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Dairy Accounting

Milk Analysis

Sampling
Estimation of milk pH by indicator
Electrometric measurement of pH
Determination of milk acidity
Alcohol test
Clot-on-boiling test
Butterfat determination
Determination of milk specific gravity
Determination of total solids (TS) and solids-not-fat (SNF) in milk
Determination of moisture content of butter

To facilitate the recording and accounting of milk supplies each supplier should be given a code number. This code should have two elements:

  1. A code for the producer and
  2. A code for the dairy cooperative or peasants' association of which the producer is a member; a register of producers and their corresponding codes should be kept at the dairy centre.

The code is recorded in the milk record book, with the weight of milk received, and also on the sample bottle. The supplier should be given a copy of this record each week. He/she should be informed each day of the quality of the milk delivered.

Once the products to be made from the milk have been decided and the prices of the products determined, milk price can be calculated as follows, assuming that butter and cottage cheese are the chosen products*:

  1. Calculate the value of 1 kg of butterfat from the known price of butter, e.g. EB 10.

    2. Butter comprises 80% butterfat. Other constituents are regarded as having no commercial value. Therefore, the price of 1 kg of butterfat

    = 10 × 100/80 = EB 12.5

    * The method given here is a "rule-of-thumb" approach — for further details, see Appendix 1.

  2. Calculate the value of 1 litre of skim milk.

    3. Cottage cheese made from fermented skim milk has a value of EB 1.50/kg. Since there is about an 8-fold concentration of casein in the manufacture of cottage cheese from skim milk, assume an average yield of 1 kg of cottage cheese from 8 litres of skim milk. Therefore, each litre of skim milk has a value of 18 cents.

  3. Calculation of the value of milk received: Assume the producer delivers 100 litres of milk containing 4% butterfat.
    1. Calculate the weight and value of butterfat received:

      2. The specific gravity of milk is 1.032 kg/litre. Therefore, the weight of milk received

      = 100 litres × 1.032 = 103.2 kg

      Weight of fat received can be calculated by multiplying the weight of milk received by the fat content:

      =103.2 × 0.04 = 4.128 kg

      Value of butterfat purchased from the producer is equal to the weight of butterfat received multiplied by the price per kg of butterfat:

      = 4.128 × EB12.5 = EB51.60

    2. Calculate the volume and value of the skim milk. While the actual recovery of skim milk may be greater, in commercial practice it is normally assumed that 80% of the whole milk is recovered as skim milk.

      3. In this case, we therefore recover 80 litres of skim milk with a value of 18 cents/litre.

      Value of skim milk = 80 × 0.18 = EB 14.40

    3. To obtain the total value of the milk received, add the values obtained in 3.1 and 3.2:

      4. EB 51.60 for butterfat

      EB 14.40 for skim milk

      EB 66.00

Therefore, the average value of 1 litre of milk is 66 cents.

It is important to note that, since the butterfat is the most valuable commercial fraction, milk price will vary in proportion to butterfat content.

It is assumed that butterfat content can be estimated. In large dairy plants, milk price is based on the content of the major milk constituents. For small-scale milk processors, this is not normally feasible and payment should be based on fat content.

Production costs and depreciation are deducted proportionally from milk price. Other deductions may also be made when calculating the price paid to the producer for milk.

Milk Analysis

Milk analysis is carried out to determine:

Sampling

A representative sample is essential for accurate testing. Milk processors usually pay for milk or cream on the basis of butterfat analysis, and a single butterfat test may be used to determine the butterfat content of thousands of litres of milk or cream. Therefore, an accurate and representative sample must be obtained.

Milk must be mixed thoroughly prior to sampling and analysis to ensure a representative sample. If the volume of milk is small, e.g. from an individual cow, the milk may be poured from one bucket to another and a small sample of milk taken immediately. But if large volumes of milk are handled, the milk or cream must be mixed by stirring. However, it is very difficult to obtain a representative sample of milk or cream when a large volume is dumped into a large container. In such a case the milk must be stirred thoroughly and small samples taken from three or more places in the container. For best results, milk or cream must be sampled when it is at a temperature between 15 and 32°C. If the cream is too cool it will be thick and viscous and will be difficult to sample.

Sour milk or cream, in which casein has coagulated, must be sampled frequently. Sampling sour milk follows the same procedure as for fresh milk. If the milk or cream has been standing for a long time and a deposit has formed on the surface and sides of the container, it should be warmed while agitating before a sample is removed.

For certain analyses, milk samples can be preserved and stored to await analysis. Samples of milk or cream for butterfat analysis can be preserved using formalin, corrosive sublimate or potassium dichromate. For general analyses, formalin is preferred, because the other two increase the solids content of the milk, influencing total solids determination.

