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Part 3


A village milk processing unit usually involves a group of milk producers living within a given area near the unit. It is therefore reasonable that the majority of the producers will deliver their own milk in the morning direct to the processing unit. The milk from the evening milking is retained by the family for home consumption.

For producers who live further away or small farmers who do not wish to make the trip with a small quantity of milk, collection can be arranged within a radius of no more than 10 kilometres.

A milk collector on a bicycle picks up the milk at collection points, which are simple shelters within which the milk can be kept in the shade.


At the milk collection point, the milk collector tests milk density with the lactodensitometer, tests milk acidity, and measures milk volume using a measuring can. The milk is then filtered and poured into the milk can.

The entire milk collection round should take no longer than two hours.

The milk collector has a little notebook in which he enters the volume and density of the milk delivered by the producer. Payments for the milk are made twice a month.

The person responsible for milk collection may either be paid by the village cheese unit or by the producers themselves. For purposes of simplification, the second choice is preferable.


Reception of the milk delivered by the farmers themselves or by the collector should take place in the very early morning.

The producers' milk is weighed, density checked, the milk filtered, and then poured into milk pails.

Milk density is measured as shown in the diagram below:

Interpretation of readings

For cow's milk an approximate estimation of density can be read as follows:

Lactodensitometer readingResult
1.028 to 1.033Normal milk
less than 1.028Diluted milk
1.033 to 1.037    Skimmed milk


When milk from all the producers has been poured into the milk cans, a sample is taken from each can and mixed together to obtain an average sample. This sample is tested for milk acidity and fat content.

1. Milk Acidity

Put 10 ml of the milk sample into a glass.

Add 3 to 4 drops of phenol-phthalein.

With the dropper, add the NAOH solution drop-by-drop into the glass until a stable pink colour results.

Read on the graduated column the number of ml used. This gives the acidity of the milk in Dornic degrees.


• 1°D is 1 mg of lactic acid in 10 ml of milk or 0.1 g/l

2. Fat Content

  1. Put 10 ml of sulphuric acid in the butyrometer.

  2. Add 11 ml of milk from the average sample.

  3. Add 1 ml of amyl alcohol.

  4. Shake the butyrometer to dissolve the milk elements.

  5. Put the butyrometers in the centrifuge. Centrifuging should continue five minutes.

  6. Next plunge the butyrometers vertically, cork down, into a water bath, temperature 65°–70° C, and leave them for five minutes.

  7. The butyrometer, cork down, should be perfectly vertical when taking the reading at eye level. Read the graduation mark at the base of the meniscus, i.e. at the base of the curved upper surface of the fat column. In the example that follows, the degree reading is 3.6. The milk fat content then is 3.6 percent, or 36 g of fat per litre of milk.

The different stages of this test are illustrated on the following page.

The laboratory will then enter the results in the milk analysis notebook.


DateQuantity of milk received (litres)Milk acidityAverage fat content %Observation
12 February38522°D3.6-
13 February40521°D3.7-
14 February39521°D3.6-
15 February37222°D3.8-
16 February38720°D3.7-
17 February38421°D3.9-

We know therefore that on 17 February, for example, the dairy received 384 litres of milk with a butterfat content of 3.9 percent. In making Edam cheese, for example, milk with a fat content of 2.6 percent is required. By standardization, the fat content can be reduced from 3.9 percent to 2.6 percent. In order to do this, the amount should be calculated of the milk required to be skimmed and remixed with the milk received to bring the butterfat content to the desired amount.

3. Calculation for milk standardization: Pearson square method

The desired percentage of butterfat is written in the centre of the square and, in the two left-hand corners, the percentages of butterfat in the available ingredients.

0 percent is the fat content of the skimmed milk.
3.9 percent is the fat content of the milk received. Substraction along the diagonals will give two values representing, respectively, the amounts of the ingredients to be used.

Thus by blending 2.6 litres of milk with a 3.9 percent fat content and 1.3 litres of skimmed milk with zero fat content, 3.9 litres of milk with a 2.6 percent fat content is obtained.

