Slamming tequila frauds

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  • Published: Nov 17, 2008
  • Author: Steve Down
  • Channels: HPLC
thumbnail image: Slamming tequila frauds

The different varieties of tequila, which are classified according to their aging procedures, can be differentiated by a simple HPLC procedure which looks for a set of five compounds that originate from the oak aging casks.

The popular party drink tequila originates from Mexico where it is produced from the blue agave plant (Agave tequilana Weber). Although it looks like a cactus, the plant is actually a member of the lily family but the agave farmers do not let it grow naturally. When the long central stalk begins to grow, they are cut off, encouraging the plant to produce a large pineapple-shaped bulb that can weigh up to 100 pounds. It is this bulb which is harvested.

Centuries ago, the local people made a drink called pulque which had earthy, vegetal tones but a low alcoholic content of about 3%. This did not satisfy the Spanish settlers who arrived in the 16th Century, who tried to distil the drink to make it stronger. A palatable drink was only obtained after the agave pulp was cooked, giving a sweeter juice that was fermented, then distilled to produce tequila.

The modern drink comes in several classifications based on the length of time the fresh spirit is aged in oak casks. The unaged drink known as tequila blanco (also called silver or white tequila) is stored only in stainless steel tanks for up to 60 days. It can be produced from pure agave sugars or by the addition of sugar cane to make mixtos. Reposado (rested) tequila is stored in oak casks for 2-9 months to acquire flavour from the wood and anejo (old) tequila is aged in the casks for at least one year.

The aging step changes the colour and characteristics of the drink but is also susceptible to abuse. Unscrupulous individuals have been known to add oak extracts or caramel colouring to tequila blanco to produce false aged tequilas for which they can charge more. But how can the fake types of tequila be distinguished quickly and decisively? Scientists in Mexico believe that they have come up with the solution.

Jesus Cervantes-Martinez and Ana Celia Munoz-Munoz from the Center for Research and Assistance in Technology and Design of the State of Jalisco (CIATEJ) with Adam Charles Grenier from Peace Corps Mexico and Humberto Gutierrez-Pulido from the University of Guadalajara turned to HPLC. They reasoned that the phenolic-type compounds that leach into the drink from the oak barrels would act as markers of aging and might even be able to distinguish between the reposado and anejo varieties.

Initially, 11 potential aging markers were targeted, including well-known wood-derived phenols such as gallic acid, syringic acid, vanillin and sinapinaldehyde. Samples of each of the different tequila types were treated the same way. An initial SPE preconcentration was used to increase the detector signals and the methanolic extracts were analysed by HPLC with diode array detection.

Separation was effected on an octadecylsilica column with a gradient of acetonitrile in aqueous methanol acidified with formic acid. The detection wavelength for 5-(hydroxymethyl)furfural and phenolics was 280 nm while that for phenolic aldehydes and scopoletin was 320 nm. Under these conditions all 11 compounds were detected and separated in less than 60 minutes.

The detection limits ranged from 0.62-4.09 µg/mL and broad linear calibration ranges were observed, ranging from 2.5-40 to 10-500 µg/mL for individual compounds. Good recoveries were also achieved at 84.2-108.5%, except for gallic acid which was low at about 65%.

The chromatograms for tequila blanco (pure or mixto) displayed peaks for three compounds only, corresponding to furfural, 2-acetylfuran and 5-methylfurfural. They are produced from the Maillard reaction during agave cooking and none are regarded as oak aging markers.

The contents of the 11 target compounds in the aged drinks compounds increased with aging time, being higher for reposado than anejo. None were found in tequila blanco. Using discriminant analysis to aid differentiation, with the concentrations as variables, five of the marker compounds were found to be the most statistically significant discriminators between the three tequila types. Using gallic acid, 5-(hydroxymethyl)furfural, vanillic acid, syringic acid and scopoletin, 99.5% of the total data variability was described.

An ANOVA test using the final values of the discriminant function for each tequila type confirmed the results. Reposado and anejo varieties were clearly distinguished, as were the pure reposado and reposado mixto types.

The HPLC method can be used by quality control labs to classify the blanco, reposado and anejo varieties of tequila in an easy and simple method that is amenable to automation. The same method can also be used in forensic labs to identify fake aged tequilas by the absence of the phenolic aging markers.

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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