Common Preservatives in Milk and Methods for Their Detection

Milk is one of the most consumed and nutritionally complete foods in the world – and unfortunately, one of the most frequently adulterated. Because raw milk is highly perishable, unscrupulous suppliers sometimes add chemical preservatives to extend shelf life, cut refrigeration costs, and maximize profits. While some of these substances occur naturally in low concentrations or have limited approved uses, their illegal addition to milk poses serious public health risks. Understanding which preservatives are used, how they work, and how they are detected is essential knowledge for anyone in dairy science, food safety, or quality assurance.

Why preservatives end up in milk 🔗

Raw milk has a very short shelf life – typically around 48 hours when refrigerated below 7°C. Formalin and other chemical preservatives can extend this shelf life by many folds, making them financially attractive options for suppliers managing long distribution chains or lacking cold storage. The problem is particularly acute in developing countries during summer months when temperatures accelerate microbial growth. Though the addition of any preservative to milk is not legally permitted in countries like India, dealers continue to add chemicals such as formaldehyde, hydrogen peroxide, and boric acid to prevent spoilage and sell entire batches without incurring additional business costs.

The most commonly detected preservatives in milk include formalin, boric acid and borates, benzoic acid and sodium benzoate, salicylic acid, mercuric chloride, potassium chromate, and hydrogen peroxide. Each has distinct chemical properties, different mechanisms of antimicrobial action, and specific laboratory tests for detection.

Formalin (formaldehyde) 🔗

Formalin is a 40% aqueous solution of formaldehyde gas (HCHO), with about 10% methanol added to prevent polymerization. It is arguably the most dangerous preservative found in milk. Formaldehyde is classified as a Group 1 carcinogen by the International Agency for Research on Cancer (IARC), with proven potential to cause cancer in humans. It works by reacting with macromolecules such as DNA, RNA, and proteins, forming cross-links that destroy microbial cells – but it does the same to human tissue. Even at low concentrations, formalin can delay digestion and react with food proteins, reducing their digestibility. Higher exposures can cause severe respiratory, liver, and kidney damage.

Detection methods for formalin 🔗

Hehner Test: This is the simplest and most widely used field test for formalin. Formalin reacts with iron in the presence of concentrated sulphuric acid to produce a violet colour. In practice, 2 ml of the milk sample is taken in a test tube, and concentrated H₂SO₄ with traces of ferric chloride is carefully poured down the side. A violet to purple ring forming at the interface of the two liquids confirms the presence of formalin.

Hehner-Fulton Test: This is a variation of the Hehner test performed in acidic medium. Even a very small quantity of formalin can be detected by this test due to the intensity of the violet colouration produced with FeCl₃.

Chromotropic Acid Test: This is a more sensitive confirmatory test. Chromotropic acid – a saturated solution of 1,8-dihydroxynaphthalene-3,6-disulphonic acid in 72% sulphuric acid – reacts with formaldehyde to produce a characteristic purple colour, detectable even at trace concentrations. The colour intensity corresponds to the amount of formalin present.

Boric acid and borates 🔗

Boric acid (H₃BO₃) and its sodium salt, sodium borate (Na₃BO₃ – commonly known as borax), are mild antiseptics historically used in food preservation. In milk, they inhibit microbial growth and act as a pH buffer. However, their regular consumption is far from safe. Boric acid accumulates in the body over time, and even small amounts consumed regularly can build up to dangerous levels, leading to kidney damage, reproductive issues, and developmental problems in children.

Detection of boric acid 🔗

Turmeric Paper Test: This is a quick, inexpensive screening method. A strip of turmeric paper is dipped into the acidified milk sample (pH 1-2). If boric acid is present, the paper turns orange to red upon drying. When subsequently dipped in concentrated sodium hydroxide, it turns green-black, confirming the presence of boric compounds. For quantification, chromatographic methods are used.

Benzoic acid and sodium benzoate 🔗

Benzoic acid is a food-grade preservative that naturally occurs in some fruits. Its sodium salt, sodium benzoate, is more water-soluble and commonly used in food products. The optimum pH range for its antimicrobial activity is 2.5 to 4.0, meaning it works best in acidic conditions. While it is permitted in many processed foods, its unauthorized addition to milk is illegal. Benzoic acid can trigger allergic reactions including asthma in sensitive individuals, and when it combines with vitamin C in the presence of heat and light, it can form benzene, a known carcinogen.

Detection of benzoic acid 🔗

FeCl₃ Test: Benzoic acid is first extracted from milk serum using diethyl ether (as it is soluble in ether but not water). In alkaline medium, benzoic acid reacts with ferric chloride (FeCl₃) to produce ferric benzoate – a salmon-red or buff-coloured precipitate. This simple colour-change test is effective for routine screening.

Modified Mohler Test: In the Modified Mohler Test, the ether extract is treated with concentrated H₂SO₄ and a crystal of potassium nitrate (KNO₃). A red-brown ring is formed at the junction of the two liquids in the presence of benzoic acid. The colour diffuses on mixing and turns greenish-yellow on gentle heating, serving as a useful confirmatory indicator.

