Milk quality and adulteration tests
Testing milk at collection and in the lab: platform tests, hygiene and mastitis tests, residues, adulterants like water and urea, and an added water sum.
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Milk passes through many hands between cow and consumer: farmer, collection centre, transporter, chilling plant, processor and retailer. At every step, its quality can fall through poor hygiene, delays without cooling, or illness of the animals, and, sometimes, it can be deliberately adulterated: watered down, or mixed with substances to fake its richness or hide spoilage. Milk testing protects consumers' health, ensures fair payment to honest farmers and keeps dairy plants running.
Milk quality tests and adulteration detection are standard practical topics in dairy technology, food safety and veterinary public health.
Platform tests at collection
Quick tests at the point of collection decide whether to accept a can or tanker:
- Organoleptic inspection: smell, appearance, colour and taste; off-odours, clots or abnormal colour lead to rejection.
- Temperature: milk should normally arrive chilled, often at or below about 4 to 6 °C at bulk collection.
- Clot-on-boiling (COB) test: milk that curdles on heating is too acidic to process.
- Alcohol test: mixing milk with ethanol (often 68 to 75 per cent) shows whether it will withstand heat processing; flocculation indicates developed acidity, colostrum or mastitis milk.
- Lactometer reading, and fat and SNF measurement by Gerber method or electronic milk analysers for payment.
- Sediment test for visible dirt.
Hygiene and health tests
- Total bacterial count (plate count, or rapid methods such as flow cytometry).
- Dye reduction tests: methylene blue reduction time (MBRT) and resazurin; bacteria use up oxygen and reduce the dye, so the faster the colour change, the more bacteria. Long reduction times indicate good-quality milk.
- Somatic cell count (SCC): white blood cells rise in mastitis; many countries and buyers set limits for bulk milk, such as 400,000 cells per mL in the European Union. The cow-side California Mastitis Test gives a quick indication.
- Antibiotic residue tests: rapid screening kits detect residues of drugs such as beta-lactams and tetracyclines, which can cause allergies, contribute to resistance and spoil cheese and yoghurt fermentations.
- Aflatoxin M1 testing, as aflatoxin in cattle feed passes into milk.
Common adulterants and how they are detected
| Adulterant | Purpose | Detection |
|---|---|---|
| Water | increase volume | freezing point, lactometer reading, lower SNF |
| Skim milk or removal of fat | sell fat separately | low fat, higher lactometer reading |
| Starch, flour | restore viscosity and SNF after watering | iodine turns blue-black |
| Cane sugar, glucose | raise lactometer reading | resorcinol (Seliwanoff) test, glucose strips |
| Urea | raise apparent protein (nitrogen) and SNF | p-dimethylaminobenzaldehyde gives a yellow colour |
| Salt | raise lactometer reading | silver nitrate test |
| Neutralisers (sodium carbonate, bicarbonate) | mask acidity of stale milk | rosolic acid test, ash alkalinity |
| Preservatives (formalin, hydrogen peroxide) | prevent spoilage | colour reactions specific to each |
| Detergents | emulsify added vegetable fat | bromocresol purple test |
| Vegetable fat or other animal fats | replace milk fat | fatty acid profile, butyro-refractometer, Reichert–Meissl value |
| Melamine | fake protein content | chromatography and mass spectrometry |
The 2008 melamine scandal in China, in which melamine was added to milk and infant formula to inflate protein tests, sickened around 300,000 infants and killed several. It led to stricter controls worldwide and showed why protein tests based only on nitrogen can be fooled.
Paying for quality
Many dairies pay farmers by fat and SNF (or fat and protein), with bonuses for low bacterial counts and somatic cell counts and penalties or rejection for antibiotic residues or adulteration. Paying for quality rewards clean, cooled, honest milk and makes adulteration far less attractive than improving herd management and milking hygiene.
Laboratory and modern methods
- Mid-infrared (MIR) milk analysers measure fat, protein, lactose and solids in seconds, and can flag unusual spectra suggesting adulteration.
- Cryoscopes measure freezing point precisely.
- Chromatography, mass spectrometry and DNA tests detect specific adulterants and the species of origin (such as cow's milk in "pure" buffalo or goat milk).
How much water was added?
Genuine cow's milk in a region has an average freezing point of −0.525 °C. A tanker sample freezes at −0.472 °C.
Percentage of added water ≈ (normal freezing point − sample freezing point) ÷ normal freezing point × 100, using the depressions below 0 °C:
- = (0.525 − 0.472) ÷ 0.525 × 100 = 0.053 ÷ 0.525 × 100 ≈ 10.1 per cent added water.
Cross-check with SNF. If genuine milk from these farms averages 8.6 per cent SNF, 10 per cent dilution would lower it to about 8.6 × 0.90 ≈ 7.7 per cent. A sample showing both a high freezing point and low SNF supports the finding. Because freezing points vary slightly with breed, season and feed, many standards allow a small tolerance before declaring water added.
Key points
- Platform tests: organoleptic, temperature, clot-on-boiling, alcohol, lactometer, fat and SNF, sediment.
- Hygiene tests: plate counts, MBRT and resazurin dye reduction, somatic cell counts (and CMT), antibiotic residues, aflatoxin M1.
- Adulterants include water, starch, sugar, urea, salt, neutralisers, preservatives, detergents, foreign fats and melamine; each has specific tests.
- Freezing point (about −0.525 °C for cow's milk) is the best test for added water.
- MIR analysers, cryoscopes, chromatography and DNA tests support modern quality control.
What the exams ask
- School-leaving and entrance papers: common milk adulterants and simple tests to detect them.
- Degree and research-fellowship entrances: platform tests, MBRT and resazurin, somatic cell counts, freezing point and added water calculations, and specific chemical tests for adulterants.
- Dairy technology and food safety examinations: quality payment systems, residue monitoring, legal standards and modern analytical methods.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Can I test milk for water at home?
A simple lactometer gives a rough indication, and a drop of milk that runs quickly on a slanting surface without leaving a trail may be watered. Reliable detection needs freezing point measurement.
Why is urea added to milk?
It raises the measured nitrogen and solids-not-fat, making watered milk appear richer. Urea occurs naturally in small amounts, so tests look for levels well above normal.
Why are antibiotic residues a problem in milk?
They can cause allergic reactions, contribute to antimicrobial resistance and stop starter cultures from working in cheese and yoghurt. Milk from treated cows must be withheld for the stated withdrawal period.
Sources
- Codex Alimentarius Commission. Code of Hygienic Practice for Milk and Milk Products (CXC 57-2004).
- International Dairy Federation and ISO. ISO 5764 | IDF 108: Milk: Determination of Freezing Point: Thermistor Cryoscope Method. ISO, 2009.
- Handford, C. E., Campbell, K. and Elliott, C. T. "Impacts of milk fraud on food safety and nutrition with special emphasis on developing countries." Comprehensive Reviews in Food Science and Food Safety 15, 130–142 (2016).
- Gossner, C. M.-E. et al. "The melamine incident: implications for international food and feed safety." Environmental Health Perspectives 117, 1803–1808 (2009).
- Regulation (EC) No 853/2004 laying down specific hygiene rules for food of animal origin. European Union, 2004.
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