Chemical Test Procedures for Quality Assurance in Dairy Products

DairyPulse 30 June 2024
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Listed here are the conventional chemical test procedures that are carried out on dairy products to assure quality of the products. You will realize that I have not indicated the lactoscan equipment in any of these tests.

The machine can execute some of the tests indicated here automatically and give you a printout of the results or display the results on its screen. Since it is not possible for everybody to access the lactoscan, these methods remain very instrumental and in some cases, they have to be done to confirm the lactoscan results.

These tests remain to be the most reliable quality assurance measures in the industry going by the local industry standards.

1. Chemical test procedures for determining the C.I.P detergent strength

The purpose of this method is to ensure accurate determination of concentration of detergents used for Cleaning In Place (C.I.P).

This work instruction covers the procedure for determination of strength of lye (NaOH) and Nitric acid (NHO3)

Necessary Apparatus and Reagents

Preparation Instructions

  1. Calculate the acid concentration by multiplying the titer value by the factor 0.233
  2. To determine the acid strength, titrate the acid sample against 0.1 N NaOH solution until the colour changes from clear to purple
  3. Calculate the % concentration by multiplying the titre by the factor 0.133
  4. Record the titer value obtained
  5. Add 2-3 drops of phenolphthalein indicator and titrate against 0.1 N Hcl solution until the pink colour disappears.
  6. Draw 3 ml of lye solution using the 10 ml pipette and transfer into a beaker cup
  7. Fill the 50 ml burette with 0.1N Hcl solution and adjusted to the 0.0ml mark
  8. To determine lye concentration, titrate the lye sample against 0.1N Hcl

2. Chemical test procedures for determining density

The purpose of this method is to ensure uniform practice in determination of density of both raw and pasteurized milk.

Apparatus/Equipment

  • Lactometer jar
  • Milk lactometer (with a valid Calibration certificate).
  • Thermometer range 10 -110°C (calibrated)

Preparation Instructions

  • Raise the temperature of the milk sample to 40°C; keep the sample at 40°C for 5 minutes. During this time, mix the sample by rotating and inverting. Cool to 20°C, mix again.
  • Fill the lactometer jar with sample milk and gently insert the lactometer into the milk.
  • Wait for at least 2 minutes (until the setup settles) and do not disturb the apparatus.
  • Read the density at the level of meniscus formed between the milk and the lactometer stem. At the same time record the milk temperature.
  • If the time of taking the reading the temperature was not equal to the calibration temperature, correct the reading by adding the value specified in the table below, which corresponds to hydrometers calibrated at a temperature of 20°C
  • Remove the lactometer gently and drain off the milk into a sample bottle.
  • Carry out butter fat test on milk.
  • Calculation of Total Solids (T.S).

T.S = {(Corrected Lactometer Reading)/4} + (1.22 x B. Fat) + 0.14

S.N.F = T.S. – B.F

T.S. = CLR – 1 (1000 ÷ 4) + (1.2 x B.F) + 0.14

S.N.F. = T.S. – B.F (Richmond formula for finding S.N.F)

3. Resazurin Test

Resazurin test is conducted to ensure milk (pasteurized and raw) has the microbial load within specifications.

Required Equipment and Reagents

  • Water bath at 37°C ±0.5°C
  • Sterile test tubes.
  • Sterilized rubber stoppers
  • Resazurin solution -0.005% made after every 8 hrs.
  • Sterile 1ml pipettes
  • Comparator with standard resazurin disc

Preparation Instructions

  • Prepare the water bath and set the temperature at 37°C
  • Pour out 10mls of milk sample into the sterile test tube (most of them come already calibrated to the 10ml mark)
  • Add 1ml of resazurin solution into the contents of the test tube.
  • Stopper the test tube with clean, sterilized to avoid any possible contamination.
  • Mix the sample gently until the dye is uniformly distributed
  • Place the test tube in the water bath at 37°C and read after 10 min, 30 minutes then 1st, 2nd and 3rd

Acceptable Standards

Only resazurin 5 and 6 after 10 min is acceptable for raw milk, for pasteurized milk only the milk that finishes 3 hrs with the resazurin 6 is acceptable.

4. Chemical test procedures for determining the acid content in dairy products

Acidity test is carried out to determine the amount of lactic acid in raw and pasteurized milks, fermented milks, and cream.

