Milky, milky: Melamine test

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  • Published: Jun 1, 2014
  • Author: David Bradley
  • Channels: Raman
thumbnail image: Milky, milky: Melamine test

Organic nitrogen of the fraudulent kind

Rapid and sensitive detection of melamine in milk with gold nanoparticles by Surface Enhanced Raman Scattering

The toxic, nitrogen-rich compound melamine is known to have been added fraudulently to food products, including pet food and baby milk. The reason being that it is a nitrogen-rich organic molecule and can therefore spoof the high-nitrogen component of proteins without the expense of the product actually containing nutritious protein, it then gives a false signal in food quality tests that determine protein levels based only on nitrogen concentration. Now, Italian researchers have developed a novel SERS, surface enhanced Raman spectroscopy, technique to reveal the presence of melamine as an adulterant in foodstuffs quickly, accurately and easily.

SpectroscopyNOW has previously reported on interesting techniques for the determination of melamine. The problem came to a head, first in the USA when pet food was found to be adulterated and then in a food health scare in China when many babies were hospitalised having inadvertently been fed adulterated baby milk products. Several hundred thousand infants were affected and several infants are known to have died. Health authorities and food agencies are now alert to the potential for products to be adulterated, but there is always a need for an improved, faster and more accurate test for this compound in food and drink products destined for human or animal consumption. The Codex Alimentarius Commission sets a safety tolerance of 1 milligram per litre of powder infant milk "formula" and 2.5 milligrams per litre for other foods and animal feed.

Flame retardant food adulterant

Melamine, 1,3,5-triazine-2,4,6-triamine, is a base produced as a white, crystalline powder, that is only slightly soluble in water, and is used in fire-retarding polymer resins. Its high nitrogen content releases nitrogen gas when it burns, stifling flames. Melamine is also used in melamine-formaldehyde resin and nitrogen-rich agricultural fertilizers. Obviously, it is banned as a food additive, not least because it is a potentially lethal renal poison causing acute kidney failure as it releases cyanurate crystals in the kidneys.

Andrea Mario Giovannozzi, Francesca Rolle, Michela Sega and Andrea Mario Rossi of the Thermodynamic Division, at the Istituto Nazionale di Ricerca Metrologica, in Turin, Italy and Maria Cesarina Abete and Daniela Marchis of the National Reference Centre for the Surveillance and Monitoring of Animal Feed, also in Turin, have exploited the selective binding of gold nanoparticles (AuNPs) with this analyte to allow them to develop a test based on SERS.

Viable analytical alternative

The interaction between melamine and AuNPs causes them to aggregate, which allows Raman hot spots to form and thus induces a very large enhancement in the signal due to any melamine in a sample. The team used an external standard calibration method to quantify and validate the method for linearity, sensitivity, repeatability and recovery. The researchers, writing in the journal Food Chemistry, report good linearity at melamine concentrations in the range 0.31 to 5.0 milligrams per litre in milk with a limit of detection of 0.17 milligrams per litre. "This method does not require a long extraction procedure (total analysis time can be lower than 30 minutes) and can be reliably used for melamine detection in the milk matrix in accordance with the European law limits," the team reports.

"SERS is emerging as a new technique for analytical methods that can be suitable for high-throughput screening analysis and could become a valid alternative to the classical analytical methodologies based on gas chromatography or high-performance liquid chromatography coupled mass spectrometry," the team says. Rapid and automated analysis with 96-well plates is also possible, the researchers add.

"Future developments will be about the application of SERS based methodologies in other food matrices and also in water," Giovannozzi told SpectroscopyNOW. "In fact, melamine is often used to decrease the level of cyanuric acid in the swimming pool/spa industry, and its toxicity concentration level has to be accurately controlled. Our methodology could be succesfully applied even in this field."

He adds that, "The recent and potential advances in SERS developed at INRIM are also related to the determination of food origin, which may have an economic impact on detecting food fraud, particularly concerning adulterated olive oil and counterfeit wine and in addressing consumer demands for clearer labelling and trasparency."

Related Links

Food Chem, 2014, 159, 250-256: "Rapid and sensitive detection of melamine in milk with gold nanoparticles by Surface Enhanced Raman Scattering"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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