Methods for Testing Milk Quality

Every litre of milk that enters a dairy processing plant must pass a series of quality checks before it is accepted for further use. These are called platform tests – rapid, on-the-spot evaluations conducted at the milk collection point to decide whether incoming milk is fit for processing. From checking heat stability to detecting hidden adulterants, each test addresses a specific quality concern. Understanding these tests is fundamental to ensuring that only safe, high-quality milk reaches consumers.

Why milk testing matters at the platform 🔗

Raw milk is highly perishable and can degrade rapidly due to bacterial activity, improper storage, or deliberate adulteration. According to DairyPulse, quality assurance for milk and milk products requires a concerted effort from all stakeholders across the dairy value chain, because dairy products are highly susceptible to bacterial contamination and spoilage. Platform tests provide immediate, low-cost results that allow processors to accept or reject milk before it contaminates an entire batch. More detailed confirmatory chemical analysis – measuring titratable acidity, fat content, and adulterant presence – follows once the milk is received into the processing facility.

Clot on Boiling (COB) test 🔗

The Clot on Boiling test is one of the oldest and most straightforward methods for assessing milk stability. The procedure is simple: a small amount of milk is heated to boiling in a test tube or spoon, and the sample is observed for any coagulation, curdling, or precipitation.

What the COB test detects 🔗

The COB test is specifically designed to identify milk with excessively high acidity. As milk ages, bacteria convert lactose into lactic acid, increasing the overall acidity. According to Unacademy’s dairy microbiology resource, when developed acidity exceeds 0.20-0.26% lactic acid, the proteins become unstable under heat and separate out as “calcium caseinate salt,” causing visible coagulation during boiling. The COB test is therefore also referred to as the heat stability test or coagulation test.

A negative COB result – where no clotting occurs – indicates that acidity is within acceptable limits and the milk is stable for heat processing. A positive result (visible curd or clot formation) signals that the milk has developed excessive acidity and must be rejected for processing. It is worth noting that this test cannot detect heavy bacterial contamination in freshly drawn milk when acidity is still below the 0.20-0.26% threshold, which is why it is used alongside other tests.

The alcohol test 🔗

The alcohol test is a more sensitive alternative to the COB test and is particularly important for products that undergo intense heat treatment, such as UHT milk, evaporated milk, and milk powders. As noted by Safe Milk Labs, dairy industries typically use three different concentrations of ethanol – 68% v/v, 65% v/v, and 60% v/v – depending on the intended use of the milk, with milk that passes the 68% ethanol test considered of superior quality.

How the alcohol test works 🔗

Equal volumes of milk and ethyl alcohol solution are mixed together and immediately observed for coagulation. DairyPulse explains that alcohol acts as a dehydrating agent, removing water from the system and disrupting the hydration forces that keep milk proteins suspended. If the milk already has elevated acidity due to microbial activity, the proteins flocculate due to the combined effect of hydration force destabilization and charge neutralization. If the tested milk is of good quality, there will be no coagulation, clotting, or precipitation.

Beyond acidity, the test also detects other causes of protein instability. The Dairy Knowledge Portal notes that milk with high developed acidity, or having calcium and magnesium compounds in greater than normal amounts, will coagulate when alcohol is added. Increased albumin levels (as in colostrum milk) and elevated salt concentrates (as in mastitis milk) can also trigger a positive result. A variant called the alcohol alizarin test – where the dye alizarin is added alongside alcohol – provides additional information by indicating the actual acidity level through a colour change.

Titratable acidity test 🔗

While the COB and alcohol tests give pass/fail results, the titratable acidity test provides a quantitative measure of the total acid content in milk. This makes it a more precise diagnostic tool, especially for monitoring milk quality over storage or controlling fermentation in products like yoghurt.

Principle and procedure 🔗

According to AgriMoon, the test is based on the principle that an acid of a known volume and strength will completely neutralize an alkali with equal volume and strength. A measured milk sample is taken in a conical flask, distilled water is added, and a phenolphthalein indicator is introduced. The sample is then titrated against 0.1 N sodium hydroxide (NaOH) until a faint pink colour appears and persists. The volume of NaOH used is directly proportional to the acidity, expressed as percentage lactic acid.

Interpreting the results 🔗

AgriMoon’s titratable acidity guidelines indicate that normal milk acidity ranges from 0.10% to 0.20% lactic acid. A value below 0.12% may suggest neutralization with alkaline substances, while any value above 0.20% indicates developed acidity from bacterial action on lactose – a sign of spoilage. The IGNOU eGyanKosh dairy technology course material specifies the standard acidity range for fresh milk as 0.14% to 0.16%, with values below 0.12% suggesting possible adulteration with neutralizers and values above 0.16% indicating bacterial development of acidity.