Equipments and reagents

  • 50 ml burette (with 0.1ml calibrations)
  • N9 NaOH
  • Beaker
  • 10 ml pipette
  • Phenolphthalein indicator – 2.5%

Instructions

  • Fill the burette to the mark. Remove with tissue any NaOH at the tip of the burette.
  • Using a blowout pipette take 10 ml of the milk sample and transfer into the beaker
  • Add 2-3 drops of phenolphthalein indicator into the sample
  • Mix the indicator into the sample gently
  • Titrate the sample quickly and continuously until the first permanent faint pink colour appears and persist for at least 1 sec.
  • Read off the volume of sodium hydroxide used and divide by 10 to get the percentage acidity, which is expressed as percentage lactic acid.

N/B: Natural acidity in fresh milk is due to presence of phosphates, calcium, and carbon dioxide. Developed acidity is a result of microbial activity.

5. Chemical test procedures for determining sediment content

Sediment test is conducted to check the amount of solid dirt in raw and pasteurized milk

Equipments and Reagents

  • sediment tester
  • thermometer (range 10 -110°C)
  • sediment test discs
  • distilled water
  • standard sediment grading chart

Preparation Instructions

  • Take a ½ litre of milk sample and adjust the temperature to 40°c and then cool to 20°c
  • Clean the sediment tester thoroughly with filtered water
  • Place a clean filter pad, with the name of the supplier into position.
  • Put the ½ litre milk sample into the tester and let all the milk pass through the sediment disc/ pad.
  • Remove the filter pad and place it on a clean surface (parchment paper) to dry in a dust free environment for at least 10 minutes.
  • Grade the milk using the standard grading card

6. Hydrogen Peroxide Test

Conducted to check presence of hydrogen peroxide in Milk

Equipment/Apparatus

Hydrogen Peroxide Test Strips/ Sticks

Preparation Procedure

Dip the mashed part of the strip in a sample of milk

Observations/Results

Colour change to blue indicates presence of hydrogen peroxide. If colour does not change then hydrogen peroxide is not present in the milk sample

7. Clot on Boiling (CoB) Test

This test is conducted to check the stability of milk proteins, whether it can withstand heat treatment

Equipment/Apparatus

Test Procedure

  • Put a sample of milk in an aluminum container
  • Heat the milk sample over the flame of the Bunsen burner

Results/Observations

  • If the milk clots, it is COB positive (cannot withstand heat treatment).
  • If it does not clot, then it is said to be COB negative

8. Butterfat Content Testing (Gerber Method)

Gerber butterfat test is conducted on milk to determine the amount of butterfat it contains

Apparatus/Equipment and Reagents

  • Butyrometer (Dr. N. Gerber 0 – 8%)
  • 10.94 ml bulb pipette
  • Self-heating centrifuge at 65°C (1100 – 1200rpm)
  • Automatic dispensers for 1ml – amyl alcohol
  • Automatic dispensers for 10ml – sulphuric acid
  • Sulphuric Acid (1.815 – 1.820 g/ml at 20°C) – Corrosive!
  • Amyl Alcohol (0.810 – 0.815 g/ml at 20°C)

Preparation Procedure

  • Dispense 10 ml of sulphuric acid via an automatic dispenser in to the butyrometer. (Avoid wetting the neck of the butyrometer)
  • Pipette 10.94 ml of well mixed milk sample at room temperature and put it into the butyrometer gently along the side of the butyrometer so as to avoid mixing the milk with acid  (Avoid wetting the neck of the butyrometer)
  • Add 1 ml of Amyl Alcohol. (Avoid wetting the neck of the butyrometer)
  • Cork the butyrometer, shake and keep inverting the butyrometer until a deep chocolate color is observed.
  • Put the butyrometer in the centrifuge at 65°C and spin for 5 min
  • Read the results and record.