Estimation of milk pH by indicator

A rough estimate of pH may be obtained using paper strips impregnated with an indicator. Paper strips treated with bromocresol purple and bromothymol blue are sometimes used on creamery platforms as rejection tests for milk. Bromocresol purple indicator strips change from yellow to purple between pH 5.2 and 6.0, while bromothymol blue indicator papers change from straw yellow to blue-green between pH 6.0 and 6.9.

Electrometric measurement of pH

Electrometric determination of pH depends on the potential difference set up between two electrodes when they are in contact with a test sample. A reference electrode whose potential is independent of the pH of the solution and an electrode whose potential is proportional to the hydronium ion concentration of the test sample are used. Saturated calomel electrodes are usually used as reference electrodes, and glass electrodes are used to measure pH.

Instruments which measure the current produced by the difference in potential between the glass and calomel electrodes are called pH meters.

Preparation of the pH meter

  1. The pH meter should be kept in a dry atmosphere.
  2. Before using a new glass electrode, or a glass electrode which has been stored for some time, soak the electrode in N/10 HC1 for about 5 hours.
  3. Care should be taken not to scratch glass electrodes against the sides of beakers or other hard surfaces during storage or testing.
  4. The level of saturated potassium chloride in the calomel electrode should be checked before making pH measurements.
  5. Crystals of potassium chloride should be present in the solution within the electrode.
  6. The rubber stopper or cap on the filling arm of the calomel electrode should be removed before making a test.

Standardising and using the pH meter

  1. Rinse the electrodes with distilled water and wipe them gently with tissue or filter paper.
  2. Set the temperature; use the control knob of the meter to set the temperature of the buffer used to standardise the meter.
  3. Standardise the pH meter against a buffer solution of known pH. Use a buffer solution with a pH as close as possible to that of the test solution.
  4. Turn the range selector to the pH range covering the pH of the buffer control until the pointer of the meter reads the pH of the buffer.
  5. Set the range switch to zero.
  6. Before measuring the pH of the test sample, rinse the electrodes with distilled water and dry them.
  7. Set the temperature control knob to the temperature of the sample.
  8. Place the test sample in position and allow the electrodes to dip into the solution.
  9. Switch the range selector knob to the proper range and read the pH.
  10. Rinse the electrodes after use and keep the electrode tips in distilled water between tests.

Always follow the manufacturer's instruction for the particular instrument.

Determination of milk acidity

The production of acid in milk is normally termed "souring" and the sour taste of such milk is due to lactic acid. The percentage of acid present in dairy products at any time is a rough indication of the age of the milk and the manner in which it has been handled. As mentioned earlier, fresh milk has an initial acidity due to its buffering capacity.

Apparatus

Reagents

I. Using N/10 sodium hydroxide

Procedure

  1. Fill the burette with N/10 NaOH and make sure there are no air bubbles trapped in the lower part.
  2. Adjust the level of NaOH in the burette to the top mark – the lowest reading being at the upper end.
  3. If milk, skim milk or buttermilk is to be tested, place 18 g in the cup using a 17.6 ml pipette. If cream is to be tested, use a 9 ml pipette (for cream weighing about 1 g/ml).
  4. Add 3 to 5 drops of phenolphthalein to the sample in the cup.
  5. Note the reading of the NaOH in the burette at the lowest point of the meniscus.
  6. Allow the NaOH to flow slowly into the cup containing the sample and stir continuously. When a faint but definite pink colour persists, the end-point has been reached.
  7. Take the reading of the burette at the lowest point of the meniscus. Subtract the first reading from the second to determine the number of millilitres of alkali (NaOH) required to neutralise the acid in the sample.

Calculation

Percent lactic acid = ml N/10 alkali × 0.0009 × 100/grams of sample

II. Using N/9 sodium hydroxide: Milk, skim milk and buttermilk

Apparatus

Same as for I.

Reagents

Procedure

  1. Put 10 ml of milk in a porcelain dish.
  2. Add 0.5 ml of 1.6% solution of phenolphthalein.

Titrate with N/9 sodium hydroxide and follow the same procedures as in I.

Calculation

Percent lactic acid = W/V

Where W = volume of N/9 NaOH required (ml) and

III. Using N/9 sodium hydroxide: Cream

Procedure

  1. Put 10 ml of cream in a porcelain dish.
  2. Add 10 ml of water with the same pipette.
  3. Add 0.5 ml of 1.6% phenolphthalein.
  4. Titrate with N/9 NaOH.
  5. Calculate as in II.

For determination of acidity of cream serum, the fat percentage of the cream should be known, and the calculation is as follows:

Acidity of serum = (acidity of cream × 100)/100 – % fat

Alcohol test

The alcohol test, together with the acidity test, is used on fresh milk to indicate whether it will coagulate on processing. Milk that contains more than 0.21 % acid, or calcium and magnesium compounds in greater than normal amounts, will coagulate when alcohol is added.

