By Maureen Moroney
Wine Chemistry Worshop, June 2018
Phenolic compounds such as tannins and anthocyanins are important for the re, mouthfeel, color, and “age-ability” of a wine.
Tracking olics at various stages throughout the winemaking process can tell us many things: they can tell us about the maturity/ripeness of the fruit, they can tell us about the extraction profile in a fermenting wine, and they can tell us about how the wine is developing as it ages.
Phenolics are a complicated topic, both in terms of the best way to measure them as well as how we then interpret and apply those data. Analysts and winemakers alike are still struggling to fully wrap our minds around them. So a complicated, labor-intensive method for testing them rarely seems worthwhile, especially for a small winery.
One alternative method for a quick estimate of the major classes of phenolic compounds in wine is known as the Skogerson or Skogerson-Boulton assay. It does not require any special chemical reagents or any long incubation times, but only a UV-Vis scanning spectrophotometer and a special sample cuvette.
The principle behind the Skogerson assay is that it is based on a predictive mathematical model of the results that would be obtained by the industry-standard Harbertson-Adams precipitation assay. Kirsten Skogerson tested a large library of wines from many grape varieties and regions using the H-A assay, and then also used a scanning spectrophotometer to measure their UV-Vis absorbance spectra in the wavelength range of 230-900nm. She then took the absorbance values obtained on the spec and built a complex mathematical formula that could be applied to those values and would result in calculated values with a high degree of correlation to the results obtained by the H-A assay. The formula was optimized based on a wide range of wines, to help ensure that it was not useable for only one specific style or variety.
The Skogerson assay uses the following basic steps:
1. Dilute the wine sample as needed.
2. Fill the sample cuvette, and place it in the spec.
3. Using the spec’s “scanning” mode, collect the UV-Vis absorbance data from 230nm to 900nm.
4. Enter the absorbance data collected, plus the sample dilution rate and cuvette path length, into the Skogerson calculation spreadsheet (available free online). Many specs have a USB port to allow quick transfer of data.
5. View results.
One thing to keep in mind if you’re considering using this method in your winery is that while it was designed to be applicable for a wide range of wines, it was still built based on Vitis vinifera varieties, which have a different phenolic profile than typical cold-hardy Midwestern grapes and wines. In addition, since phenolics primarily impact sensory perception, having a numerical value assigned to them is most meaningful in a sensory context. So, for both these reasons, it is advisable to “calibrate” your palate to the results generated, and to keep a comparative database, in order to better understand what they can tell you about your wines.
Original research article: skogerson_2007_-_rapid_determination_of_phenolics.pdf