9. Chemical test procedures for determining butter moisture content

This test is used to determine the moisture content in butter by evaporation method

Apparatus/Equipment

Preparation Procedure

  • Weigh the Kohman cup and record the weight – A
  • Weigh 10 g of butter (from the centre of the sample) in to the Kohman cup – B
  • Heat the sample on a Bunsen burner swirling to avoid charring until all the moisture escapes. When the bubbling stops and colour changes to golden brown, stop heating – C
  • Allow the cup to cool to the original temp
  • After cooling the sample is reweighed – D
  • The difference in weight is the moisture content

For instance; say, Weight of Kohman cup – A = 40g

Weight of sample  = 10g

Weight of cup + butter sample – B = 50g

Weight of cup + sample after heating and cooling –D = 48.4g

Moisture content = B – D ÷ 10 x 100 = 50 – 48.4 ÷ 10 x 100 = [1.6 x 100]/10

Moisture content = 16%

10. Chemical test procedures for determining salt content in butter

This test is conducted in salted dairy products to determine the salt concentration in these products

Apparatus and Reagents

  • A pair of tongs
  • Kohman cup
  • Bunsen burner
  • Beaker
  • 250 ml volumetric flask
  • 6 ml pipette
  • Silver nitrate
  • Potassium chromate
  • Distilled water
  • Thermometer

Preparation Procedure

  • Warm the distilled water to a temperature of between 40 – 50°C
  • Using the heated butter sample whose moisture content has been determined, rinse the butter contents from the Kohman cup with the warm distilled water as you put it put into the volumetric flask until the 250 ml mark.
  • Leave it to settle so that the fat floats and the curd sediments
  • 6 ml of the solution is pipetted from the middle and put in a beaker
  • Add one drop of potassium chromate indicator
  • Titrate it against Silver nitrate until endpoint.

11. Chemical test procedures for determining free fatty acids in butter

This test is conducted to determine the amount of fatty acids present in milk.

Equipment/Apparatus and Reagents

  • Conical flask, 250ml
  • Measuring cylinder 50ml
  • Pipette 10 ml
  • Weighing balance of 0.0gm precision
  • Ethanol-Ether mixture (prepared by mixing equal volumes of 95% ethyl alcohol and diethyl ether, and neutral to phenolphthalein.
  • Sodium Hydroxide 0.1N.
  • Phenolphthalein indicator 2.5% in 95% ethyl alcohol.

Preparation Procedure

  • Weigh a sufficient amount of butter in a beaker or a flask
  • Melt indirectly using a Bunsen burner.
  • Weigh 10 grams of the melted butter, carefully avoiding the aqueous phase in to a beaker
  • Add 50ml of FFA (Diethyl ether/ Ethanol) reagent to the melted butter
  • Add 5 – 6 drops of phenolphthalein indicator
  • Titrate against N/9 NaOH up to endpoint
  • Read off the titre

Results/Observations

Multiply the titre value by 0.282 to get FFA (0.282 is a constant factor)

12. Peroxidase Test

This test is conducted to verify the effectiveness of high temperature (>80°C) pasteurization of cream and milk used to make fermented milk products.

Apparatus/Equipment and Reagents

  • Red and blue litmus paper,
  • Test tubes (medium size),
  • Stop watch,
  • Water bath at 80°C
  • 1n Hcl in pipette or dropping bottle,
  • 1% NaOH in pipette or dropping bottle,
  • 2% H2O2 in pipette or dropping bottle,
  • 2% paraphenylenediamine in pipette or dropping bottle.
  • Keep reagents in cold storage

Preparation Procedure

0.2% H2O2 for Storch’s test: Dilute 7 ml of 3% hydrogen peroxide solution (Ph.Nord III, 1963,p.302) with 98 ml of distilled water. Stabilize with 0.1 ml concentrated sulphuric acid. Store in amber bottle. Will keep several months if stored in a cool place.

2% Paraphenylenediamine: Dissolve 2g paraphenylenediamine (for analysis) in 100 ml of cold distilled water by shaking filter. Store in amber bottle. Will keep one week. A precipitate will occur in course of one to two days, which should be filtered off.

The principle involved here:

Raw milk contains an enzyme, milk peroxidase, which can accelerate the oxidation process of the hydrogen peroxide. The peroxidase enzyme is destroyed by heating. At 80°C the enzyme will become inactive in 21/2 seconds. At higher temperatures the inactivation will take shorter time, at lower temperatures it will take longer.

Test procedure

Whole milk. Approximately 5 ml of milk is tested with litmus paper in a test tube. If the reaction of the milk is not about neutral, pH is adjusted with 0.1N Hcl or 1%NaOH. Add one drop of 0.2% H2O2 and 2 drops of paraphenylenediamine. Shake, check the time it takes for the blue colour to appear using a stopwatch.

