Milk testing is an essential component of quality assurance in manufacturing. Dairy processing plants, whether small, medium-sized or large-scale, need robust quality assurance labs with all the milk testing equipment necessary for effecting quality.
There are equipment and supplies for rapid tests as well as sophisticated machinery for complex analysis. Milk quality assurance begins at the farm. The quality should be maintained (preferably in a cold value chain) throughout the process until the product reaches the final consumer.
The infographic below shows some of the rapid tests you can do to ascertain the quality of milk and milk products.
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Common mandatory quality assurance tests at the dairy plant:
|C.O.B||Burner (source of heat).|
|Alcohol /Alcohol-alizarin||Alcohol gun|
Measuring cylinder (for standardizing ethanol).
Dropper (for indicator)
Test tubes (10ml mark).
1 ml pipette
|Gerber butterfat||Gerber Centrifuge.|
Gerber butyrometers with appropriate stoppers.
Gerber milk pipette.
Acid dispenser/Gerber acid pipette.
1ml pipette for amyl alcohol.
Distilled water (not a must)
Measuring cylinder (for holding sample).
|Phosphatase||Water bath (controllable).|
Test tubes and pipettes (1, 5, 10ml)
Filter paper (No. 2).
|Sodium carbonate pellets|
Disodium p-nitrophenyl phosphate
|Appropriate culture media for the test.|
Water bath (most likely).
Test tube brushes
Common microbiological tests and culture media used in the dairy quality assurance include:
|Microbiological test||Suitable Culture Media (use any of them).|
|Yeast and Moulds|
|Coliform||1. Lauryl Tryptose (lactose) Broth.|
2. Brilliant Green Lactose Bile (BGLB) Broth.
3. MacConkey Broth.
4. Lauryl Tryptose Mannitol Broth with Tryptophan
5. Tryptone water
6. EC Medium
7. Improved Formate Lactose Glutamate Medium
|Standard Plate Count||1. Plate Count Agar (PCA)|
2. Standard Methods Agar (SMA)
Some of the milk testing equipment used in the process include:
1. Electric Gerber Centrifuge Machine
Gerber Centrifuges are manufactured by Gerber Instruments, a company that was founded in 1892 to produce precision equipment for chemical analysis on dairy products. They are the most prominent manufacturer in the dairy industry and their products have become the benchmark for quality analysis.
Butter fat content determination is an essential analytical procedure in the dairy industry. Given the principle of butterfat analysis, Gerber centrifuge becomes a basic equipment in the process hence a basic equipment in every laboratory.
Gerber Centrifuges are designed to meet very high safety standards. In every centrifuge, you will find the following features:
- Gerber centrifuges come in different capacities (18, 12, 24, etc).
- Each machine comes with a motor that has elastic bearing feature to ensure smooth run. The centrifuge also comes with anti-vibrator pads to ensure the machine remains grounded during operations.
- The centrifuge has an automatic shutoff feature that will snap into action when the machine detects any imbalance. In some devices, the covering glass will break to alert you.
- The Gerber centrifuge has a cover lock that ensures the machine will not run unless it is properly secured.
- Gerber centrifuges come with a strong metallic sheet with acid-resistant paint. This ensures the machine does not get corroded in case of acid spillage.
- Every Gerber centrifuge has a mechanical brake for safe operations. The machines are calibrated to run for 5 minutes.
- Each unit is heated to eliminate the need for water bath after centrifugation.
What you can do with a Gerber Centrifuge:
- Gerber butterfat content determination
- Determination of solubility through Gerber and ADPI methods.
There are also manual centrifuges for butter fat determination you can consider if you are not connected to the grid.
2. Gerber Butyrometer
A Gerber butyrometer is a special measuring equipment for determining the fat content of dairy products. There are special butyrometers for fresh milk and cream.
A butyrometer works with a Gerber centrifuge and is an indispensable instrument in the butter fat determination process.
When you buy this item for laboratory use, do not forget the butyrometer stand.
