What is melamine?
Melamine is a synthetic triazine compound and an organic base (C3N6H6, CAS number 108-78-1) with the chemical name 2,4,6-triamino-1,3,5-triazine. It is an important industrial chemical and has been used as a component in various products since the late 1930s. The best known use of melamine is in combination with formaldehyde to produce melamine resin, a very hard-wearing thermo-setting plastic. Melamine resin has been widely used to make durable tableware, worktops and whiteboards. Melamine is important in a number of other applications, including fire retardants, fertilizers, pigments and glues.
Melamine also has analogues produced by successive deamination reactions. These are ammeline, ammelide and cyanuric acid. It is now generally accepted that melamine in food, especially in combination with an analogue, can have potentially serious health consequences for animals and for humans.
What foods can be contaminated?
Melamine contamination in food first became a food safety issue when the chemical was detected in pet foods linked to kidney failure in thousands of dogs and cats in North America in 2007. However, since then it has become apparent that a similar incident affecting an estimated 6,000 dogs in Asia in 2004 and first attributed to mycotoxin contamination, was also likely to have been caused by melamine. Other earlier incidents in Asia, Europe and South Africa have also since come to light. An investigation of the 2007 incident found that melamine and its analogue cyanuric acid were present in wheat gluten and rice protein concentrate imported from China by the pet food producer and used as a thickening and binding ingredient. Levels of between 0.2% and 8% were detected in batches of the two ingredients. This would correspond to a dose of around 400 mg/kg per day for animals fed with the contaminated pet food. Since then, melamine has been found in animal feed samples and in animal tissues at low concentrations. It has also been found in beverages, including coffee and orange juice, at levels of up to 2 mg/kg, but this is thought to be as a result of migration from plastic cups at high temperatures. Very low levels of melamine are thought to be occasionally present in some processed foods as a result of migration from packaging or processing equipment. It is also possible that melamine could be generated at very low levels as a by-product of processing.
In 2008, melamine was found in dairy products from China, especially powdered milk used to make infant formula associated with widespread kidney disease in babies. Samples of dairy products (including infant formula) were reported to contain melamine at concentrations between 0.09 mg/kg and 6,196 mg/kg. Melamine has also been found in liquid milk in China (highest concentration 8.6 mg/kg) and a wide variety of other products made using Chinese sourced milk powder. These include chocolate and milk-based confectionery, biscuits and other bakery products, coffee and tea whiteners and milk-based beverages. It was also detected in Chinese fresh eggs at concentrations of 3.1 to 4.7 mg/kg. Contaminated foods have been found all over the world, particularly in other Asian countries, but also in the EU, the USA, Canada and Australia. Reported contamination levels were very variable, ranging from 0.38 mg/kg to 945 mg/kg in dairy products and from 0.6 mg/kg to 6,694 mg/kg in processed foods and food ingredients. A great many food products were withdrawn from sale as a result of melamine contamination.
There is little information on the likely dietary exposure that would result from such levels in processed foods, but the European Food Safety Authority (EFSA) has estimated that chocolate with high levels of contaminated milk powder could result in an exposure of 1.35 mg/kg bodyweight per day, more than six times the current World Health Organisation tolerable daily intake (TDI) of 0.2 mg/kg bodyweight for children and adults. It is thought that infants in China, fed exclusively on contaminated formula, could have been exposed to melamine at levels up to 200 times higher than the TDI.
How does it affect human health?
Melamine and its analogues are not particularly toxic compounds when considered individually. An oral LD50 of more than 3,000 mg/kg has been reported for rats. Both animals and infants affected by melamine-contaminated foods suffered from kidney damage, especially kidney stones and in some cases kidney failure. However, toxicology studies have shown no acute renal toxicity caused by melamine, although high doses have diuretic properties in animals. It is not genotoxic, or teratogenic, and does not cause skin irritation except at high doses. However, some animal studies of chronic toxicity have shown that kidney and bladder calculi (stones) can form when high levels (1% or more) of melamine are included in the diet over a long period. These calculi can result in bladder cancer in rats as a result of irritation.
