The Ways UV-Vis Expedites Phenolics Analysis

Editor’s note: This Wine Talk article was originally published in the September/October issue of Vintner Magazine.

Phenolics are a broad class of compounds encompassing the simpler non-flavonoids as well as the more complex anthocyanins, tannins, and pigmented tannins.

Red wine phenolic extraction during fermentation is of particular interest when making pressing decisions, optimizing flavor and color profiles while minimizing bitterness. The most comprehensive phenolic assay in wine is the Harbertson-Adams assay (HA). It entails multiple spec measurements timed throughout extractions and careful protein precipitation. Though it provides quite comprehensive data on phenolics, anthocyanins, and polymeric pigment, it is time-consuming.

Thankfully for those of us in the laboratory, mathematical models taking advantage of phenolic Ultraviolet and Visible (UV-Vis) range spectral features have been developed which significantly enhance throughput and ease of measurement. This type of method, in which a general observable (UV-Vis Spectrum) is related to the direct measurement by another method (results of HA), is referred to as a secondary method.

The primary method is the Harbertson-Adams Assay. The secondary measurement of UV-Vis Spectrum is then related to the primary using a statistical model. The model is developed by analyzing hundreds or thousands of samples by both methods and then generating the model which relates them. The Bolton-Skogerson model demonstrated the utility of such a method using Partial Least Squares Regression (PLS) years ago, leading to additional advanced statistical models based on these principles. This technique is used in other areas of wine analysis and forms the basic strategy underlying technologies offered by the Anton Paar Alcolizer and Foss WineScan, for instance.

There is far more work involved in creating this model than is feasible for a single winery laboratory. However, once the model is built, one can obtain the results of the more complex primary method by using the far simpler, more rapid secondary method, and the algorithm. Associate Professor Jim Harbertson’s lab (Yes, that Harbertson) at Washington State University (WSU), with a grant provided by the Washington Grape and Wine Research Program to their newly formed Viticulture and Enology Department, built and continue to maintain models that offer this secondary method to the wine industry at large.

While an experienced technician can usually process 20 to 40 samples per day through the entirety of the HA Assay, at Beringer Vineyards, one of Treasury Wine Estates’ iconic Napa Valley wineries, we process a few hundred samples in a single morning during harvest. Using the Thermo Scientific Evolution 201 UV-Vis Spectrophotometer with an autosampler, we need only centrifuge our samples well, and program the instrument to collect a full spectrum from 230 – 700 nm. Using a 0.1 mm pathlength flow cell, we don’t even need a dilution, and thorough centrifugation eliminates the need for filtering.

The spectra are then uploaded to a portal at WSU’s Viticulture and Enology Department. In moments, anthocyanin, tannin, and iron reactive phenolics (HA results) are returned and can be reported immediately to winemaking. With this method in place, it actually takes longer for us to collect the samples from our cellar than it does to process them in the lab.
Our chemistry team combines Thermo Fisher’s technology with WSU’s modelling to provide phenolic data to our winemaking staff that is accurate, and rapid enough to be immediately actionable. As wine scientists, it is up to us to provide the best tools to our winemakers, so they can add that information to their suite of tools.

This is just one of the many technological advancements implemented here at Beringer that has helped Treasury Wine Estates establish their brands as some of the most admired premium and luxury brands worldwide.

Jonathan R. Cave, Ph.D. is an Analytical Chemist for Treasury Wine Estates.

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