Beyond freshness assessment, titratable acidity is also used to monitor storage conditions – since microorganisms produce acidity when milk is stored in conditions that favour their growth – and to control manufacturing processes where a specific acidity level must be achieved, such as during yoghurt fermentation.

Testing for preservatives and adulterants 🔗

Research published in Scientific Reports identifies milk as one of the most adulterated foods in developing countries. Adulterants are added for economic gain – to increase volume, improve apparent quality parameters, or extend shelf life. The most dangerous of these are chemical preservatives and thickeners that pose direct health risks to consumers.

Hydrogen peroxide 🔗

Hydrogen peroxide is sometimes added to milk as an unauthorized preservative to extend its shelf life. A review in Springer’s Food Safety and Risk journal confirms that peroxides in milk can cause gastrointestinal complications, including gastritis and intestinal inflammation. To detect hydrogen peroxide, paraphenylene diamine (3 drops) is added to 5 ml of milk and shaken; a change in colour to blue confirms its presence. Alternatively, adding 10-15 drops of vanadium pentoxide reagent to a milk sample will produce a pink or red colour if hydrogen peroxide is present.

Formaldehyde (formalin) 🔗

Formaldehyde, commonly used as formalin (40% solution), is added to milk to preserve it for extended periods. Auriga Research notes that formalin is highly toxic and causes damaging effects on the liver and kidneys. For detection, concentrated sulphuric acid is carefully poured along the side of a test tube containing 10 ml of milk, without shaking. If a violet or blue ring appears at the intersection of the two liquid layers, formaldehyde is present. This test requires careful handling due to the corrosive nature of concentrated sulphuric acid.

Starch 🔗

Starch – along with wheat flour, arrowroot, and rice flour – is added to milk to increase the solid-not-fat (SNF) content and to restore the apparent thickness of milk that has been diluted with water. Detection is straightforward: 3 ml of milk is boiled in a test tube, cooled to room temperature, and 2-3 drops of 1% iodine solution are added. The appearance of a blue colour confirms the presence of starch, due to the well-known starch-iodine complex formation. The blue colour disappears on boiling and reappears on cooling, which can be used as a confirmatory observation. According to research published in PMC, excessive starch in milk can cause diarrhoea due to the effects of undigested starch in the colon.

How these tests fit into a quality assurance system 🔗

Each of these tests occupies a specific position within the milk quality assurance chain. The COB and alcohol tests are conducted rapidly at the platform during milk receipt to make immediate acceptance or rejection decisions. Titratable acidity provides a more precise, quantitative backup during laboratory analysis after the milk has been received. Adulterant and preservative detection tests are used either as routine checks or when suspicious samples are flagged by platform tests or organoleptic inspection.

Together, they form a layered quality control system – moving from rapid screening to detailed confirmation. No single test can cover all quality concerns, which is why dairy processors use this combination of methods. Regulatory bodies such as the Food Safety and Standards Authority of India (FSSAI) and international agencies like the World Health Organization (WHO) set acceptable limits for acidity, adulterant concentrations, and preservative levels, and these tests are the primary tools for ensuring compliance.

What do you think? Given that platform tests like the COB and alcohol tests have been used for decades, are they sufficient on their own to guarantee milk safety, or should more sensitive chemical tests be made mandatory at the collection point itself? And with adulterants like formaldehyde and hydrogen peroxide posing serious health risks, how can smaller dairy cooperatives in rural areas be equipped with accessible, low-cost testing tools to protect consumers?

References

1. https://dairypulse.org/milk-quality-tests/
2. https://unacademy.com/content/kerala-psc/study-material/dairy-microbiology/clot-on-boiling-test/
3. https://safemilklabs.com/alcohol-test-a-vital-tool-for-assessing-raw-milk-quality/
4. https://dairypulse.org/blog/article/quality-tests-milk-b15/
5. https://www.dairyknowledge.in/dkp/article/alcohol-test-assessing-raw-milk-quality
6. https://agrimoon.com/acidity-test/
7. https://egyankosh.ac.in/bitstream/123456789/9633/1/Unit-7.pdf
8. https://www.nature.com/articles/s41598-022-17851-3
9. https://link.springer.com/article/10.1186/s40550-016-0045-3
10. https://aurigaresearch.com/milk-adulteration-how-to-check-adulteration/
11. https://pmc.ncbi.nlm.nih.gov/articles/PMC9372070/
12. https://www.fssai.gov.in/
13. https://www.who.int/news-room/fact-sheets/detail/food-safety