3. Milk Sampler / Scoops
A milk sampling dipper /scoop is a metallic cylindrical container with a long handle used to obtain milk sample from a homogeneous bulk.
It comes in various capacities depending on the purpose of the sample and the container from which the sample is drawn. It is very handy when sampling milk from churns and other medium-sized bulk storage tanks.
4. Milk Sampling Bottles
Milk sampling bottles can be made of either glass or plastic. Their capacities vary depending on the size of sample to be obtained for analysis.
They are designed to ensure maximum hygiene for the sample.
5. Laboratory Water Bath
Water baths contain heated water and are used to incubate samples at constant temperatures over time, warm reagents, and melt media/substrate. They can be either digital or analog types with an option to control the temperature to the desirable level.
Depending on the application, different types of water baths available include:
Vibrating water baths
These water baths constantly shake to allow the bacterial cultures under incubation mix with air for optimal performance.
Circulating water baths
They constantly circulate the water for constant temperatures that are necessary for serological and enzyme mediated reactions.
Non-circulating water baths
They do not have the feature for circulating the water. They are not very accurate in controlling the temperature.
- Water bath water is rarely changed. Make sure you raise the temperature of the water to over 90°C at least once a week to decontaminate the system.
- Monitor the water level to avoid exposing the heating element. Adjust the water level using distilled water and avoid adding salt as it will cause the element to rust.
- Do not use water bath with moisture sensitive reactions as it is likely to affect the results.
- Do routine management checks to ensure that the instrument is working correctly. The calibration should be accurate to avoid giving skewed results.
6. Gerber Lactometer
A hydrometer works with Archimedes’ principle, which states that a floating body displaces an equivalent of its weight of the fluid on which it floats.
Lactometers are special hydrometers used to check the specific density of fluids. Gerber lactometers are specially made for determining the density of milk and is very sensitive to changes in milk solids.
The sensitivity of a Gerber lactometer is due to its size and the lead shots at the base that are calibrated to detect the slightest changes in density.
One can judge the quality of milk fairly easily with a Gerber lactometer since milk has a specific value for density.
The specific gravity (density) of milk is read directly from the stem of the lactometer according to the formula below. This value is also the corrected lactometer reading at calibration temperature of the lactometer.
How to correct the lactometer readingIn case you are taking the lactometer reading at temperatures other than calibration temperature, use the correction factor to get the corrected lactometer reading.
If you record the lactometer reading at a temperature other than the calibration temperature, you must correct the reading. For every 1°C above the calibration temperature, add 0.2 to the lactometer reading.
On the other hand, subtract 0.2 from the lactometer reading for every 1°C drop in temperature from the calibration temperature.
7. Milk Churn
They are standardized and come in different capacities for easy handling and cleaning.
Most milk cans are made of either stainless steel or aluminium material. These metal is properly welded for maximum durability and the seamless body makes them easy to clean for hygienic purposes.
8. Brix Refractometer
In yoghurt and other fermented milk products, brix is useful in determining the viscosity of the product. Many consumers prefer more viscous fermented milk products.
Brix refractometer is also useful in determining the level of immunoglobulins in colostrum for feeding calves.
The video below shows how to use the brix refractometer to determine colostrum quality.
You can find the interpretation chart for colostrum quality here, courtesy of Crystal Creek.
Rapid tests for determining milk adulteration
Type of Adulteration
How to Detect the Adulterant
1. Salt in milk
Mainly done to increase the density of milk (usually after adding water to increase volume)
|Into 1ml milk sample in a test tube, add 5 ml of 0.1N silver nitrate followed by 2-3 drops of potassium dichromate. Observation of yellow color indicates presence of salt while chocolate or reddish brown color shows salt is absent.|
2. Starch in milk
Added to increase the density of milk.
|Add a few drops of 1% iodine solution to 5ml milk sample in a test tube. Formation of blue-black color indicates the presence of starch.|
3. Glucose /Invert sugar
Done to increase the consistency of milk (to make it look rich)
|Into 1 ml of milk sample in a test tube, add 1 ml of modified Barfoed’s reagent and heat in a boiling water bath for 3 minutes then cool rapidly under running tap water. Glucose is present if deep blue color is observed.|
· Dipping a Diacetric strip into the milk sample for 30 sec to 1 min should produce a color change on the strip for a positive test and no change if glucose is absent in the milk sample.
