placed in boiling water; dipping in warm water is sufficient.

Butter worker/keeler: Place the sieve, scoop and spades on the butter worker or keeler. Pour hot water over all of them and scrub well to remove all traces of grease. Scald with boiling water and leave to air. Treat the steel part of the butter worker with vaseline to prevent rusting.

Storage of butter

Surplus good-quality butter can be stored, but should contain more salt than usual—at least 30 g/kg. Low moisture content is desirable. The butter must be packed in clean containers, such as seasoned boxes or glazed crocks, and stored in a cold room or in a cold, airy place. If a box is used, it should be lined with good-quality polythene. The container should be filled to capacity from one churning. The more firmly butter is packed, the better; it may be covered with a layer of salt, but this is not essential. The container should be securely covered with a lid or a sheet of strong paper.

Overrun and produce in butter-making

Overrun

An enterprise engaged in butter-making must be able to measure the efficiency of the process, i.e. by measuring the yield of butter from the butterfat purchased.

First, the theoretical yield of butter has to be estimated. Butter contains an average of 80% butterfat. Thus, for every 80 kg of butterfat purchased 100 kg of butter should be produced, or for every 100 kg of butterfat purchased 125 kg of butter should be produced.

The difference between the number of kilograms of butterfat churned and the number of kilograms of butter made is known as the overrun. This difference is due to the fact that butter contains non-fatty constituents such as moisture, salt, curd and small amounts of lactic acid and ash in addition to butterfat.

The overrun is financially important to the dairy industry and constitutes the margin between the purchase price of butterfat and the sale price of butter. The dairy unit depends largely on overrun to cover manufacturing costs and to defray expenses incurred in the purchase of milk.

As stated above, the maximum legitimate overrun is 25%. In commercial operation, however, it is not possible to establish the degree of accuracy that is assumed in the calculation of theoretical overrun, and the actual overrun shows the difference between the amount of butter churned out and the amount of butterfat bought.

Overrun is affected by:

Notes:

1 The need for care when sampling milk is referred to in the section dealing with butterfat testing. For example, if careless sampling and testing results in a reading of 3.6% butterfat against an actual content of 3.2% butterfat, what will be the effect on the overrun from 100 kg of milk?

Fat paid for = 100 × 0.036 = 3.6 kg of butterfat.

Maximum theoretical yield of butter = 3.6 × 1.25 kg = 4.5 kg

Fat received = 100 × 0.032 = 3.2 kg

Maximum theoretical yield = 3.2 × 1.25 = 4 kg

Our overrun therefore is

Butter made = 4 kg

Butterfat paid for = 3.6 kg

Overrun = 4/3.6 =1.11=11%.

Thus, carelessness at the testing stage can result in serious manufacturing losses. Losses at any stage in the process should be avoided. If overrun is low, each step of the process should be checked carefully in order to trace the loss.

A more comprehensive calculation of overrun is given in Appendix 1.

2 The non-fatty constituents of butter are moisture, salt and curd. In most of the principal butter-producing countries the percentage of moisture in butter is limited to 16%. Salt content varies largely according to market requirements and can be as high as 3%. Curd content is fairly uniform at 0.5–0.75%.

Any practice that increases the percentage of non-fatty constituents in butter automatically lowers the percentage of fat and increases the overrun. It is because of this that most countries legislated for a minimum of 80% butterfat in butter.

Butter composition also affects overrun. If the moisture content of butter is 14% instead of 16%, 2% more of the total weight must be provided by butterfat. This reduces the theoretical overrun from 25% to 21.95%.

Produce

Another method for estimating the efficiency of a process is to measure the number of litres of milk required per kilogram of butter produced.

For example, how many litres of milk containing 4% butterfat are required to make 1 kg of butter?

In 1 kg of butter there is 0.80 kg of butterfat.

In the milk we have 4 kg fat/ 100 kg or per 100 litres/1.032.

Therefore we have:

1 kg fat in 100/(1.032 × 4) = 24.22 litres

or 0.8 kg. fat in 19.38 litres

Therefore 19.38 litres of milk containing 4% fat will be required to make 1 kg of butter. Thus the efficiency of operation can also be checked by calculating output.

The fat content of the whole milk, skim milk and buttermilk should be checked daily. The moisture content of the butter should be checked for each batch. The accuracy of weighing scales and other measuring devices should be checked regularly.

