Measuring Phenolics in Grapes

We measure Anthocyanins in grapes to evaluate their maturity and to anticipate the subsequent concentration of phenolics in wine. The goal is to measure the effect of the weather and viticultural practices and to improve harvest timing.

The process has been developed by Enotrex LLC (copyright 2010-2014) and provided by Gianni Colantuoni for research purposes. The central idea is that by pressing unripe grapes and exposing the pomace/skins to a warm alcohol solution, we can simulate the phenolic extraction process that the ripe grapes will undergo later, yet estimate their Anthocyanin levels now. This page has the following sections:

  • Setup: describes the laboratory equipment required to prepare the simulated wine samples

  • Preparing the phenolics extraction buffer: describes how to prepare the alcohol solution, which will extract the phenolics and create a simulated wine

  • Process: describes the measurement process in detail

  • Calculations: shows how to interpret the measurements

Setup

The setup consists of

  1. A Phenolics Extraction Buffer, an alcohol solution, to create a model wine from grape samples.

  2. A Press; we use a pneumatic laboratory press specially designed for this application

  3. A temperature-controlled water bath

  4. A standard Spectral Analysis Setup as used for phenolics measurements with WineXray

Here is an annotated picture:

 Preparation of Phenolics Extraction Buffer

The materials needed are:

  • Potassium Hydrogen Tartrate (KHTa; 99%; manufactured by SIGMA-ALDRICH; Catalog Number 243531)

  • Ethanol (CH3CH2OH, i.e., EtOH; Alcohol, Anhydrous, Reagent; manufactured by J. T. Baker; Catalog Number 9401-22)

  • De-ionized water (DI H2O)

  • Hydrochloric Acid (HCl; 1.0N and/or 0.1N Volumetric Solution; manufactured by J. T. Baker; Catalog Numbers 5620-02, 5611-02)

The composition of the buffer is 5 g/l Potassium Hydrogen Tartrate, 12% Ethanol v/v; pH 3.3. Follow these steps to prepare 500mL of the Phenolics Extraction Buffer:

  1. Place 400 ml of de-ionized water in a 1000 ml graduated beaker.

  2. Weigh and add 2.5 g of Potassium Hydrogen Tartrate.

  3. Measure and add 80 ml of 100% Ethanol.

  4. With a magnetic heater/stirrer, stir for five (5) minutes; do not worry if all the Potassium Hydrogen Tartrate does not dissolve.

  5. Use a pH meter while stirring the solution to monitor its pH.

  6. Allow the pH to stabilize, and then adjust the pH of the solution to 3.3 using Hydrochloric acid.

  7. Bring the volume to 500 ml with de-ionized water.

Store the buffer at room temperature; it is to be used within 30 days



Process

The measurement process has nine steps for each sample:

  1. Collect 100 berries in a plastic bag. Count the Berries ( NB = 100) and weigh them (WB in grams)

  2. Crush the berries in the bag. Assure all berries are broken and the pulp and seeds are visibly separated from the skins.

  3. Put the crushed berries sample into the press (calibrated for a 10-second close, 2-minute hold, and 10-second release). Activate the press.

  4. Pour the pressed juice into a measurement container and measure the volume and weight of extracted juice: (VJ in mL, WJ in grams) and its approximate Sugar content per 100 g of solution (BX in Brix), pH (pH), and Titratable Acidity (TA in g/L).

  5. Open the press and remove the compressed skins and seeds cake into a holding jar labeled "Extraction Jar" Then pour a given volume VJ of the "Extraction Buffer" into the Extraction Jar and mix well. Essentially replace the juice pressed out with the Extraction Buffer

  6. Incubate the Extraction Jar for 2 hours in a water bath temperature controlled at 55 dC, then shake it and let it cool down for 15 minutes.

  7. Empty the entire contents of the Extraction Jar into the press

  8. Activate the press, and allow the juice to flow into a measurement container labeled "Incubated Juice." Measure the volume of the Exposed Juice (VEPB in mL). Discard the compressed seeds and skins cake.

  9. Measure the phenolic compounds in the Extracted juice and in the Incubated Juice. Record Total Anthocyanins (JA and tEA in ppm Malvadin Equivalent) using the WineXray platform.



Calculations

Enter the measurements into the database using the "INPUT: Berry Test" layout.


The picture shows the measurements and results from the Berry Tests done on September 10, 2019, for all six vineyard sections. The following table shows for a different set of samples, the inputs (yellow fields) and result fields as calculated in the database (red fields) in weight per average berry [mg/Berry] or [% of average Berry Weight

 

Calculations

 The formulae for calculating the results are shown to the right of each variable. Note: if the volume of Buffer added is equal to the volume of Extracted Juice, then the estimated Total Extractable Anthocyanins TEA = JA + tEA (Anthocyanins in Juice + Anthocyanins extracted by buffer).

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Last updated: March 4, 2023