4. Sugar in milk
Added to increase milk density after adulteration with water
|Take 3 ml of milk in a test tube and add 2 ml of conc hydrochloric acid followed by 0.1g of resorcinol. Place in a water bath for 5 minutes. Red color indicates presence of sugar in milk.|
5. Water in milk
Added with the intention of increasing the milk volumes
|Put a drop of water on a slanting polished surface. Pure milk is more viscous and leaves a white trail while milk with added water flows immediately leaving no trail.|
· Use the lactometer test. Adulterated milk has lower than normal density.
6. Hydrogen Peroxide in milk
Usually done to preserve raw milk during transit
|The fastest method is through the use of hydrogen peroxide strips. You should observe a blue color on the strip for a positive sample.|
· Add 5 ml of milk sample into a test tube followed by 3 drops of Paraphenylene Diamine and then shake thoroughly. Color change to blue shows positive hydrogen peroxide in milk.
· Add 10 ml of milk sample in a test tube followed by 10-15 drops of Vanadium Pentoxide then mix thoroughly. Observation of pink/red color indicates hydrogen peroxide present.
7. Neutralizer (sodium bi-carbonate) in milk
Added in milk to counter developed acidity due to microbial activity.
|Into 5ml of the milk sample in a test tube, add 7ml of rectified spirit followed by 4-5 drops of rosalic acid. Milk with neutralizer will produce red color after this treatment.|
8. Borax and boric acid in milk
|Into 5ml milk sample in a test tube, add 1ml of conc hydrochloric acid and then dip a turmeric paper into the resultant solution. Dry the paper in a watch glass at 100 °C. A change in color of the turmeric paper to red indicates presence of boax or boric acid in milk.|
9. Urea in milk
|Into 5 mL of the milk sample, add 5 mL p-Dimethyl Amino Benzaldehyde and observe color change. A distinct yellow color indicates added urea while slight yellow color indicates natural urea in the milk.|
· Into 5ml milk sample, add 0.2ml urease (20mg/ml) and mix thoroughly by shaking. Add 0.1ml of 0.5% bromothymol blue (BTB) and observe color change. Distinct blue color formed within 10-15 minutes show added urea present in milk. A faint blue color is as a result of natural urea in milk.
· Into 5ml milk sample, add 5ml of 24% TCA to precipitate milk solids. Filter off the precipitate and take 1ml of the filtrate into which you add 0.5ml of 2% sodium hypochlorite, 05ml of 2% NaOH, and 0.5ml of 5% phenol solution. Mix the contents thoroughly and observe the color change. You will see a blue-green color for added urea while whey of pure milk will remain colorless.
10. De-fatting / skimming / fat removal
|Density of milk shoots up when tested with a lactometer.|
11. Detergents in milk
Has bacteriostatic effect in milk
|Mix equal amounts of the milk sample and water then shake thoroughly. Lathering indicates the presence of detergent in milk.|
· Into 5ml of the sample, add 0.1ml of 0.5% bromocresol purple (BCP) solution and mix well. Violet color shows detergent is present while pure milk shows a faint violet color.
· Into 5ml milk sample, add 1ml of methylene blue followed by 2ml chloroform. Swirl the contents for 15 seconds then centrifuge at 1100rpm for 3 minutes. Intense blue color at the base shows detergent present while intense blue color at the top shows detergent absent.
· For pulverized soap, add 10 ml of the milk sample into a test tube followed by 10ml of hot water then 1-2 drops of phenolphthalein indicator. Pink color observed shows presence of soap.