Butter quality

Butter quality can be discussed under two main headings:

The compositional quality of butter can be further divided into two subsections:

Compositional quality

The chemical composition of butter is determined at the processing stage when the salt, moisture, curd and fat contents of the product are regulated. Once these parameters have been set there is little one can do to change them. The microbiological quality of butter is also determined during the production and processing stages.

Chemical composition affects butter yield, while butter of poor microbiological quality will deteriorate rapidly and become unacceptable to consumers. The butter may also contain pathogens. Cleanliness at all stages of production is, therefore, essential.

Organoleptic quality

The organoleptic quality of butter can be described as the customer's reaction to its colour, texture and flavour. It has been said that the consumer tastes with his or her eyes, and it is true that a person's initial impression of a food will often determine whether or not he or she will buy it. It is important, therefore, to produce butter that has an even colour, clean flavour and close texture. It is also important that it be free from defects such as loose moisture. It should be packed attractively, both to attract customer attention and to retain its quality.

Butter produced carelessly and without the use of preservatives has a very short shelf life. Preservation of butter quality can assist the smallholder in two ways:

The first step the producer can take to ensure a high-quality product is to make it in a clean, hygienic manner.

This results in fewer spoilage organisms being present in the butter. Another step is to take care in the handling and storage of the butter.

The use of permitted preservatives is by far the most effective means of maintaining butter quality when used in conjunction with the above precautions. Salt—sodium chloride—is an excellent preservative, and salting butter to 3% extends its storage life: salted butter can be stored for up to 4 months without significant deterioration. A salt concentration in excess of 3% gives little advantage and can adversely affect the flavour of the butter.

Aside from the influence of salt on the flavour and keeping quality of the butter, adding salt is of economic importance as it increases overrun.

Adding salt to butter disturbs the equilibrium of the emulsion (the butter). This, in turn, changes the character of the body and alters its colour. Unless the butter is subjected to sufficient working to regain the original equilibrium of the emulsion, it will tend to have a coarse, leaky body and uneven colour.

Salt is added to butter most commonly using the dry-salting method, in which dry salt is sprinkled evenly over the butter and worked in.

Butter must be adequately worked if it is to be stored for a long time. First, working distributes the salt uniformly in the moisture and this helps inhibit microbial growth. Secondly, it distributes the salt solution into many tiny droplets rather than fewer large ones. For a given level of microbial contamination, the microbes will be more isolated in small droplets and will have less of the butter's nutrients available to them for growth.

After salting, the butter should be stored in a clean container, and the container sealed. It should then be stored in a cool, dark place.

Ghee, butter oil and dry butterfat

These products are almost entirely butterfat and contain practically no water or milk SNF. Ghee is made in eastern tropical countries, usually from buffalo milk. An identical product called samn is made in Sudan. Much of the typical flavour comes from the burned milk SNF remaining in the product. Butter oil or anhydrous milk fat is a refined product made by centrifuging melted butter or by separating milk fat from high-fat cream.

Ghee is a more convenient product than butter in the tropics because it keeps better under warm conditions. It has low moisture and milk SNF contents, which inhibits bacterial growth.

Milk or cream is churned as described in the sections dealing with churning of whole milk or cream. When enough butter has been accumulated it is placed in an iron pan and the water evaporated at a constant rate of boiling. Overheating must be avoided as it burns the curd and impairs the flavour. Eventually a scum forms on the surface: this can be removed using a perforated ladle. When all the moisture has evaporated the casein begins to char, indicating that the process is complete. The ghee can then be poured into an earthenware jar for storage.

A considerable amount of moisture and milk SNF can be removed prior to boiling by melting the butter in hot water (80°C) and separating the fat layer. The fat can be separated either by gravity or using a hand separator. The fat phase yields a product containing 1.5% moisture and little fat is lost in the aqueous phase.

Alternatively, the mixture can be allowed to settle in a vessel similar to that used in the deep-setting method for separating whole milk. Once the fat has solidified the aqueous phase is drained. The fat is then removed and heated to evaporate residual moisture. Products made using these methods exhibited excellent keeping qualities over a 5- month test period.

