One for the vine: testing tannins with absorption measurements

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  • Published: Dec 1, 2010
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: One for the vine: testing tannins with absorption measurements

Analysing complexity

At Christmas time, there's no need to be afraid...of testing your tannins, at least now that US researchers have developed a sensor capable of discriminating between the different tannins present in red wine. The sensor could be used to fingerprint a wide variety of red wines with a view to confirming provenance and authenticity.

The chemical structures and the concentrations at which they are found in any living thing are present due to its unique blend of environmental exposure and genetics. This is no different for grapevines, yeast, and trees whose wood is made into wine barrels. The chemical complexities of these various organisms, whether arising from nature or nurture, are a matter of grave concern for those who produce, sell and drink the products of its fruit.

Alona Umali, Sarah LeBoeuf, Robert Newberry, Siwon Kim, Lee Tran, Whitney Rome, Tian Tian, David Taing, Jane Hong, Melissa Kwan and Eric Anslyn of the University of Texas at Austin, and Hildegarde Heymann of the Department of Viticulture and Oenology, University of California Davis, point out that analysis of complex mixtures such as these represents a significant part of the chemical endeavour. "Complex mixtures are ubiquitous in many fields of chemistry and biology, and hence, complex mixture analysis is common in pharmaceutical sciences, medical diagnostics, the food industry, as well as environmental sciences," the team says in a forthcoming issue of the RSC journal Chemical Sciences. "Analyses in [various] cases employ classical analytical techniques, such as HPLC, mass spectrometry, electrochemistry, UV-vis spectrometry, as well as hyphenated modes of these analytical methods," the team adds.

Sensor arrays, e-noses and e-tongues

The use of arrays or differential sensing has recently come to the fore as a more potent approach to the analysis of complex mixtures than linear assays or traditional one-dimensional analysis, however. The team further explains that arrays of sensors have been developed inspired by our senses of taste and smell in which cellular sensors work in parallel to generate a pattern of signals from a stimulus, which then affects our perception of the "sample".

In array sensing, the differential approach means that individual sensors are not required to be specific to a given analyte, indeed they are designed to be "cross reactive" and so can function variably with different the analytes. As such, differential sensing has been used to developed very powerful analogues of our olfactory system in the form of electronic noses for analysing gaseous mixtures, or aromas, and as electronic tongues, for "tasting" liquid samples. Sensors based on polymers, chemoresistive metal oxides and optical fibres have been developed and have been used widely.

The researchers point out that, "One of the major components of red wines is a group of polyphenolic compounds derived mainly from the skin of the grape called tannins, which are oligomers of flavonoids and non-flavonoids. Their structures are primarily dictated by the grape's biochemistry, making this class of compounds diagnostic for differences among wine varietals."

Tannin analysis

Many red wine tannins are yet to be characterised, so the team suggests that the development of an analytical array for assaying wine would not only be useful for quality and regulatory control of wine but could assist in research into these important components of a widely imbibed product. Previous studies have used infrared and nuclear magnetic resonance spectroscopy as well as combinations of these techniques with chromatography and utilised electrochemical sensing elements and electrodes.

The US team has side-stepped the issues surrounding electrode construction and sample preparation require for electrochemical sensing by turning to an array of peptidic, colorimetric sensors that can discriminate the flavonoids, the hydrolysates of tannins found in wines, with UV-vis absorbance indicators amenable to pattern-recognition protocols. "The basis of discrimination is the differential binding of analytes to arrays of sensors, rather than the analytes' structural properties as measured by spectroscopy or electrochemistry," the team explains.

They have now demonstrated that the approach using a simple array of peptides, metals, and indicators can produce a characteristic fingerprint for a given wine and allow varietal identification irrespective of the vintner. The same approach to analysis might also be applied to metabolomics and fingerprinting of other complex mixtures.



The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

At Christmas time, there's no need to be afraid...of testing your tannins, at least now that US researchers have developed a sensor capable of discriminating between the different tannins present in red wine. The sensor could be used to fingerprint a wide variety of red wines with a view to confirming provenance and authenticity.

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