Hand-held Grape Color Measurements Cannot Predict Enologix Anthocyanin Levels of Wines
You might be better off using your own eyes to attempt such a quantitative determination
by Doug McKesson
August 28, 2019 CALIFORNIA — The law was discovered by Pierre Bouguer before 1729, while looking at red wine, during a brief vacation in Alentejo, Portugal. This centuries-old law of analytical chemistry is the Beer-Lambert Law used to determine anthocyanins at Enologix.
Researchers and vendors now unveil "instant" photometric measurements of anthocyanins in the vineyard, not validated by the Association of Offical Analytical Chemists (AOAC). Do the new measurements obey the Beer-Lambert Law?
New instant measurements would let the vendor set standards for at the center of bottled wine prices without the Beer-Lambert Laws and without showing the relation between the unique measurements and bottled wines.
The Enologix concentrations of total, free and complexed Anthocyanins are related to and are a significant predictor of quality—whether measured by bottled wine 100-point scores, prices, or sales volumes since the 1990s.
The instant measurements released over the last decades would reverse protections set by Enologix measurements in the 1990s. New, non-standard instant measurement would let each worker and vendor set standards for conditions at the center of bottled wine sales without first showing the relation between the unique measurements and price, quality, and sales metrics.
Enologix points to all the leading papers in the use of "instant" reflectance measurements. For example a recent paper by entitled, "Grape Colour Phenotyping: Development of a Method Based on the Reflectance Spectrum," contains data which shows (Figure 7) the reflectance measurements do not obey the Beer-Lambert Law. Moreover, the new measurements accounted for only 53.4% of the Beer-Lambert measurements. The maximum error between samples could be as high as 94% for Enologix measurements.
Enologix has become a standard. There was a time not so long ago that we had to convince those in our industry that the science of phenolics was valid, and their contribution to wine quality was real. We know we are nearing the "general acceptance" stage of technology development when we see technologies and devices come to market that attempts to circumvent wet chemical methods of grape and wine analysis.
We have been watching the academic research community make attempts to determine anthocyanin content in grape clusters using indirect reflectance and fluorescence-based measurements using hand-held instruments for well over a decade. These measurements do have some demonstrated use in measuring chlorophyll in leaves, such as determine Nitrogen levels and the presence of disease. However, the application of this technology to determine color accumulation in grape clusters is no better than using our own eyes to make a quantitative determination.
Forgive me for getting techie for a moment. Fluorescence involves the use of brief flashes of UV light to excite a chromophore, which then reflects a color at another frequency as it returns from the excited state to the normal state. Fluorescence and reflectance measurements require a dark environment without extra stray light (much like a spectrophotometer does to measure absorbance in a sample). There are vast differences between vineyard and indoor measurements due to the difference in the ambient stray light, as documented in the literature.
Most of the studies utilize the sensors primarily as an investigative tool in an attempt to map color variation throughout a given vineyard. Comparisons to laboratory analysis of anthocyanins in grape skins via destructive methods appear to be made simply as a point of reference. To date, there are no published studies that propose the use of these sensors as a replacement for laboratory analysis, nor as a predictor of actual anthocyanin content as determined by wet chemical methods.
Reflectance does not equal absorbance when it comes to analytical chemistry. Reflectance is measuring what is NOT absorbed, and absorbance is measuring what is absorbed. For absorbance, we have the Beer-Lambert Equation, and for reflectance, we have nothing connected to molar concentrations. These instruments are measuring the reflected darkness or red-brown color of the grape.
However, Reflectance spectra have been shown to classify cultivar phenotypes ("white grapes" vs. "pink grapes" vs. "red grapes" using reflectance spectra cluster analysis combined with cluster and discriminant analysis. As we all know, grape quality and maturity include much more than just Total Anthocyanin measurement. Total Phenol, Tannin, and Complex Anthocyanin levels play a huge role in quality, and they vary independently during the grape ripening process. These measurements and our knowledge of how these levels change over time make our grape analysis the cutting-edge technology in our industry.
Enologix grape measurements are modeled using standardized fermentation protocols that are the same year after year so that customers can predict the anthocyanin extraction in a given year, within about 90% accuracy. Winemakers are now empowered to ask questions such as, "What is the extractability going to be like this year? Do I have to change my fermentation protocols?"
These are the questions we are working with customers to answer every year. Today, our challenge is convincing them how the changes in growing season weather are going to impact these measurements and their future wine quality, which is a much more enjoyable endeavor.