Assuming reception of 384 litres of milk on 17 February, with an average fat content of 3.9 percent, it would be necessary to skim:

A hand-operated separator with an hourly capacity of 60–200 litres may be used for milk standardization.

Approximately 100 litres of milk (98 litres, to be exact) are skimmed to provide 16 litres of cream, as shown in the above illustration.


Heat-treatment of milk is a most important factor in the quality of the finished product.

After standardization, milk must be heat-treated. This means bringing it to a minimum temperature of 63°C for 30 minutes.

The various equipment suggested for heat-treatment can bring the temperature up to 63°C from between 40 to 60 minutes.

The diagram shows the heat-treatment profile:

The milk must be constantly stirred to maintain a homogeneous temperature throughout the heating and heat-treatment stages. The milk stirrer and the thermometer are important items of equipment.


When the milk has been kept at a temperature of 63°C for 30 minutes, it is then cooled down to bring it to a suitable temperature for cheese-making (approximately 35°C).

The milk is cooled either by immersing the milk cans in a tank with cold running water, or by running cold water through the double sides of the cheese vat.

During the cooling, as during the heating period, the milk must be stirred constantly.

Bearing in mind heat losses, cooling water should be shut off before the desired milk temperature is reached. For instance, if the milk temperature for cheese-making is 32°C, cooling water must be shut off when the milk temperature has reached 35°C.


1. Preparation of starter cultures

One person only must be responsible for the preparation of starter cultures.

The simple method for preparing starter cultures is illustrated.

The preparation of the starter cultures requires suitable strains of bacteria and a milk of good bacteriological quality.

The starter culture used should be a commercial lyophilized one. These strains keep relatively well and it is, therefore, advisable to maintain a three-months' supply.

Following the heat-treatment of milk for cheese-making, a certain amount of milk for the preparation of the starter culture is retained in the milk can or in the jacketed vat in order to maintain the heat-treatment of 63°C for an additional 15–30 minutes. The milk is then poured into one-litre bottles and into a 5-litre container and covered with a clean cloth.

a) Imported lyophilized starter culture

The contents of the vial are poured into a bottle of milk and shaken well to mix the powder and milk together.

b) Mother culture

In order to avoid contamination by air, both the bottle containing the mother culture and the 5-litre container of starter culture are placed in a small wooden cupboard.

c) Starter culture for cheese-making

The mother culture to be used for cheese-making (mesophilic culture) is placed in the culture cupboard.

The incubation temperature should be 20 – 22°C for 15–16 hours.

The acidity of the mother culture will then be 80 to 90°D. Use the mother culture to inoculate, at 2 percent, a second mother culture and 5 litres of milk for cheese-making.

The preparation of starter cultures for cheese-making is shown below.


If the starter cultures are prepared with great care, one strain of commercial culture may be used for one to two months by means of successive subculturings.

d) Yoghurt-making cultures

Starter cultures for yoghurt-making are thermophilic bacteria. They must be cultured therefore at a temperature of 40 – 45°C for three to four hours.

After heat treatment of the milk to which the yoghurt bacteria are to be added, the milk is cooled to 45°C and then poured into 1 litre bottles. These are then set into a water tank at 45°C to stabilize the bottled milk temperature.

The lyophilized commercial strain for yoghurt-making is then put into the bottles.

The bottles of milk are left in the water bath at 45°C for three to four hours.

The preparation of cultures for yoghurt-making is shown below.

2. Preparation of rennet

Rennet, like starter cultures, may be imported. However, unlike commercial starter strains, rennet can be made locally. Since laboratory equipment is needed for making rennet, it should therefore be undertaken with the advice of a laboratory in the capital city - that for example of a university.

a) Obtaining abomasa

Abomasa should be obtained from preferably unweaned calves.

Assuming that the annual amount of calf rennet to be obtained is some 120 litres of liquid rennet at a strength of 1/10 000 and a yield of approximately 2 abosama for 1 litre of rennet, 240 abosama will be required per annum.

b) Preparation of abosama

The abosama should be washed and the fat and veins removed.