Cheese-making

Cheese is a concentrate of the milk constituents, mainly fat, casein and insoluble salts, together with water in which small amounts of soluble salts, lactose and albumin are found. To retain these constituents in concentrated form, milk is coagulated by direct acidification, by lactic acid produced by bacteria, by adding rennet, or a combination of acidification and addition of rennet.

Rennet coagulation theory

Rennet, a proteolytic enzyme extracted from the abomasum of suckling calves, was traditionally used for coagulating milk. Originally, the abomasum was itself immersed in milk. The extraction of rennet that could be stored as a liquid was the first step towards refining this procedure.

This was followed by purification and concentration of the enzyme. The purified enzyme was originally called rennin, and is now called chymosin.

On weaning, the chymosin of the suckling calf is replaced by bovine pepsin. With the decrease in the practice of slaughtering calves, chymosin became scarce, resulting in a search for chymosin substitutes. Rennet is a general term currently used to describe a variety of enzymes of animal, plant or microbial origin used to coagulate milk in cheese-making.

Rennet transforms liquid milk into a gel. While the process is not fully understood, rennet coagulation is thought to take place in two distinct phases, the first of which is regarded as being enzymatic, the second non-enzymatic. The first, or primary phase, can be illustrated as:

                   water

Casein     ————>    para casein + glycomacropeptide

Since k-casein stabilises the other caseins and its hydrolysis leads to the coagulation of the casein fraction, the primary phase can also be expressed as:

                      water

ĸ-casein     ————>  para ĸ-casein + glycomacropeptide

The effect of milk coagulants on the other caseins is thought to be negligible at this stage.

The second, or secondary, phase is the non-enzymatic precipitation of para casein by calcium ions. Para casein, in association with the calcium ions, is thought to produce a lattice structure throughout the milk. This traps the fat and whey is gradually exuded. The coagulum then contracts, a process known as syneresis. This is accelerated by increasing the temperature and reducing pH to as low as pH 4.6.

Rennet also has a tertiary action on milk proteins. This occurs during cheese ripening, during which rennet hydrolyses milk proteins. If the desired hydrolysis is not obtained, the cheese becomes bitter. While a wide variety of proteolytic enzymes coagulate milk, the tertiary action of many of these on milk proteins causes undesirable flavours in cheese, which limits the range of coagulants that can be used.

Cheese varieties

Many cheese varieties are manufactured around the world but they are all broadly classified by hardness (i.e. very hard, hard, semi-soft and soft) according to their moisture content.

Cheese is usually made from cows milk, although several varieties are made from the milk of goats, sheep or horses. Flow diagrams for the manufacture of the varieties discussed are shown in Figures 14 to 17.

Queso blanco (White cheese)

Queso blanco is a Latin-American fresh, white cheese. It is usually made from milk containing 3% fat, using an organic acid, without starter or rennet.

Procedure

  1. Take fresh whole milk and determine its fat content. If the fat content is higher than 3%, standardise using skim milk.
  2. Transfer the standardised milk to a cheese vat, preferably a double-jacketed standard cheese vat, and heat to 82°C.
  3. While the milk is being heated measure out lemon juice of pH about 2.5 in a measuring jar. About 3 ml of lemon juice should be added per 100 ml of milk.
  4. Dilute the lemon juice with an equal amount of clean, fresh water.
  5. When the milk temperature reaches 82°C, add the diluted lemon juice carefully and uniformly while stirring. For even distribution of the juice, add in three separate amounts.
  6. The curd precipitates almost immediately. Continue to stir for 3 minutes after adding the juice, then allow the curd to settle for 15 minutes
  7. Drain the whey through a metal sieve or cheese cloth.
  8. While draining the whey, stir the curd to prevent excess matting.
  9. Distribute a total of about 3.5 to 5 kg of salt to 100 kg curd, in three applications.
  10. Prepare a cylindrical or square hoop by lining with cheese cloth and scoop the salted curds into it.
  11. Press the curd overnight at room temperature.
  12. Remove the pressed cheese and cut into blocks of 0.5 or 1 kg.

Queso blanco is made without starter or rennet. A variety of acidulants can be used for its manufacture. Heating the milk to 82°C pasteurises the milk and denatures the whey proteins, so that they are recovered with the curd.

This increases cheese yield. The cheese has good keeping quality and is thus suitable for manufacture in rural areas.

Expected yield: 1 kg of cheese from 8 kg of milk (12.5%).