Apparatus

Reagents

The only reagent needed is a 75% alcohol solution. This is usually prepared from 95% alcohol by mixing with distilled water in the proportion of 79 parts of 95% alcohol to 21 parts of distilled water.

Procedure

  1. Put equal volumes of milk and 75% alcohol in a test tube.
  2. Invert the test tube several times with the thumb held tightly over the open end of the tube.
  3. Examine the tube to determine whether the milk has coagulated: if it has, fine particles of curd will be visible.

Clot-on-boiling test

Acidity decreases the heat stability of milk. The clot-on- boiling test is used to determine whether milk is suitable for processing, as it indicates whether milk is likely to coagulate during processing (usually pasteurisation). It is performed when milk is brought to the processing plant — if the milk fails the test it is rejected.

The test measures the same characteristics as the alcohol test but is somewhat more lenient (0.22 to 0.24% acidity, as opposed to 0.21 % for the alcohol test). It has the advantage that no chemicals are needed. However, its disadvantage is that at high altitude milk (and all liquids) boils at lower temperature and therefore the test is even more lenient.

Apparatus

Reagents

None

Procedure

  1. Place about 5 ml of milk in a test tube (the exact amount is not critical), and place the test tube in boiling water for 5 minutes.
  2. Carefully remove the test tube and examine for precipitate. The milk is failed if any curd forms.

Butterfat determination

The main tests used to determine the fat content of milk and milk products are the Gerber and Babcock tests. Automated methods for testing milk are now used in central laboratories and at large processing centres.

The Gerber test

The procedures outlined below are used to determine the butterfat content of milk, skim milk, buttermilk, cream and whey.

Milk

Apparatus

The apparatus required for butterfat content analysis comprises:
  1. Gerber butyrometer calibrated to read 0–8% or 0–5% and graduated at 0.1 % intervals.
  2. Butyrometer stoppers.
  3. Milk pipette — volume to match the butyrometer in use.
  4. 10 ml double-bulb pipette* for pipetting sulphuric acid.
  5. 1 ml bulb pipette* for pipetting amyl alcohol.
  6. Thermometer to read 1–100°C
  7. Water bath.
  8. Gerber centrifuge.

*Alternatively, automatic dispensers can be used for delivering 10 ml of sulphuric acid and 1 ml of amyl alcohol.

Reagents

Procedure

  1. Mix the milk sample (temperature about 20°C) thoroughly, taking care to minimise incorporation of air. Allow the sample to stand for a few minutes to discharge any air bubbles. Mix gently again before pipetting.
  2. Pipette or dispense 10 ml of sulphuric acid into the butyrometer.
  3. Pipette the required volume of milk into the butyrometer. Care must be taken to avoid charring of the milk, by ensuring that the milk flows gently down the inside of the butyrometer. It then rests on top of the acid.
  4. Pipette or dispense 1 ml of amyl alcohol.
  5. Clean the neck of the butyrometer with a tissue or dry cloth.
  6. Stopper the butyrometer tightly using a clean, dry stopper. Shake and invert the butyrometer several times until all the milk has been absorbed by the acid.
  7. Then place the butyrometer in a water bath at 65°C for 5 minutes.
  8. Centrifuge for 4 minutes at 1100 rpm.
  9. Return the butyrometer to the water bath for 5 minutes, ensuring that the water level is high enough to heat the fat column.
  10. Read the fat percentage. If necessary, the fat column can be adjusted by regulating the position of the stopper.

Hazards

Precautions

Skim milk, buttermilk and whey

Apparatus

Standard Gerber butyrometers designed for testing skim milk. The rest of the apparatus is the same as that used for whole milk.

Reagents

The same reagents are required as for whole milk.

Procedure

The procedure is the same as for whole milk up to and including the first centrifuging. The butyrometers are then placed in the water bath at 65°C, stoppers down, for 1 to 2 minutes and again centrifuged for 4 to 5 minutes. Then they are placed in the water bath for 2 to 3 minutes and read. A check reading is made after they are placed in the water bath for 2 to 3 minutes. The readings obtained must be corrected as follows:

Percentage read on the

Cream

Apparatus

The apparatus required for whole milk, except for the butyrometers and the 11 ml pipette, is supplemented by certain additional items for testing cream. The test bottles are standard Gerber cream butyrometers. Other items include a balance for weighing to 0.001 or 0.005 g; a stand to support the butyrometers on the balance or a stopper weighing funnel, and a wash bottle containing warm (30–40°C) distilled water.

Reagents

The same as for whole milk.

Procedure

  1. Mix the sample thoroughly, though cautiously, to avoid frothing. If the sample is very thick, it should be warmed to between 37.8° and 50°C to facilitate mixing.
  2. Weigh 5 g of cream into the butyrometer.
  3. Add about 6 ml of warm distilled water from the wash bottle.
  4. Add 10 ml of sulphuric acid and I ml of amyl alcohol.