Testing Cream. The procedure remains the same as for whole milk.

Interpretation

  1. 1 second = Instantaneous intense blue colour which turns dark blue in 30 seconds indicates 100% raw milk or inadequately pasteurized milk to which more than 20% of raw milk has been added, or milk which has only been heated to approximately 73 – 74°C in the pasteurizer for 21/2 seconds.
  2. 2.5 – 10 seconds = Lighter blue colour appearing in the course of 5 – 10 seconds, assuming a dark blue colour in 30 seconds, but lighter than described under 1, indicates inadequately pasteurized milk to which 4 – 20% of raw milk has been added or milk which has only been heated to 74°C – 75°C in the pasteurizer for 21/2
  3. 15 – 20 seconds = Faint grayish colour appearing in 15 – 20 seconds, assuming a faint grey-blue tint in 30 seconds, lighter than described under 2, indicates adequately pasteurized milk/ cream to which 2 – 3% raw milk/ cream has been added, or milk/ cream which has only been heated to 75 – 77°C in the pasteurizer for 21/2 seconds.
  4. 20 – 25 seconds = Slight greyish tint appearing in 20 – 25 seconds, assuming a light grey colour in 30 seconds, indicates adequately pasteurized milk to which 1 – 2% raw milk/ cream has been added or milk/ cream which has been heated to 77° – 78°C in the pasteurizer for 21/2 seconds.
  5. No colour developing within 30 seconds indicates properly pasteurized milk/ cream (addition of less than 0.5% raw milk/cream cannot be demonstrated with Storch’s test).

Any colouring after 30 seconds is of no significance. Adequately pasteurized milk/ cream will always colour after prolonged standing owing to spontaneous oxidation of the paraphenylenediamine.

13. Viscosity Test Procedure

Conducted on milk and milk products to check consistency/viscosity

Required Apparatus

A rheometer would be ideal. In its absence, however, use the following:

  • Viscometer stand
  • Viscometer
  • Timing device (stop watch)
  • Sample (100ml)
  • Apertures (3mm and 5.5mm)
  • Beaker

Test Procedure

  • Clamp the viscometer to the stand at a fixed height.
  • Select the aperture that is appropriate for the sample to be tested and screw it to the viscometer.
  • Adjust, as necessary, the temperature of the sample. (20°C – 25°C)
  • Place a beaker beneath the viscometer.
  • Hold the aperture with the index finger.
  • Fill sample into the viscometer cup.
  • Remove the finger allowing the sample to flow, simultaneously switch on the stop watch.
  • Record the time taken for the sample to cross the bottom mark.
  • For improved assurance and precision, perform three tests, record the results from all three, and use the average as a best estimate of actual viscosity.
  • Clean the viscometer immediately after each use.

14. Brix Determination

Measures the refractive index of milk and milk products

Requirements

  • Plastic pipette
  • Refractometer
  • Soft tissue
  • Sample (0.1ml)
  • Distilled water

Procedure

  • Press the ON/ OFF key.
  • Using a plastic pipette, fill the sample well with distilled water.
  • Press the ZERO key.
  • Gently absorb the water with a soft tissue.
  • Using a plastic pipette, drip sample (0.1ml) onto the prism surface.
  • Press the READ key. Measurement is displayed in units of percentage BRIX.
  • Remove sample from the sample well by absorbing on a soft tissue.
  • Using a plastic pipette, rinse prism and sample well with distilled water. Wipe dry.

15. Homo-Efficiency (H.E) test procedures

To determine the measure of creaming of fresh milk after pasteurization

Equipment/Apparatus

  1. Refrigerator
  2. Measuring cylinder (200 – 250ml)
  3. Apparatus for butter fat determination
  4. Suitable pipette

Test Procedure

  • Place 200 – 250mls of pasteurized milk in measuring cylinder for a minimum of 12 hours in a refrigerator.
  • Without disturbing the sample, draw 10% milk from the top of the cylinder.
  • Mix thoroughly and determine the butter fat content of the above sample and call it, A.
  • Mix thoroughly the remaining milk sample and determine its butterfat content, B.
  • H.E = 100 – {[(Butterfat in A) – (Butterfat in B)]/Butterfat in B} x 100

Interpretation of results

ReadingObservation
 96 – 100%Very good quality milk
90 – 95%Good quality milk
< 90%Poor quality milk
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