Milky melamine

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  • Published: Sep 1, 2010
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: Milky melamine

Chinese scientists have published in the journal Talanta details of a quick and simply UV-Vis colour change technique for detecting melamine contamination in milk products. The test might help prevent future fraud using this toxic compound.

Melamine was in the news in 2004, 2008 and again in 2009 when it was found as a deliberate contaminant in baby milk formula in China, pet food in the USA when it was blamed for thousands of pet deaths, and then again in baby milk products. In the last episode, several hundred thousand infants were affected and several infants are known to have died; more than a dozen in 2004 and six in 2009.

Melamine is an organic compound, a base, with the formula C3H6N6, 1,3,5-triazine-2,4,6-triamine. It is produced as a white, crystalline powder, is only slightly soluble in water, and is used in fire retardant polymer resins. Its high nitrogen content releases nitrogen gas when it burns, stifling flames. The compound is also used in melamine-formaldehyde resin and nitrogen-rich agricultural fertilizer. Although it is banned as a food additive, it is known that excessive ingestion of melamine will cause insoluble melamine cyanurate crystals to form in the kidneys potentially leading to acute renal failure.

Following the adulteration incidents, a safety limit for melamine of 2.5 milligrams per kilogram in milk and milk-based products was instigated by the US Food and Drug Administration and European Union regulatory authorities. The maximum residue level of melamine allowed in infant formula and other dairy products in China is now 1 and 2.5 milligrams per kilogram. "There is an urgent need for establishing a rapid and reliable analytical method, which is capable of detecting melamine in milk-based products at level desired by regulatory authorities," researchers say.

It is the high nitrogen level - 66% nitrogen by mass - of melamine that gives it the analytical characteristics of protein molecules in some simple tests for protein used in the food industry. There is no excuse for spoofing proteins with a potentially lethal compound. Diluting a product, the usual approach to food fraud, is highly unethical and causes malnutrition, but adding melamine is poisoning.

Currently, liquid chromatography-tandem mass spectrometry, gas chromatography-mass spectrometry, capillary zone electrophoresis, micellar electrokinetic chromatography, capillary electrophoresis with diode array detection, high performance liquid chromatography with UV detection, chemiluminescence, infrared spectroscopy, Raman spectrometry, fluorescence and spectrophotometric absorption have all been tried as approaches to melamine detection. They have been successful to varying degrees and all of those techniques are highly sensitive, but are at the same time time-consuming and labour-intensive given the pre-treatment commonly required.

Now, Chinese researchers have developed a colour test for melamine in milk using label-free gold nanoparticles. Liangqia Guo, Jianhai Zhong, Jinmei Wu, FengFu Fu, Guonan Chen, Xiaoyan Zheng and Song Lin of the Ministry of Education Key Laboratory of Analysis and Detection for Food Safety at Fuzhou University, Fujian Provincial Key Laboratory of Analysis and Detection Technology for Food Safety, in Fuzhou, and Fujian Provincial Central Inspection Institute, describe their test as a "simple, rapid, field-portable colorimetric method for the detection of melamine."

The test works as melamine can induce the aggregation of gold nanoparticles NPs causing them to change colour from a wine-red to purple, a sufficiently distinct colour change for colorimetric detection. "The proposed method can be used to detect melamine in liquid milk and infant formula with a detection limit of 1.0 and 4.2 ppm, respectively," the team says. Importantly, it works very quickly, the colour change is complete within 30 minutes, and is visible to the naked eye.

Obviously, a colour change could be detected spectroscopically in an automated system. Indeed, detection can be as low as 0.15 ppm of melamine in liquid milk and 2.5 ppm of melamine in infant formula when using UV-vis spectroscopy, the team says. The approach also has the big advantage of requiring no complex pre-treatment of the sample.



Credit: Fu et al, Talanta/Elsevier
Melamine colour testing

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