The remaining procedures are the same as for whole milk.

Cheese

Fat determination in cheese is carried out in a similar manner to that for milk.

Apparatus

Gerber cheese butyrometer stamped "3 g cheese". Other apparatus same as for Gerber milk fat analysis.

Reagents

Procedure

  1. Weigh out 3 ± 0.01 g of cheese on a counter-balanced piece of grease-proof paper.
  2. Dispense 10 ml sulphuric acid into the butyrometer. Add 3 ml of water carefully so that it rests on the acid.
  3. Wrap the 3 g of cheese in the grease-proof paper to form a cylinder that fits into the butyrometer.
  4. Add a further 4 to 5 ml of water.
  5. Add 1 ml of amyl alcohol.
  6. Stopper the butyrometer securely and shake to dissolve the cheese. (It may be difficult to dissolve the cheese. If difficulty is experienced, place the butyrometer in the heated water bath and remove periodically for mixing until the cheese is fully dissolved.) Cheese butyrometers are centrifuged and read as for milk and cream.

Determination of milk specific gravity

Specific gravity is the relation between the mass of a given volume of any substance and that of an equal volume of water at the same temperature.

Since 1 ml of water at 4°C weighs 1 g, the mass of any material expressed in g/ml and its specific gravity (both at 4°C) will have the same numerical value. The specific gravity of milk averages 1.032, i.e. at 4°C 1 ml of milk weighs 1.032 g. 

Since the mass of a given volume of water at a given temperature is known, the volume of a given mass, or the mass of a given volume of milk, cream, skim milk etc can be calculated from its specific gravity. For example, one litre of water at 4°C has a mass of 1 kg, and since the average specific gravity of milk is 1.032, one litre of average milk will have a mass of 1.032 kg.

Apparatus

( L / 1000 ) + 1 = specific gravity (sp. gr.)

Thus, if L = 31, specific gravity = 1.031.

Procedure

  1. Heat the sample of milk to 40°C and hold for 5 minutes. This is to get all the fat into a liquid state since crystalline fat has a very different density to liquid fat, and fat crystallises or melts slowly. After 5 minutes, cool the milk to 20°C.
  2. Mix the milk sample thoroughly but gently. Do not shake vigorously or air bubbles will be incorporated and will affect the result.
  3. Place the milk in the cylinder. Fill sufficiently that the milk will overflow when the lactometer is inserted.
  4. Holding the lactometer by the tip, lower it gently into the milk. Do not let go until it is almost in equilibrium.
  5. Allow the lactometer to float freely until it reaches equilibrium. Then read the lactometer at the top of the meniscus. Immediately, read the temperature of the milk. This should be 20°C. If the temperature of the milk is between 17 and 24°C, the following correction factors are used to determine L:

Temp. (o C)   17      18     19     20      21     22      23     24

Correction   –0.7   –0.5   –0.3   —    +0.3   +0.5    0.8   1.1

e.g. The lactometer reading is 30.5 and the temperature is 23°C.

Corrected lactometer = Lc = 30.5 + 0.8 = 31.3

Calculations

All calculations always use Lc, the corrected lactometer reading. To calculate the specific gravity, divide the corrected lactometer reading by 1000 and add 1.

In our example: Sp. gr. = ( 31.3 )/ 1000 + 1 = 1.0313

Determination of total solids (TS) and solids-not-fat (SNF) in milk

The total solids content of milk is the total amount of material dispersed in the aqueous phase, i.e. 

SNF = TS – % fat.

The only accurate way to determine TS is by evaporating the water from an accurately weighed sample. However, TS can be estimated from the corrected lactometer reading. The results are not likely to be very accurate because specific gravity is due to water, material less dense than water (fat) and material more dense than water (SNF). Therefore, milk with high fat and SNF contents could have the same specific gravity as milk with low fat and low SNF contents.

TS = (Lc)/ 4 + (1.22 × fat %) + 0.72

SNF = TS – fat %

Or = Lc / 4 + (0.22 × fat%) + 0.72

It should be noted that the relationship between Lc and TS varies from country to country depending on milk composition. The above formulae are called the Richmond formulae and were calculated for Great Britain.

Determination of moisture content of butter

Apparatus

Procedure

  1. Weigh 10 g of butter into the cup. Heat the butter over a low flame until it ceases foaming and a light-brown colour appears. When heating the sample, place the container on the asbestos-centre wire gauze on a tripod. This distributes the heat evenly across the bottom of the cup.
  2. After the moisture is driven from the butter, allow the sample to cool and reweigh.

Calculations

Percentage moisture content of the butter is calculated as:

Moisture % = (Original weight – final weight )/ Original weight × 100

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