12. Ammonium Sulphate in milk
Added to increase milk density
|Add an acidifier (e.g. citric acid or TCA) into 10ml of hot milk and separate the whey using a filter paper (grade 42 preferable). Add 0.5ml of 5% barium chloride into 5ml of the separated whey. Formation of a white precipitate indicates presence of ammonium/zinc/sodium/magnesium sulphate.|
· Into 2ml of the milk sample, add 0.5ml of 2% NaOH solution, followed by 0.5ml of 2% sodium hypochlorite solution, and 0.5ml of 5% phenol solution. Heat the resultant contents of the test tube for 20 seconds in a boiling water bath. If bluish color turns to deep blue, ammonium sulphate is present. Unadulterated milk shows pink color that gradually turns blue after 2 hours.
13. Formalin in milk
Added to increase the shelf life of raw milk
|Take 10 ml of milk in a test tube and add 5 ml of concentrated sulphuric acid from the sides of the wall without shaking. Presence of violet or blue ring at the interface of the two layers indicates the presence of formalin.|
· Add 1ml of 10% ferric chloride and top up the volume to 500ml using conc hydrochloric acid. Take 5ml of this solution into 5ml of milk sample in a test tube and boil in a water bath for 4 mins. Brown color indicates formalin present.
· Take saturated solution of 1, 8- dihydroxynaphthalene-3, 6- disulphonic acid in about 72% sulfuric acid to make chromotropic acid solution. Add 1ml of the resultant solution in 1ml of milk sample in a test tube. A brownish-pink color indicates formalin present.
14. Synthetic milk
Can be due to accidental/intentional addition of paint, oil, urea, alkali, etc. into the milk
|Synthetic milk has a soapy feeling when rubbed between the fingers, turns yellow on heating, and has a bitter after taste.|
You can also test for protein properties of synthetic milk via urease strips (ELISA). Use the corresponding colors from the color chart to determine the quantity of urea present in the milk.
15. Coal / tar dyes in cheese, milk, milk powder, etc.
|Add 5 ml of dilute sulphuric acid (or conc HCl) into 10ml of the sample in a test tube and mix thoroughly. Pink/reddish color indicates presence of coal dyes. If you are unable to observe any color change, dilute the HCl with distilled water.|
· To test for azo dyes in milk, add a few drops of HCl in the sample. Positive test should give a pink color.
· To test for annatto in milk, change the milk pH to alkaline by adding sodium bicarbonate the dip a strip of filter paper in the mixture and let it stand for 2 hours. Annatto is present in the sample if the filter paper changes to red.
· Testing for any added color, mix 10ml of the milk sample with 10ml of diethyl ether and mix thoroughly. Presence of yellow color on the ethereal layer shows presence of added color.
16. Margarine in ghee
|Mix equal amounts of melted margarine sample and conc HCl with a pinch of sugar. Mixt thoroughly for a minute and let it stand for five more minutes. If you observe a pinkish red color in the lower layer, margarine is present in that ghee sample.|
This test relies on acid reaction with sesame oil, which is an ingredient of margarine
17. Benzoic and salicylic acid
|Acidify 5 mL milk sample with sulfuric acid and then add 0.5% ferric chloride solution drop-wise. Mix thoroughly and observe color change. Presence of buff color shows benzoic acid present while violet color indicates the presence of salicylic acid in milk.|
Further reading about quality assurance of dairy products
- Bector B. S., Ram M., Singhal O. P. Rapid Platform Test for the Detection/Determination of Added Urea in Milk. Indian Dairyman. 1998;50(4):59–62.
- Deibel, C. T., & Deibel, R. H. (2008). Laboratory Analysis of Milk and Dairy Products. In R. C. Chandan, Dairy Processing & Quality Assurance (pp. 529-574). Hoboken, NJ, United States: John Wiley & Sons, Inc.
- DE Souza EMT, Arruda SF, Brandao PO, Siqueira EM, Almeida D. Electrophoretic Analysis to Detect and Quantify Additional Whey in Milk and Dairy Beverages. Ciênc Tecnol Aliment. 2000;20(3):314–7.
- Sharma R. S. R., Barui A. K., Ed. Rapid Methods for Detection of Adulterants in Milk Chemical Analysis of Value Added Dairy Products and Their Quality Assurance. National Dairy Research Institute; 2011.