Marvellous time for monitoring moonshine

A portable IR device that can quickly and easily determine the strength of alcoholic drinks proves more effective than laboratory-based FTIR spectroscopy. The device could be useful in law enforcement and in industry fraud.

"Ever since Gay-Lussac's time, the alcoholic strength by volume (%vol) has been determined by using densimetric measurements," explain Dirk Lachenmeier and colleagues at the testing agency Chemisches und Veterinäruntersuchungsamt in Karlsruhe, Germany. However, hydrometer-type alcohol meters although still widely used in industry do not have the precision required of the regulatory laboratory. A typical procedure would involve distillation followed by pycnometry. This approach is labour intensive and so can be expensive.

Various methods have been tried unsuccessfully, including titration methods, enzymatic analysis, sequential injection analysis, and liquid or gas chromatographic methods. None of these have gained wider utility in commercial laboratories beyond niche applications because they have no advantage over conventional densimetric reference.

An alternative that has been widely adopted is the use of infrared (IR) spectroscopy with a multivariate regression analysis, which can be used to quickly screen samples in less than 2 minutes with no sample preparation, but Fourier transform (FT) IR or near-IR instruments are expensive and require matrix-dependent calibration. "During the last 20 years, most of the research on alcoholic strength determination was focused on spectroscopy," the team says.

"While infrared spectroscopic techniques are today routinely applied in larger alcohol testing laboratories (e.g., our institute has successfully used FTIR screening for every sample since 2004)," the team adds, "the relatively large investment for the instrument currently prohibits the application in smaller laboratories and small-scale manufacturers (e.g. wineries, distilleries, or breweries)."

Lachenmeier and colleague Rolf Godelmann, working with Markus Steiner, Bob Ansay, Jurgen Weigel and Gunther Krieg of Unisensor Sensorsysteme GmbH, also in Karlsruhe, explain that their flow-through infrared device avoids time-consuming sample preparation steps but is still more effective even than densimetry and FTIR spectroscopy. "We apply a much simpler device consisting of a patented multiple-beam infrared sensor in combination with a flow-through cell for automated alcohol analysis, which is available in a portable version that allows for on-site measurements," the team says.

Legitimate brewers and distillers alike as well as regulatory authorities and tax inspectors on the track of illicit alcohol from home stills - need to be able to measure the alcohol, ethanol, content of products. Writing in Chemistry Central Journal, the researchers show that their technique is just as accurate, and more sophisticated, than widely used lab-based methods. The team tested the device, which is portable to allow for on-site measurements, on a range of alcoholic samples.

Their tests on 260 different alcoholic drinks showed that the flow-through infrared device was much easier to handle than typical reference procedures, while time-consuming sample preparation steps such as distillation were unnecessary. Reproducibility with six different wine samples, was 0.05% vol and the relative standard deviation was below 0.2%. They also tested wines partway through the fermentation process, and unrecorded (non-commercial or illicit) alcohol samples, with good results. The test took less than a minute per sample.

"The device gives the opportunity for mobile on-site control in the context of labelling control of wine, beer and spirits, the process monitoring of fermentations, or the evaluation of unrecorded alcohols," explains Lachenmeier. He adds that the device can also be used easily in developing country settings, with results that compare well to a more sophisticated lab set up. Beers and sparkling drinks present an issue, but only in that the carbon dioxide "fizz" has to be removed, by degassing using filtration and ultrasonic treatment, prior to making the measurements.

The authors add that unrecorded alcohol accounts for a quarter of all alcohol consumed worldwide, but in most cases, "not even the most basic chemical composition such as alcoholic strength is known for these beverages."