Fourier Transform IR Spectroscopy Samples Wine
An old saying asserts, "There is truth in wine," but winemakers need to know what else besides veritas there is in vino. The alcohol content of wine is routinely monitored for quality control and regulatory purposes, but a wine's worth depends on a complex mix of volatile constituents, including ethanol, sugars, organic acids and tannins.
Using single-bounce attenuated total reflectance, researchers collect infrared spectral data from samples of wine to measure alcohol and nine other constituents, including acids and sugars. The setup is easier to use than other approaches to Fourier transform infrared spectroscopy for this application, the developers suggest. Courtesy of Thermal-Lube Inc.
Traditionally, measuring the mix involves high-performance liquid chromatography and other techniques. Each component of the wine must be analyzed separately, which takes anywhere from 5 to 30 minutes per component per sample. Now researchers at McGill University in Sainte Anne de Bellevue, Quebec, Canada, have developed a method based on Fourier transform infrared (FTIR) spectroscopy that takes the measure of a wine almost as quickly as taking a sip.
"Our technology can analyze all the components in a wine sample simultaneously, with an analysis time per sample of only one to two minutes," said Robert A. Cocciardi, who was part of the research team and who has gone on to work at Thermal-Lube Inc. of Pointe-Claire, Quebec. The company, which is commercializing dedicated FTIR systems, has developed and is marketing Spirit, an analyzer based on the work by Cocciardi and Ashraf A. Ismail, an associate professor in McGill's food science and agricultural chemistry department.
Cocciardi said that the analysis performed on wine is a function of regulation and the size of the winery. As a result, what is measured varies with location. The Société des Alcools du Québec, for example, requires the measurement of 12 or so constituents for wines sold in the province. France has similar regulations, and transmission FTIR wine analyzers from Foss Analytical AS of Hillerød, Denmark, have proved popular there and in Germany.
Because of the high cost and the required calibration, however, the technology may not meet the needs of smaller laboratories. The McGill researchers thus set out to develop a solution. They settled on a system based on single-bounce attenuated total reflectance. In this technique, a drop of wine is placed on the surface of a crystal, typically zinc selenide. When an infrared beam enters the crystal at the proper angle, it reflects off the sample-crystal interface, penetrating the wine by approximately 1 µm. That bounce probes the sample, picking up its infrared fingerprint.
The researchers, Cocciardi said, chose single-bounce attenuated total reflectance over the alternatives for several reasons, including its higher energy throughput, which improves the signal-to-noise ratio. Another advantage, he added, is that the method is easier to use. Once the analysis is completed, the crystal is wiped clean and rinsed with water, and the system is ready for the next sample. In contrast, transmission FTIR requires that samples be pumped through a 30- to 50-µm-diameter cell. The small opening makes the cell difficult to clean, and it can become clogged by particles.
A drawback for single-bounce attenuated total reflectance, Cocciardi acknowledged, is that the path length through the sample is only 1 or 2 µm. However, by carefully selecting the infrared regions to be analyzed, the researchers achieved results comparable to those from transmission FTIR.
The Thermal-Lube system, which is based on this research, measures alcohol as well as nine other wine constituents, including various sugars and acids. Acids, Cocciardi said, are of interest to aficionados and casual wine drinkers alike.
"A wine with a high value for volatile acidity means that the wine is turning into vinegar and is no longer good," he said.
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