Figure 14. Manufacturing steps for Queso blanco cheese.

Halloumi

Halloumi is the curd, formed by coagulating whole milk using rennet or similar enzymes, from which part of the moisture (whey) has been removed by cutting (bleeding), warming and pressing.

Procedure

  1. Heat the milk to 32–35°C.
  2. Add rennet or a similar enzyme according to the manufacturer's directions, while stirring the milk.
  3. Hold the milk at 32–35°C until the curd sets.
  4. Check for setting of the curd by applying pressure to the edge of the milk where it comes in contact with the vat, using a spatula or a knife with a round tip. If the curd is set it comes away clean from the wall of the vat.
  5. After coagulation, the curd is cut into 3–5 mm cubes using vertical and horizontal knives.
  6. Hold the curd in whey for about 20 minutes, stirring gently and continuously, and then allow it to settle.
  7. Drain the whey and scoop out the curd into a hoop lined with cheese cloth. Press the curd.
  8. While the curd is in the press, heat the whey to about 80–90°C. This precipitates the whey proteins, which can then be removed and pressed to make a whey cheese (anari).
  9. Take out the pressed curd, cut it into pieces of 10 × 10 × 3 cm and heat at about 80°C in hot whey. Continue heating until the pieces of curd float on the surface of the whey and become soft and elastic.
  10. Remove the pieces of curd when still warm and either press in the hands, folded or unfolded, and rub in a little dry salt mixed with dried leaves of Mentha viridis (spearmint).
  11. When the pieces are cold, put them in containers filled with cool, boiled whey brine and store in a cool place to ripen for about 30 days.
  12. After ripening put in an airtight container and store in a refrigerator at less than 12°C. The cheese will keep for several months under these conditions. Halloumi cheese is best after 40 days but can also be consumed just after manufacture.

Note: 15% salt concentration in whey brine is normally used.

Expected yield: 1 kg of cheese from 9 kg of milk (11 %).

    Figure 15. Manufacturing steps for Halloumi cheese.

    Domiati–Gybna beyda

    Known as Domiati in Egypt and Gybna beyda in Sudan, this is a hard, white cheese.

    Procedure

    1. Heat fresh milk to 35°C and add enough salt to give 7 to 10% salt solution in the milk.
    2. Add enough rennet to coagulate the milk in 4 to 6 hours.
    3. Once set, transfer the coagulum to wooden moulds lined with muslin.
    4. Allow the whey to drain overnight.
    5. On the following day, pack the cheese in tins and fill the interspaces with whey.
    6. Seal the tins by soldering.

    Notes:

    1 and 2. In some areas rennet is added before salting. In this procedure, salt is not added until a coagulum has formed. If salt is added before rennet it is not advisable to add more rennet to shorten the coagulation time, as this reduces the quality of the cheese.

    6. Whey expulsion continues during storage and the cheese hardens.

    Expected yield: 1 kg of cheese from 7 kg of milk (15%).

    Figure 16. Manufacturing steps for Domiati/Gybna beyda cheese.

    Feta

    This is a brine-pickled cheese. It can be made from milk of cows, sheep or goats. Feta can be made without starter and can also be made from standardised milk. The procedure described here is for the manufacture of a feta-type cheese without starter or additives.

    Procedure

  1. Standardise the milk to 3% fat, heat to about 32°C and allow to ripen for one hour before adding rennet.
  2. Add commercial rennet at the rate of 25 ml/ 100 litres of milk. Leave the milk until a firm clot has formed—this usually takes 40 to 50 minutes.
  3. Cut the curd into 2- to 3-cm cubes to facilitate whey drainage. Allow 15 minutes for the whey to separate. Stir intermittently during this time.
  4. Allow the curds to settle and decant the supernatant whey.
  5. Transfer the curds and some whey to cheese moulds lined with muslin. Place the lid on the mould and invert at half-hourly intervals in the first few hours to facilitate whey drainage.
  6. Allow the curd to settle overnight.
  7. On the following day, cut the curd mass into blocks of suitable size and sprinkle them with salt.
  8. Place the salted blocks in a 15% brine solution to give 6–8% salt in the cheese at equilibrium.

The high salt concentration retards bacterial activity. However, air should be excluded from the brining container to prevent the growth of moulds.

