Detecting melamine in food

Anne L. Fischer, anne.fischer@laurin.com

Researchers at Purdue University are using infrared lasers and light spectroscopy methods to rapidly detect tiny amounts of melamine in baby formula. The technique is performed with equipment that’s readily available to health officials and businesses and could go far in detecting the cancer-causing chemical in a variety of food products.

Melamine is a synthetic chemical that’s used in cleaning products, in the production of resins such as those used in plastic plates, and in fertilizers and pesticides. According to the Centers for Disease Control and Prevention in Atlanta, exposures to high levels of melamine – or exposure to lower doses combined with other chemicals – has caused urinary tract and kidney problems in animals, and long-term exposure can cause cancer. The US doesn’t allow melamine to be used as a food ingredient, but in China it has shown up in pet food, infant formula, dairy products, candy and more. It likely was used to increase the protein content of the food, according to a Dec. 25, 2008, article in the New England Journal of Medicine.

After the discovery of pet food contaminated with melamine (along with cyanuric acid, a related compound), a liquid chromatography/triple-quadrupole tandem mass spectrometry detection method became widely available. This technique can detect down to 250 parts per billion, although sample preparation and cleanup are difficult, and detection time can take up to three hours. A rapid detection method was recently developed by R. Graham Cooks, a professor of chemistry at Purdue University, using a low-temperature plasma probe with tandem mass spectrometry, but drawbacks include the lack of available equipment and the question about whether it’s exportable for international use.

A new approach

A group of students at Purdue, working in a food science laboratory that houses near- and mid-IR spectrometers, found another rapid detection method. However, finding an alternative to the liquid chromatography methods was not their original purpose. “We had a new software program, and I gave them the exercise so they could learn to use the software while working on a relevant industrial problem,” said Lisa Mauer, an associate professor of food science at Purdue. The near-IR setup included a multiple-purpose analyzer from Bruker Optics of Billerica, Mass. Spectra were collected in the mid-IR region using a Fourier transform-IR spectrometer from Thermo Scientific of Madison, Wis.

They applied two sampling techniques: diffuse reflectance spectroscopy and multibounce attenuated total reflectance. The near-IR instrument had a sample loading wheel, so it was fully automated and very simple, according to Mauer. By reflecting IR laser beams off unadulterated samples of powdered infant formula and toward a detector, they calculated how much of the energy was absorbed by the sample, creating an absorbance spectrum unique to that sample. The spectra were analyzed using the new lab software from Bruker Optics. They then collected the same data for pure melamine and compared the two. Because the melamine structure is very different from the formula’s, they were able to detect down to 1 part per million (ppm) of melamine. US federal guidelines allow for 1 ppm in formula and 2.5 ppm in other products.

The method developed at Purdue is fast and accurate. As reported in the Journal of Agricultural and Food Chemistry, the factorization analysis differentiated unadulterated formula from samples with 1 ppm of melamine with no misclassifications. The near-IR method took two minutes, while the mid-IR method took five.

Although this new technique is capable of detecting down to the critical 1 ppm level and yields rapid feedback, the current challenge is that the partial least-squares analysis and factorization models are not matrix-independent. New calibration models are needed so that different brands or formulations can be analyzed. Because the study was conducted as a lab training exercise, “It’s not something the students will continue on,” Mauer noted. However, each student has an individual project as well, so even lower detection methods might emerge from Purdue’s department of food science.