Following the widespread deaths among dogs and cats fed with melamine-contaminated feed in 2007, an investigation into the toxic mechanism involved was undertaken. The results of this study suggest that the effects observed in animals were caused by the formation of crystals in the urine, leading to kidney and bladder calculi, blocking the renal tubules in severe cases and causing potentially fatal kidney failure. The crystals were composed of a stable, insoluble melamine/cyanuric acid complex, which formed a lattice-like structure held together by hydrogen bonds. It has been reported that the two compounds are absorbed separately in different regions of the gut, because melamine is a base and has a much lower pKa value than that of cyanuric acid. The insoluble complex is then thought to form and produce the crystals within the kidneys.
The mechanism responsible for the kidney problems experienced by infants fed on contaminated formula in China during 2007-2008 is now thought to be somewhat different. Cyanuric acid was not present in significant quantities in the urinary tract calculi obtained from the affected children. Instead the stones were found to consist of melamine and uric acid. It is thought that these stones formed in infants rather than adults because they typically have higher levels of uric acid in their urine. There is still some uncertainty about the exact toxicological mechanisms involved. The incident is reported to have affected at least 294,000 children (source WHO). Some 51,900 of these required hospital treatment and at least six deaths have been associated with the contamination.
EFSA initially applied a TDI of 0.5 mg/kg bodyweight for melamine, based on the data available, but this figure was reduced in 2010 to 0.2 mg/kg bodyweight in line with the WHO TDI set in 2008. WHO has also recommended a TDI for cyanuric acid of 1.5 mg/kg bodyweight.
Where does it come from?
Melamine is reported to be widely available in China as a by-product of the plastics industry. Media reports suggest that it was added to certain food ingredients and to milk because of its very high nitrogen content. This would give a falsely high result in tests designed to determine protein content and cause the material to be assigned a higher quality rating and commercial value. It has been estimated that the addition of 1g of melamine to 1 litre of milk would raise the apparent protein content by approximately 0.4%. If this is indeed the case, then melamine is an adulterant and has been deliberately added to milk, wheat gluten and rice protein concentrate in a fraudulent attempt to increase profits and disguise watered-down, or poor quality products.
Melamine in food may also come from other sources, especially plastic packaging, or processing equipment, but usually only at levels not harmful to health. It is also produced in animals as a metabolite of the insecticide cyromazine, which is widely used to prevent insect damage to fruit and vegetables.
Is it stable in foods?
There is little information as yet as to the stability of the melamine/cyanuric acid complex, but its poor solubility and its survival at high temperatures during pet food processing and powdered milk production suggests that it is relatively stable. However, the complex is thought to dissociate at low pH.
How can it be controlled?
Since melamine at detectable levels is likely to have been added as an adulterant, its presence should not be acceptable under any circumstances.
For food processors
Since melamine contamination appears at present to be associated mainly with food ingredients from China, food manufacturers should exercise caution when sourcing ingredients. Traceability to the point of origin is essential. Materials such as milk powder, dried egg products and high-protein ingredients should be purchased only from known low-risk sources.
The only practical control for melamine in foods at present, other than careful sourcing, is testing and analysis of all ingredients that carry a risk of contamination. A number of chemical methods have been developed, based on gas chromatography or HPLC. However, both the European Commission and the FDA recommend a gas chromatography – mass spectrometry method for the analysis of melamine in foods. A number of commercial laboratories can analyse samples for clients using this method.
Recently, a method based on enzyme-linked immunoassay (ELISA) has been developed and is available commercially. This is suitable for screening ingredients and can be carried out by smaller laboratories.
Are there rules and regulations?
In Europe, melamine can be used as a component in plastics and has been assigned a specific migration limit of 30 mg/kg food for materials in direct contact with foodstuffs.
Melamine is not a permitted additive or ingredient in food and therefore limits have not been set in food legislation before now. However, following the incident in China both the European Commission and the US FDA have applied a maximum acceptable limit of 2.5 mg/kg for melamine in imported foods, particularly foods containing powdered milk from China, and 1 mg/kg in infant formula. In 2010, the Codex Alimentarius Commission adopted the same recommended limits.
The legislation position could change as more information becomes available and should be regularly reviewed.
Where can I learn more?
WHO/FAO expert meeting to review toxicological aspects of melamine and cyanuric acid, 1-4 December 2008 (full report)
FDA – The melamine story (video link)