Wine is produced by transforming sugars into alcohol during fermentation of grape must. It is composed mainly of water, ethanol, sugars, and acids but can also contain other chemical compounds that vary in concentration and can be very influential on the sensory properties of the final product. Measuring grape characteristics is a basic requirement for vineyard improvement and optimum production of desired wine styles. There is a strong need for timely information that can be used for berry maturity assessment and classifying vineyard blocks, but existing analytical methods are insufficient for the demands of production. Estimated worldwide annual grape and wine productions are over 60,000 million tons and 25,000 ML. Even simple analyses like total soluble solids (TSS expressed as °Brix) and acidity require samples to be sent to a laboratory with delayed results. More complex quality analyses, such as grape color, nitrogen, and phenolics are not even considered on a large-scale basis because of the cost and time involved. Thus, there exists a need for cost-effective and timely methods for measuring parameters in wine from grape harvesting all the way through to the final quality assessment. One such method that has been studied is NIR spectroscopy.
Parameters of Interest in Grapes
Parameters of interest in grapes include total anthocyanins (correlated to color), TSS, and pH. Grapes are traditionally harvested based on TSS concentration, mostly consisting of glucose and fructose. TSS refers to the total soluble constituents of the grape and because these are primarily sugars, °Brix is used when determining the sucrose equivalent of soluble solids in products. If these parameters can be measured in the field before harvest, it can tell the grower the optimal time for picking the berries. Anthocyanins are naturally occurring phenolic compounds and are responsible for the color in wine grapes as well as other fruits and vegetables. The color of appearance is pH dependent. Studies have been conducted to measure these parameters as well as other acidity measurements in wine grapes with success. The best results have been obtained using local data sets based on individual ranges for different vintages, regions, and grape varieties for the parameters of interest. Some work has also been done on detecting powdery mildew & mold contamination in wine grapes. Results found that while contamination was not correlated to pH or TSS, the potential exists for discriminating infected grapes using NIR spectroscopy. This would be a valuable tool at the stage before crushing grapes for fermentation.
Wine fermentation is a complex process where grape juice is transformed by microbial action into wine. Control of the wine fermentation process is a very important step in wine production of wine, and there is a need to accurately and rapidly control both the substrate (sugars, ethanol, phenols) and product quality. Alcohol content, pH, and SSC are of vital importance during the fermentation stage. As is the case with beer and spirits, monitoring alcohol content can be used to optimize the fermentation stage regarding yield and fermentation time, resulting in savings for the wine producer. pH is an important property that makes a major contribution to wine quality. It influences the acidity, anthocyanin formation, and maturation processing as well as potentially complicating the biochemical changes during fermentation. SSC is also important in wine quality. The sediments of SSC cause changes in taste, color, flavor, and odor during the process of fermentation and storage and is a sequential phenomenon that starts immediately after fermentation. Results from studies have proven the feasibility of measuring these parameters using NIR spectroscopy, showing the potential to replace the traditional methods of refractometry and titration during fermentation. Phenolic compounds such as tannins and anthocyanins are also important quality parameters during fermentation and in the final wine product that affect the taste, color, and mouthfeel. However, reference tests on these parameters are rarely conducted on a large scale by vineyards because of the expense involved. One study collected grape varieties over three consecutive harvests and accurately predicted the concentration of some major anthocyanins and tannins in both Cabernet Sauvignon and Shiraz wines during fermentation. Being able to measure anthocyanins and tannins using NIR spectroscopy would be an invaluable tool for vineyards.
Wine grading and quality assessment after the fermentation is an important part of the winemaking process, particularly when allocating batches of wine to styles that are determined by consumer demand. The price of grapes is often determined by the quality category of the resulting wine. One issue with the current methods for determining wine quality in terms of sensory characteristics is that they are subjective, performed by winemakers, competition judges, or tasting panelists. The potential exists for NIR spectroscopy to be used as an objective method for determining wine quality. While some flavor compounds are surely below the detection limit for NIR spectroscopy, some of the more abundant organic compounds do exist in the wine matrix to offer the potential for quality assessment in this manner. Some studies have been conducted in Australia correlating NIR spectra and sensory data. While the studies were limited in their range of samples, they did show the potential for using this method. Wine grading using NIR spectroscopy could provide a rapid screening tool to add to current quality assessment methods used by winemakers. One way this could be used is for blend allocation of large batches before sensory assessment, developing profiles for blends as NIR calibrations.
More research into the interpretation of spectral data may provide insight into parameters affecting wine quality and interactions that occur within the wine matrix that determine sensory properties. Other uses of NIR spectroscopy in wine analysis that have been studied include yeast identification, product authenticity, and nutrients in vine tissue, all showing potential for development as a real-time analytical tool.
Quality Analysis, Classification, and Authentication of Liquid Foods by Near-Infrared Spectroscopy: A Review of Recent Research Developments – Wang, Sun, Pu, and Cheng, Critical Reviews in Science and Nutrition, 2017, Vol. 57, No. 7, 1524-1538
Grape and Wine Analysis – Enhancing the Power of Spectroscopy with Chemometrics. A Review of Some Applications in the Australian Wine Industry – Gishen, Dambergs, and Cozzolino, Australian Journal of Grape and Wine Research, 11, 296-305, 2005
Analysis of Grapes and Wine by Near-Infrared Spectroscopy – Cozzolino, Cynkar, Gishen, Journal of Near Infrared Spectroscopy, 14, 279-289 (2006)
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