To make a desirable wine and know what makes it taste so good, vintners run tests. The alcohol and sugar level, pH, and other factors impact the product. Various forms of light-based analysis—including infrared (IR) and near-infrared (NIR) spectroscopy—provide useful information from an easy workflow.
When asked about the main reasons for analyzing wine with IR spectroscopy, Ricardo Páscoa of the University of Porto in Portugal, says: “It is a low-cost technique that is rapid, robust, simple, and multi-parametric in one single analysis.” He added that IR remains fast even when collecting lots of information. “I found out that this technique is perfect for most of the major parameters evaluated in wine, since with a single analysis—taking only three minutes—several parameters can be determined at the same time.” Páscoa also points out that he “applied the technique directly to wine samples that were previously degassed.”
From the Department of Viticulture and Oenology at Stellenbosch University in South Africa, researcher Astrid Buica adds that IR can be applied to low volumes with this flexible technique. “There are different regions of IR that can be used depending on the application, and the instruments have different options for sample presentation: transmittance, reflectance, as well as hybrids of the two, transflectance, and interactance,” she explains. “It can be used for liquid and solid samples.”
For wine analysis, working in a lab is not always the most convenient way to analyze a sample, but IR spectroscopy offers other possibilities. “Lately, there has been a big move toward more portable options for using the equipment; for example, directly in the vineyard or in the cellar,” Buica says.
Options in analysis
Páscoa and his colleagues applied NIR spectroscopy to wine, but adjusted the method. They pointed out that water in wine limits the use of NIR for quantitative analysis because the water molecules reduce the signal-to-noise ratio. So, the scientists analyzed lyophilized wine samples with NIR to measure the level of alcohol, sugars, acidity, and more. This method proved to be effective for accurate quantification of total sugars, pH, volumic mass, and total dry extract, and circumvented the interference of water bands.
Although Buica calls IR instruments some of the “work horses” in her lab, no analytical device is perfect. “One has to always pay attention to the analyses themselves, as some parameters are determined more accurately than others,” she said. “For example, one run can give values for pH, total acidity, glucose, fructose, residual sugar, yeast-assimilable nitrogen, ammonia, free amino nitrogen, tartaric acid, malic acid, citric acid, ethanol, and volatile acidity, but some of the values are more reliable than others.” As an example, she mentions that IR analysis of malic and lactic acid could be used to monitor malolactic fermentation, which is a crucial step in making wine, but the analysis failed to provide accurate values for the concentrations of the two acids. As Buica notes, “Basically, one needs to know how to appropriately use the results.”
To test which IR-based methods were appropriate for wine samples, Buica set up calibrations of various parameters in juice from red and white grapes using a range of instruments, including benchtop and mobile devices, NIR and mid-IR wavelengths, and transmittance and reflectance modes. From this work, she concludes: “Some of the calibrations could be used for QC, while some others were not even good enough for screening.”
These examples show how light-based analysis can be used to analyze wine and where challenges arise. Overall, IR and related technologies provide potentially useful methods for analyzing wine during production and beyond.