Salicylic acid 🔗

Salicylic acid is an organic compound structurally related to aspirin. It is extracted from milk serum using ether, similar to benzoic acid. As a preservative, it works by inhibiting bacterial enzymes essential for microbial growth and reproduction. Though naturally present in some plants, its synthetic addition to milk is illegal. It can cause stomach irritation and allergic reactions, particularly in individuals sensitive to aspirin.

Detection of salicylic acid 🔗

FeCl₃ Test: Once the ether extract residue is dissolved, treatment with ferric chloride gives a violet colour in the presence of salicylic acid. This is because salicylic acid contains both a carboxyl group and a phenolic hydroxyl group – the phenolic portion reacts with FeCl₃ to give the distinctive violet response, distinguishing it clearly from benzoic acid, which gives a salmon-red precipitate under the same conditions.

Mercuric chloride 🔗

Mercuric chloride is a highly toxic heavy metal salt with no acceptable safe exposure level in food products. It is occasionally used as a preservative in milk but is illegal in most countries due to its extreme toxicity. Even trace amounts can cause severe neurological damage and kidney failure. Its use in food products is universally condemned by regulatory bodies.

Detection of mercuric chloride 🔗

Stannous Chloride Test: The milk extract is prepared the same way as for benzoic and salicylic acid – by removing casein and extracting with ether. The residue is dissolved in water and treated with a 15% stannous chloride solution in 1:1 HCl. A silky white precipitate forms in the presence of mercuric chloride, which turns grey upon further addition of the stannous chloride solution, confirming the presence of this heavy metal contaminant.

Potassium chromate 🔗

Potassium chromate differs from most other preservatives on this list in one important respect – it is used as a preservative specifically for the storage of milk samples designated for laboratory analysis, not for milk intended for direct consumption. Its solution appears yellow in colour due to the presence of chromate ions. Like mercuric chloride, chromate compounds are toxic and carcinogenic, and have no place in consumer milk.

Detection of potassium chromate 🔗

Barium Chloride Test: Potassium chromate is detected by a simple test using barium chloride (BaCl₂). When barium chloride is added to the milk ash dissolved in dilute HCl with a drop of acetic acid, yellow precipitates of barium chromate form in the presence of potassium chromate. The bright yellow colour makes this one of the more visually distinctive detection reactions.

Hydrogen peroxide 🔗

Hydrogen peroxide (H₂O₂) is a powerful oxidizing agent that kills bacteria by blocking their metabolic enzymes, preventing multiplication and delaying milk degradation. The use of H₂O₂ to activate the inherent lactoperoxidase enzyme system has historically been used to improve the quality of raw dairy products in regions where refrigeration is not widely available. In the United States, however, its direct addition to fluid milk is not permitted. When added to milk, hydrogen peroxide can decrease nutritional value by destroying vitamins A and E and generating reactive oxygen species that damage nucleic acids, lipids, and proteins. Its consumption has been linked to oxidative stress, gastrointestinal disturbances, and cellular damage.

Detection of hydrogen peroxide 🔗

Peroxide Strip Test: A hydrogen peroxide strip is dipped into the milk sample. The formation of a blue colour confirms the presence of H₂O₂, and the intensity of the colour indicates the approximate concentration.

Vanadium Pentoxide Test: Alternatively, 1g of vanadium pentoxide is dissolved in 100 ml of 6% H₂SO₄. When 10-20 drops of this reagent are added to 10 ml of the milk sample, the formation of a pink or red colour indicates the presence of hydrogen peroxide. This test is particularly useful when strip tests are unavailable.

Why detection matters – and what the science says 🔗

Standard qualitative tests like the Hehner test, turmeric paper test, and FeCl₃ tests are valuable screening tools for field conditions and routine inspection. However, they have limitations in detecting very low concentrations. Advanced techniques such as Near Infrared (NIR) spectroscopy combined with multivariate analysis have been developed to detect and quantify formalin adulteration in milk with high precision. Similarly, gold nanoparticle-based methods using Tollen’s reagent can detect formalin at very low concentrations and are being explored as cost-effective quality control tools. For hydrogen peroxide, paper-based analytical devices using enzyme-catalysed reactions with guaiacol have shown excellent sensitivity down to 0.001% w/w, making them practical for field deployment.

Regulatory bodies like the Food Safety and Standards Authority of India (FSSAI) have published standardized manual methods for the analysis of milk and milk products that include procedures for detecting all the preservatives described above. These standards form the backbone of dairy quality assurance programmes nationwide.

The range of chemical tests – from the simple colour-change reactions used in field screenings to sophisticated spectroscopic methods in certified laboratories – reflects how seriously food safety agencies treat preservative adulteration. Each test is designed around the specific chemical behaviour of the preservative in question, making chemical detection both a science and a practical public health tool.

What do you think? Given that many of these illegal preservatives can only be reliably detected through laboratory testing, how should dairy supply chains be restructured to allow for routine screening at the farm or collection centre level? And with advanced detection methods like NIR spectroscopy becoming more accessible, do you think rapid on-site testing could eventually replace traditional laboratory-based methods for preservative detection in milk?

References

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