An ultraviolet (UV) light induced fluorescence study to discriminate fake tequila from genuine ones is presented. A portable homemade system based on four light emitting diodes (LEDs) from 255 to 405 nm and a miniature spectrometer was used. It has been shown that unlike fake and silver tequila, which produce weak fluorescence signal, genuine mixed, rested, and aged tequilas show high fluorescence emission in the range from 400 to 750 nm. The fluorescence intensity grows with aging in 100% agave tequila. Such fluorescence differences can even be observed with naked eyes. The presented results demonstrate that the fluorescence measurement could be a good method to detect counterfeit tequila.
Distilled spirits adulteration with water, ethanol, or methanol is a serious economical and health problem [
Different alcoholic distilled beverages, like whiskey, gin, cognac, and rum, have similar components. They also have some common sensory characteristics, mainly related to the fermentation and aging processes, but some especial and distinct compounds generally define the unique aroma of each spirit. Tequila has an aroma profile composed of smoke, almond, varnish, coffee, caramel, rancid coconut, and wet straw. It also has a lightly flowery aroma and a piquant tone in taste. These properties are not present in the aforementioned beverages. On the other hand, like whiskey, tequila shows fresh fruit, dried fruit, and fruit aromas. Also, it shows vanilla and woody aromas like rum. However, not many of other aromas found in the distilled beverages are present in tequila [
The manufacturing process of tequila involves harvesting the “piña,” the stem of the agave plant with leaves removed, followed by cooking in an oven to convert polysaccharides (inulins) to a mixture principally of fructose and glucose. Sugars are extracted by milling and pressing and then they are fermented with yeast, typically
Two categories of tequila are very well distinguished: “100% agave” and “mixed.” For the “100% agave” only pure blue Weber agave juice is used to be fermented and distilled. Mexican regulations rule that this kind of tequila should be produced and bottled only in a specific region of Mexico. Mixed tequila is manufactured with 51% of agave sugar adding up to 49% (w/v) of different sugar, typically from sugar cane [
The Mexican Official Standard for Tequila accepts the following ranges of physicochemical specifications: 35 to 55 vol.% of ethanol; 50 to 500 mg/100 mL higher alcohols (referred to as pure ethanol); 30 to 300 mg/100 mL of methanol; 0 to 40 mg/100 mL of aldehydes; 2 to 250 mg/100 mL of esters; and 0 to 4 mg/100 mL of furfural [
Different techniques have been used for the analysis of adulterants in alcoholic beverages. Near-infrared spectrometry [
This study presents the fluorescence spectra produced at four excitation wavelengths (255, 330, 365, and 405 nm) by original tequilas, ethanol, ethanol-water mixtures, methanol, and counterfeit tequilas obtained in Mexico City. The fluorescence excited at wavelengths of 365 and 405 nm is much more intense for original mixed, aged, and rested tequilas. Nevertheless, it can be visually detected in all the studied samples. That could be used to detect counterfeit tequilas.
Samples (from different bottles) of aged, rested, and silver authentic “100% agave” tequila were used in this study. The number of samples (
A portable homemade system was used to acquire fluorescence and transmittance spectra from the studied samples. The system comprises five major components: a light source, a fiber-optic probe, a spectrometer, a cuvette holder with four light ports, and a laptop or a PC. The system is depicted in Figure
Experimental arrangement.
The transmittance average values (from the
Transmittance of distilled water (DW), ethanol (ET), methanol (ME), aged tequila (AT), rested tequila (RT), mixed tequila (MX), silver tequila (ST), and a fake tequila (FT) at 255 nm, 330 nm, 365 nm, and 405 nm.
The fluorescence of all samples detected at 90° at different excitation wavelengths is presented in Figure
Fluorescence spectra of the studied samples at excitation wavelengths (a) 255 nm, (b) 330 nm, (c) 365 nm, and (d) 405 nm.
At 330, 365, and 405 nm excitation wavelengths, the fluorescence spectra, presented in Figures
Visual fluorescence from RT, FT, ET, and ME (from the left to the right) excited with 5 mW at 405 nm of laser light.
The principal chemical components in tequila are ET and water. Therefore, the fluorescence spectra of fake tequila and of ET-DW mixtures in three distinct proportions (40-60%, 50-50%, and 60-40%) were measured using excitation wavelengths of 255, 330, 365, and 405 nm. In Figure
Fake tequila fluorescence (FT) compared to ethanol and distilled water mixtures fluorescence at different excitation wavelengths. (a) 255 nm, (b) 330 nm, (c) 365 nm, and (d) 405 nm.
From Figure
As the permitted alcohol content of tequila lies between 35 and 55% [
Fake tequila fluorescence (FT) compared to ET-DW (40%-60%) and ST fluorescence at different wavelengths. (a) 255 nm, (b) 330 nm, (c) 365 nm, and (d) 405 nm.
From the results it can be concluded that the fluorescence excited at 255 nm could be effectively used to discriminate adulterated and counterfeit tequilas from the genuine ones. Also, the detection method could be improved by a complementary measurement of spectral transmittance in the 255 nm to 405 nm wavelength interval. These results suggest the feasibility of building a compact and portable fluorescence spectroscopy system for a fast counterfeit tequila detection.
The fluorescence emission at 330, 365, and 405 nm from fake tequila and silver tequila cannot be used to discriminate the fake ones. On the other hand, aged, rested, and mixed tequilas can be discriminated from fake ones and from ethanol-water mixtures using 330, 365, and 405 nm excitation through spectrometric measurements or with naked eye observation of the visible fluorescence. Finally, the fluorescence of 100% agave tequila increases in the white-rested-aged order; this could be useful to determine their maturation age.
The authors declare that there is no conflict of interests regarding the publication of this paper.