The abosama are then inflated with air to avoid the two sides touching. They are then ball-shaped, the neck and the base being tied with string.

The inflated abosama are hung in a dry, well-ventilated area. Drying should be complete after approximately one month of storage. At this stage, the flattened abosama can be kept in a dry place for a long period without signs of deterioration (about one year).

c) Soaking

When required for use, the abosamum should be sliced into thin strips, 5 mm wide.

In an-easy-to-clean basin of plastic or stainless steel, soak the strips of abosamum in a 10 percent sodium chloride (salt) and 1 percent sodium benzoate solution.

To produce a three months' batch of 30 litres of rennet, 60 abosama should be soaked in 40 litres of this brine solution.

The pH of the solution is adjusted to 4.3 with benzoic acid, after soaking for 24 hours at a temperature of from 20 to 25°C. Pour the liquid off into a container. In the same soaking basin and keeping the same abosama strips, the brine solution may be renewed four to five times before the properties of the abosama strips are completely exhausted.

Each extract obtained in this fashion is treated as described above and the first and most concentrated extract is used to standardize the strength of the rennet.

d) Treatment of the liquid

To eliminate the mucilage in suspension in the extract, the solution is reacidified with hydrochloric acid to a pH of 4.8, and allowed to settle for two hours.

The pH of the extract is raised with disodium phosphate to pH 5.5 – 5.6.

Vigorous stirring must accompany these two actions. The liquid is then filtered over Watmann paper. Filtration may take a long time and probably will require distribution of the extract over several filters.

The crude rennet extract obtained is generally a golden-yellow colour.

e) Determination of strength

Definition: Rennet strength is the number of volumes of coagulated milk clotted by one volume of rennet in 40 minutes at 35°C.

If “v” equals one volume of rennet, and “V” one volume of milk and measuring the clotting time in seconds, the calculation is:

In practice, liquid rennet strength should be 1/10 000 (1 litre of rennet clots 10 000 litres of milk at 35°C in 40 minutes).


Put 500 ml of fresh milk in an Erlenmeyer flask and plunge it in a water bath at 35°C.

Remove 1 ml of the rennet to be standardized and dilute it in 10 ml of water.

When the milk in the Erlenmeyer flask reaches a constant temperature of 35°C, pour 10 ml of diluted rennet, stirring constantly, and start the timer. Keeping the Erlenmeyer flask in the water bath, slant it while rotating it gently so that a film of milk is formed on the sides of the flask. When the liquid begins to floculate, stop the timer.

Rennet strength:

If flocculation time is 60 seconds, for example, rennet strength will be:

S = 20 000

f) Standardization

The strength of the four or five different batches, obtained by successive extractions of the rennet from the abosama is determined, thus enabling the rennet to be readjusted to a strength of 1/10 000.

For example, if 30 litres at a strength of 1/20 000 have been obtained, and the extract from the four preceding rennet mixtures gave a solution at a strength of 1/5 000, the volume of rennet at strength 1/5 000 required to prepare standard rennet at a strength of 1/10 000 is determined in the following way:

30 litres × 20 000 units + “Y” litres × 5 000 units = (30 + Y) litres × 10 000 units

600 000 units + 5 000Y = 300 000 + 10 000 Y
 Y = 60 litres.

After mixing the two extracts, 90 litres of rennet will be obtained at a strength of 1/10 000.

g) Maintenance-storage

Rennet must be stored in opaque glass bottles or in dark (blue or black-tinted) plastic containers and placed in cold storage at a temperature of 5 to 7°C. Under these storage conditions, the rennet will remain active for three months.

h) Renneting the milk

It is recommended that coagulation tests be made first on small volumes of the cheese-milk. This is to redefine the amounts of rennet to use in order to obtain the same coagulation time as obtained previously with powdered rennet of 1/100 000 strength.

Rennet preparation is illustrated on the following page.

Preparation of rennet

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