Whether celebrating Cinco de Mayo or just having another relaxing day in Margaritaville, a new chemical test could be the assurance you need that the bottle you're downing is genuine tequila.

Researchers in Mexico and Germany have devised a quality assurance procedure that can distinguish between the, ahem, real thing and faux tequila, manufactured by fraudulent sources and non-genuine starting materials. The findings provide a new shot of QC to the billion-dollar tequila market, say the researchers. "Tequila is one of the best regulated spirits in the world with strict Mexican standards and labeling regulations," says study leader Dirk Lachenmeier, of the Chemical and Veterinary Investigation Laboratory of Karlsruhe in Germany. He worked with colleagues Eva-Maria Sohnius, Rainer Attig, and Mercedes Lopez of Centro de Investigacion y Estudios Avanzados del IPN, Mexico, on the new test.

Tequila is made from the blue Agave plant, a species similar to the lily, and its production is limited to certain geographic areas, primarily to the state of Jalisco in West-Central Mexico. Although it is subject to strict production standards and labeling regulations, adulterated samples are occasionally reported, although precise figures are not known.

Top-quality tequila is produced from 100% Agave, and mixed tequila can contain up to 49% sugar before fermentation. Labeling fraud takes place when this mixed-tequila is shipped and relabeled as 100% Agave or if alcohol from other sources is added.

The researchers compared ion and gas chromatography on 31 tequila samples in the 100% Agave category with 25 mixed-tequila samples. The pure Agave product contained significantly higher levels of methanol, 2-methyl-1-butanol, and 2-phenylethanol, which thus provided the researchers with a chemical fingerprint for genuine high-quality tequila that would not be seen in adulterated products. The team points out that although toxic methanol is present, it is at levels below safety margins.

The team provided additional support for the chromatographic studies using Fourier Transform Infrared Spectroscopy (FTIR), which they say can identify fake tequila. The researchers explain that combining spectroscopic and chromatographic methods provides a more accurate identification than previous attempts that focused on other chemicals or isotopic analysis. The same test works with other Agave spirits including including mezcal (the one with the worm), sotol, and bacanora.

J Agric Food Chem, 2006, in press; http://dx.doi.org/