Testing Wine in the Mid-Infrared
To the connoisseur’s standard methods of testing wine using taste, smell and sight, scientists are adding another: mid-infrared spectroscopy. A team from Hardy Wine Co. of Reynella and from the Australian Wine Research Institute of Glen Osmond, both in Australia, has developed a technique to fingerprint wine rapidly by obtaining its mid-infrared spectrum.
A goal of the research is to develop an easy and accurate wine monitor. Such a tool would detect dilution by water, for example. Hardy Wine senior research chemist Christopher J. Bevin noted that it is a rare event when the quality of a batch of wine is compromised. “However, it does occur, and when it does, it can cause loss of product,” he said.
At present, there is no quality control method available that is recognized by the industry, easy to use, sufficiently sensitive and nondestructive. For example, conventional chemical analysis techniques are slow and costly, and refractive index monitoring is not very sensitive and is not recognized.
The researchers turned to mid-infrared spectroscopy because it has the potential to definitively identify a wine sample by its constituents. The carbon-oxygen stretch for primary alcohols, the carbon-hydrogen stretch from ethanol and the double-bond carbon-oxygen stretch for aldehydes, carboxylic acids and esters all fall within the mid-infrared range.
They used two identical mid-infrared spectrophotometers from Foss of Hillerød, Denmark. The group scanned 161 wine samples in transmission with the spectrophotometers between 926 and 5012 cm–1, averaging several passes. These findings showed instrument-to-instrument variability that could be largely eliminated if the water absorption peaks were excluded.
They compared the resulting spectra measured by one device with those measured by the other using a similarity index calculated from the data to determine whether a wine sample was the same. They achieved good results in correctly classifying wines, authenticating 98 percent of the samples across the instruments.
Plans call for implementing the similarity index check commercially within Hardy’s sites. This test will be done before and after transport, with comparison to a sample of a standard reference to ensure proper quality control. Future research will evaluate smaller instruments suitable for use in winery cellars and not just in the laboratory. Eventually, with additional research into qualitative analyses of wine, uses of the technique can expand, and applications can evolve. Bevin, for example, noted that the verification method will see further enhancements.
“The next step is to improve software to automate the authentication process as much as possible.”
Journal of Agricultural and Food Chemistry, Dec. 27, 2006, pp. 9713-9718.
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