Identifying Common Milk Adulterants and Preservatives

Milk is one of the most consumed and nutritionally complete foods in the world – and unfortunately, one of the most frequently adulterated ones. According to the Food Safety and Standards Authority of India (FSSAI), it prescribes specific standards for all types of milk to ensure quality and consumer safety. Yet adulteration remains widespread, driven largely by economic incentives – diluting milk with water and then adding cheap chemicals to restore its appearance, texture, and shelf life. Understanding how these adulterants and preservatives are chemically detected is essential for quality assurance professionals, food technologists, and anyone involved in dairy production and testing.

Why adulterants and preservatives end up in milk 🔗

Milk is highly perishable. Its rich nutrient content makes it an ideal environment for bacterial growth, which pushes some producers toward adding chemical preservatives to extend shelf life. Others add adulterants to increase volume, boost apparent protein or fat readings, or mask the flavour changes that come with dilution. Research published in the International Journal of Food Contamination identifies three broad categories of adulterants: volume extenders such as water, starch, and urea; preservatives and spoilage cover-ups such as formaldehyde, hydrogen peroxide, and boric acid; and nutritional manipulators like non-milk proteins and vegetable oils used to mimic fat and protein content.

As documented in Scientific Reports, milk is contaminated with a broad range of substances including urea, melamine, detergents, boric acid, formalin, ammonium sulphate, soaps, salt, neutralisers, maltodextrin, starch, sugars, hydrogen peroxide, caramel, and water. The health consequences are serious – ranging from gastrointestinal disorders and kidney overload to cancer risk with chronic exposure.

Detection of preservatives in milk 🔗

Preservatives are added to slow down microbial spoilage, but their use in milk is not legally permitted. The FSSAI’s Manual of Methods of Analysis of Foods outlines official chemical tests for each of these preservatives. Each test relies on specific reagents that produce characteristic and observable colour changes.

Hydrogen peroxide 🔗

Hydrogen peroxide (H₂O₂) is added to milk to extend its shelf life by inhibiting microbial growth. According to FSSAI guidelines cited in Scientific Reports, the maximum residue limit for hydrogen peroxide in milk is 0.05% v/v. Its detection involves adding a testing reagent – typically paraphenylenediamine or a peroxidase-based reagent – to the milk sample. A characteristic blue coloration confirms the presence of hydrogen peroxide. Even at low concentrations, peroxides can cause gastritis and intestinal inflammation due to their oxidative effects.

Hypochlorites 🔗

Hypochlorites (bleaching agents) are sometimes used as sanitisers and may contaminate milk if equipment is inadequately rinsed, or they may be added deliberately to suppress bacterial counts. The standard detection procedure uses starch-iodide paper or ortho-tolidine reagent. A positive result produces a blue or greenish colour, signalling the presence of free chlorine compounds. The FSSAI manual classifies detection of hypochlorites and chloramines as part of routine preservative screening in liquid milk.

Formaldehyde (formalin) 🔗

Formaldehyde is perhaps the most dangerous chemical adulterant detected in milk. As reviewed in a ScienceDirect article on milk adulterant detection, formalin is toxic, potentially carcinogenic, and causes damage to the liver, kidneys, and respiratory system. Its detection uses Hehner’s test: concentrated sulphuric acid is carefully added along the side of a test tube containing milk. A violet or purple ring at the junction of the two liquids confirms the presence of formaldehyde. Detection requires careful handling because both the adulterant and the testing reagent are hazardous.

Boric acid and borates 🔗

Boric acid is used as both a preservative and a pH buffer, but it accumulates in the body over time and can cause kidney damage and developmental issues with chronic consumption. The turmeric paper test is the standard method: a strip of turmeric-impregnated filter paper is dipped into acidified milk. According to the FSSAI official manual, the appearance of a characteristic red colour on the turmeric paper indicates the presence of boric acid or borates. Boric acid acts on the curcumin in turmeric to produce this distinct colour change.

Detection of common adulterants 🔗

Beyond preservatives, a range of chemical adulterants is added to milk to manipulate its physical and compositional parameters – particularly solid-not-fat (SNF) content, density, and apparent protein levels. The review by Azad and Ahmed in the International Journal of Food Contamination notes that cane sugar, starch, sulphate salts, urea, and common salt are all added specifically to inflate SNF readings – a dishonest way of appearing to meet quality benchmarks.

Starch 🔗

Starch is added to milk that has been diluted with water to restore its thickness and viscosity. Its detection is straightforward: a few drops of iodine solution are added to the milk sample. If starch is present, a distinct blue-black coloration appears immediately. This reaction occurs because iodine molecules become trapped within the helical structure of starch polymers, producing the characteristic colour. Research notes that excessive starch in milk can cause diarrhoea and may be particularly dangerous for diabetic patients due to accumulated undigested starch.

Sugar (sucrose) 🔗

Sucrose is naturally absent from milk, so its presence is a direct indicator of adulteration. It is added to increase SNF content or to mask the altered taste of diluted milk. Detection uses the resorcinol (Seliwanoff’s) test: the milk sample is treated with hydrochloric acid and resorcinol reagent, then boiled for a few minutes. A red or brick-red coloration confirms the presence of added cane sugar. The reaction is based on the conversion of fructose – produced by the acid hydrolysis of sucrose – into a red-coloured compound with resorcinol.