Feta cheese can be eaten after a few days or can be stored for long periods in the brine, provided that air is excluded. The cheese develops a soft, crumbly texture during ripening.

Expected yield: 1 kg of cheese from 9 kg of milk (11 %) .

Figure 17. Manufacturing steps for Feta cheese.

Cheese yield

In cheese-making, the milk fat and casein are recovered with some moisture. The yield of cheese can be expressed in kilograms of cheese obtained per 100 kilograms of milk processed. Cheese yield is influenced by milk composition, the moisture content of the final cheese and the degree of recovery of fat and protein in the curd during cheese-making.

Milk low in total solids will give a low cheese yield, while milk high in total solids will give a high cheese yield. In order to predict the theoretical yield of cheese, the fat and casein content of the milk must be known. Because of difficulties encountered in estimating casein content, the following formula is often used to estimate cheese yield:

(2.3 × fat %) + 1.4 = cheese yield (kg/ 100 kg milk)

Therefore, with milk containing 4% fat the expected yield would be:

(2.3 × 4) + 1.4 = 10.6 kg/ 100 kg milk

This formula gives an estimate of cheese yield and is applied most often to Cheddar cheese. It is useful as an immediate check on efficiency, but a universal yield factor for cheese varieties is unrealistic.

If the yield of cheese is less than expected, the following checks should be made:

The fat content of whey should be analysed for each batch of cheese made.

In estimating the profitability of cheese-making enterprises, an average annual yield of 9.5%, i.e. 9.5 kg of cheese per 100 kg of milk, is used.

Milk standardisation may be used to increase cheese yield, particularly with high-fat milk. Standardisation also gives a good return for skim milk. However, over-standardising results in coarse-textured cheese with poor flavour.

High moisture content increases cheese yield, but reduces keeping quality. Cheese loses moisture during storage if it is not properly wrapped, thus reducing cheese yield. Waxing reduces moisture loss, as does storing the cheese in brine.

Milk fermentations

Raw milk produced under normal conditions develops acidity. It has long been recognised that highly acid milk does not putrefy. Therefore, allowing milk to develop acidity naturally preserves the other milk constituents.

Bacteria in milk are responsible for acid development. They produce acid by the anaerobic breakdown of milk carbohydrate—lactose—to lactic acid and other organic acids. Anaerobic breakdown of carbohydrate to organic acids or alcohols is called fermentation.

Pyruvic acid formation is an intermediate step common to most carbohydrate fermentations:

C6H120 ———> 2 CH3.CO.COOH

However, fermentations are usually described by an identifiable end product such as lactic acid or ethyl alcohol and carbon dioxide.

A number of sugar fermentations are recognised in milk. They can be either homofermentative, with one end product, or heterofermentative, with more than one end product. The fermentations discussed are outlined in Figure 18.

Figure 18. Outline of four important lactose fermentations.

Organisms responsible:

1. Streptococci and Lactobacilli.

2. Propionibacteria.

3. Yeasts – Candida and Torula.

4. Coliform bacteria.

The factors that affect microbial growth also affect milk fermentation. Fermentation rates will generally parallel the microbial growth curve up to the stationary phase. The type of fermentation obtained will depend on the numbers and types of bacteria in the milk, storage temperature and the presence or absence of inhibitory substances.

The desired fermentations can be obtained by temperature manipulation or by adding a selected culture of micro-organisms—starter—to pasteurised or sterilised milk. In smallholder milk processing, traces of milk from previous batches are often used to provide `starter' for subsequent batches. Other sources include the container and additives such as cereal grains.

The fermentation will be established once the organisms dominate the medium and will continue until either the substrate is depleted or the end product accumulates. In milk, accumulation of end product usually arrests the fermentation. For example, accumulation of lactic acid reduces milk pH to below 4.5, which inhibits the growth of most micro-organisms, including lactic-acid producers. The fermentation then slows and finally stops.

Fermented milks are wholesome foods and many have medicinal properties attributed to them.

Fermented milks

The types of fermented milk discussed here are those made by controlled fermentation. This is achieved by establishing the desired micro-organisms in the milk and by maintaining the milk at a temperature favourable to the fermentative organism.

A variety of fermented milks are made, each dithering markedly from the other. However, a number of steps are common to each manufacturing process, and these are outlined in Figure 19.