Salt (sodium chloride) 🔗

Salt is added to milk to raise the lactometer reading in already-diluted milk, effectively masking the dilution. As per FSSAI norms cited in Scientific Reports, milk should not contain any added salt. Detection uses a silver nitrate and potassium dichromate method: a few drops of potassium dichromate are added to silver nitrate solution, followed by the milk sample. If the colour turns yellow (instead of remaining brick-red), added salt is confirmed. The presence of excess chloride ions precipitates silver chloride, altering the colour reaction.

Urea 🔗

Urea is a nitrogen-containing compound added to increase non-protein nitrogen levels, making diluted milk appear protein-rich in standard tests. According to the FSSAI Act 2006 and PFA Rules 1955, the maximum allowable limit for urea in milk is 70 mg/100 mL, since urea is a natural constituent of raw milk in small amounts. Detection uses para-dimethylaminobenzaldehyde (DMAB) reagent: when added to a milk sample, it produces a yellow colour in the presence of excess urea. As reviewed in ACS Omega, urease and bromothymol-based chemical methods are also commonly used. Excess urea in consumed milk places an additional burden on the kidneys, which must filter out the elevated nitrogen load.

Maltodextrins 🔗

Maltodextrin – a partially hydrolysed starch product – is added to milk to increase its density and SNF content at a low cost. Its detection relies on iodine-based tests similar to starch, but because maltodextrin is a partially degraded starch, the colour reaction may range from blue-grey to reddish-brown depending on the degree of hydrolysis. More precise detection can be achieved through high-performance liquid chromatography (HPLC). A review in ScienceDirect notes that chromatographic methods offer selective identification of multiple adulterants including maltodextrins.

Mineral oil 🔗

Mineral oil is occasionally found as an adulterant, added to increase fat content readings. Its detection involves the Bellier test or Sudan III dye test, where milk fat is extracted and treated with the reagent. The presence of mineral oil produces a characteristic colour that is absent in pure milk fat. Since mineral oil is not digestible and may contain toxic hydrocarbons, its presence in food is entirely prohibited by food safety regulations.

Health consequences of adulterated milk 🔗

A 2025 review in Food Chemistry confirms that fraudulent practices involving preservatives, non-milk fats, thickening agents, and nitrogen-based compounds pose severe health risks including kidney failure, gastrointestinal disorders, and long-term toxicity. The same body of research highlights that both peroxides and detergents can lead to gastritis and intestinal inflammation, while urea in excessive amounts overloads kidney function. For infants and children, who are the primary consumers of milk, even small quantities of toxic preservatives like formaldehyde or hypochlorites carry disproportionate risk.

International bodies including the WHO, FDA, and the European Food Safety Authority (EFSA) have established strict food safety standards and monitoring frameworks to protect consumer health and ensure milk quality compliance globally.

From simple tests to advanced methods 🔗

As noted in Scientific Reports, traditional laboratory techniques like the lactometer density test, freezing point test, Kjeldahl protein test, and Gerber fat test have long been used to characterise milk quality – but they are unable to detect the majority of chemical adulterants. For that, qualitative chemical tests based on colour reactions remain the first line of analysis in most dairy quality control labs because they are rapid, affordable, and require no expensive instruments.

For more precise and quantitative work, advanced techniques documented by ScienceDirect include high-performance liquid chromatography (HPLC), gas chromatography-mass spectrometry (GC-MS), Fourier transform infrared (FTIR) spectroscopy, near-infrared (NIR) spectroscopy, immunological methods like ELISA, and biosensors capable of real-time detection. These instruments offer sensitivity, speed, and the ability to quantify multiple adulterants simultaneously – though they require trained personnel and are less accessible in field settings.

The development of paper-based microfluidic devices and portable test kits – including the 3D paper-based device reported in Scientific Reports that can simultaneously detect seven adulterants including urea, detergents, soap, starch, hydrogen peroxide, sodium bicarbonate, and salt – represents an important step toward making detection accessible at every point in the supply chain, from the farm gate to the consumer’s kitchen.

What do you think? With so many chemical adulterants possible in a single glass of milk, should routine adulteration testing be made mandatory at the point of sale for all retail dairy suppliers – not just large processors? And given that colorimetric tests already exist for most common adulterants, what do you think is the biggest barrier to wider adoption of on-the-spot testing in developing countries?

References

1. https://pmc.ncbi.nlm.nih.gov/articles/PMC11482397/
2. https://link.springer.com/article/10.1186/s40550-016-0045-3
3. https://www.nature.com/articles/s41598-022-17851-3
4. https://www.fssai.gov.in/upload/uploadfiles/files/MILK_AND_MILK_PRODUCTS.pdf
5. https://www.sciencedirect.com/science/article/pii/S2214180421000386
6. https://pubs.acs.org/doi/10.1021/acsomega.4c01274
7. https://www.sciencedirect.com/science/article/abs/pii/S0308814625014530
8. https://pmc.ncbi.nlm.nih.gov/articles/PMC9372070/