Figure 19. Flow diagram of fermented milk manufacture.

Standardisation

Occasionally some fat is removed or milk SNF added. In some instances, the removal of moisture during heating increases the proportion of solids in the final product.

Heating

Milk is heated to kill pathogens and spoilage organisms and to provide a cleaner medium in which the desired micro-organisms can be established. Heating also removes air from the milk, resulting in a more favourable environment for the fermentative organisms, and denatures the whey proteins, which increases the viscosity of the product.

After heating, the milk must be cooled before it is inoculated with starter, otherwise the starter organisms will also be killed.

Inoculation with starter

Starter is the term used to describe the microbial culture that is used to produce the desired fermentation and to flavour the product. When preparing the starter, care must be taken to avoid contamination with other micro-organisms. Companies that supply starter cultures detail the precautions necessary. Care should also be taken to avoid contamination when inoculating the milk with starter.

Incubation

After inoculation the milk is incubated at the optimum temperature for the growth of the starter organism. Incubation is continued until the fermentation is complete, at which time the product is cooled. Additives may be added at this stage and the product packed. The manufacturing procedures for a number of fermented milks are given in Table 5.

Table 5. Manufacturing procedures for yoghurt, acidophilus milk and kefir.

Product

Milk

Standardise

Sterilise

Starter

Incubation
temperature

Incubation
Time

Yoghurt

Cows

Optional

95°C
30 min.

S. thermophilus
L. bulgarian

37°C

4–6 hours

Acidophilus milk

Cows

Optional

120°C
20 min

L. acidophilus

38°C

1824 hours

Kefir

Cows
Ewes
Goats
Mares

85°C
30 min

Kefir grains*

22°C

12 hours

* Kefir grains are irregular granules in which bacteria and yeast grow. When they are introduced into the milk, the micro-organisms on the granules bring about the fermentation.

Preparation of the fermentation vessel

The fermentation vessel is first washed to remove visible dirt. It is then dried and smoked by putting burning embers of Olea africana, wattle or acacia into the vessel and closing the lid. The vessel is then shaken vigorously and the lid opened to release the smoke. This procedure is repeated until the inside of the vessel is hot. Smoking flavours the product and is also thought to control the fermentation by retarding bacterial growth. While it is known that smoke contains compounds that retard bacterial growth, the precise effects of smoking on fermentation have not been investigated.

Once smoking is complete the vessel may be cleaned with a cloth to remove charcoal particles. However, in some areas the charcoal particles are retained to add colour to the product.

Milk treatment

In some processes the milk is boiled prior to fermentation. It is then allowed to cool and the surface cream removed. In other processes the milk is not given any prefermentation treatment.

Fermentation

The milk is placed in the smoked vessel and allowed to ferment slowly in a cool place at a temperature of about 16–18°C. The fermentation is almost complete after 2 days, but may be continued for a further 2 days, by which time the flavour is fully developed. The milk must ferment at low temperature, otherwise fermentation is too vigorous, with much wheying off and gas production.

The product has a storage stability of 15 to 20 days.

Concentrated fermented milks

Concentrated fermented milks are prepared by removing whey from fermented milk and adding fresh milk to the residual milk constituents. The fermentation vessel is prepared as for fermented milk. The milk is allowed to ferment in a cool place for up to 7 days, during which milk may be added daily. After 7 days a coagulum has formed and the clear whey is removed. Fresh milk is then added and, following further fermentation, whey is again removed. In this way the casein and fat are gradually concentrated in a product of extended keeping quality. The actual degree of concentration depends on the amount of whey removed and of fresh milk added.

Milk

Fat
Casein  
} Coagulum ———> Stored    
Whey proteins———>Whey———> Fed to calves

Sour-milk technology

Smallholder milk processing is based on sour milk. This is due to a number of reasons, including high ambient temperatures, small daily quantities of milk, consumer preference and increased keeping quality of sour milk.

Products made from sour milk include fermented milks, concentrated fermented milks, butter, ghee, cottage cheese and whey. Other products made are cheese and products made by mixing fermented milk with boiled cereals.

The equipment required for processing sour milk is simple and is all available locally. Milk vessels can be made from clay, gourds and wood, and can be woven from fibre, such as the gorfu container used by the Borana pastoralists in Ethiopia.

Making butter and cottage cheese from sour whole milk

The products and byproducts of butter-making from sour whole milk are shown in Figure 20.

Figure 20. Products and byproducts of butter-making from sour whole milk.

Butter-making from sour whole milk

This is a very important process in many parts of Africa. Smallholders produce 1 to 4 litres of milk per day for processing. Under normal storage conditions the milk becomes sour in 4 to 5 hours. The souring of milk has a number of advantages. It retards the growth of undesirable microorganisms, such as pathogens and putrefactive bacteria, and makes the milk easier to churn.

Milk for churning is accumulated over several days by adding fresh milk to the milk already accumulated. The churn holds about 20 litres and the amount of milk churned ranges from 4 to 10 litres. The milk is normally accumulated over 2 or more days. Butter is made by agitating the milk until butter grains form. The churn is then rotated slowly until the fat coalesces into a continuous mass. The butter thus formed is taken from the churn and kneaded in cold water.

The milk is usually agitated by placing the churn on a mat on the floor and rolling it to and from. It can also be agitated by shaking the churn on the lap or hung from a tripod.

A number of factors influence churning time and recovery of butterfat as butter:

Effect of acidity: Fresh milk is difficult to churn: churning time is long and recovery of butterfat is poor. Milk containing at least 0.6% lactic acid is easier to churn. Acidity higher than 0.6% does not significantly influence churning time or fat recovery.

Effect of temperature*: Sour milk is normally churned at between 15 and 26°C, depending on environmental temperature. At low temperatures churning time is long; butter-grain formation can take 5 hours or longer. As churning temperature increases churning time decreases. This becomes marked at temperatures above 20°C, but as little as 60% of the butterfat may be recovered as butter at 26°C. Control of temperature is therefore critical.

*It is difficult to isolate the effects of temperature and acidity on churning efficiency because while the milk is ripening it is also cooling and the fat is crystallising. Direct acidification of fresh milk increases butter yield, but allowing milk to develop acidity during a ripening period of 2 to 3 days allows considerable fat crystallisation.

Degree of agitation: Increasing agitation reduces churning time. Fitting an agitator to a traditional churn reduces churning time and increases butter yield. The percentage of fat recovered as butter is increased, with as little as 0.2% fat remaining in the buttermilk. However, the process is very temperature-dependent and churning at temperatures above 20°C results in short churning times with poor recovery of fat. The optimum churning temperature is between 17 and 19°C.

Extent of filling the churn: Churns should be filled to between a third and half their volumetric capacity. Filling to more than half the volumetric capacity increases churning time considerably but does not reduce fat recovery.

Thus, when churning whole milk, the following conditions should be adhered to:

Once the fat has been recovered, the soured skim milk contains casein, whey proteins, milk salts, lactic acid, lactose, the unrecovered fat and some fat-globule-membrane constituents.

Defatted milk is suitable, and is often used, for direct consumption. It is also used to inoculate fresh milk to encourage acid development.

Cottage cheese

The casein and some of the unrecovered fat in skim milk can be heat-precipitated as cottage cheese, known in Ethiopia as ayib.

The defatted milk is heated to about 50°C until a distinct curd mass forms. It is then allowed to cool gradually and the curd is ladled out. Alternatively, the curd can be recovered by filtering the cooled mixture through a muslin cloth. This facilitates more complete recovery of the curd and also allows more effective moisture removal. Temperature can be varied between 40 and 70°C without markedly affecting product composition and yield. Heat treatments between 70 and 90°C do not appear to affect yield but give the product a cooked flavour.

The whey contains about 0.75% protein, indicating near-complete recovery of casein. Whey can be consumed by humans or fed to animals.

The cottage cheese comprises 79.5% water, 14.7% protein, 1.8% fat, 0.9% ash and 3.1 % soluble milk constituents. It has a short shelf-life because of its high moisture content. Shelf-life can be increased by adding salt or by reducing the moisture content of the cheese. Storing the product in an air-tight container also extends storage life.

Equipment: Skim milk can be heated in any suitably sized vessel that is able to withstand heat. Heating can be direct or indirect. A ladle or muslin cloth can be used for product recovery.

Expected yield: The yield depends on milk composition and on the moisture content of the product, but should be at least 1 kg of cottage cheese from 8 litres of milk (12.5%).

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