Evaluating the Phenolic Potential of Grapes
A Climate Change Environment for the Wine Industry
In a climate change environment, every harvest presents a new challenge for winemakers. One of their main concerns is achieving the desired equilibrium of sugars and acids while maximizing the phenolic potential in the grapes.
Optimizing Analysis Solutions to Determine Phenolic Potential
Many wineries use the ‘Glories Index’ as the primary analysis to quantify the phenolic potential of their grapes.
This is the case with Cape Mentelle Winery in Australia, which monitors the phenolic potential of all their Cabernet and other red varieties during the ripening process to determine the optimal harvest time and achieve the best quality in their future wines.
Principle of the Glories Method
The Glories Index is a widely used metric in viticulture and enology to assess grape ripeness and phenolic maturity, particularly for red grape varieties. It measures the levels of anthocyanins (responsible for colour) and tannins (key to structure and aging potential) in grape skins. The index helps winemakers decide the optimal time for harvest.
Results Provided by the Method
- Sample pretreated at pH 1: Total potential in anthocyanins
- Sample pretreat at pH 3.2: Total potential in extractable anthocyanins in the pH of the wine
- EA%: Anthocyanins extractability
- RPT: Total polyphenolic richness of the grapes
- MP%: Seed maturity, representative of the contribution of seed tannins
Interpretation Of Anthocyanins Extractability (EA%)
A higher ratio (>0.8) suggests advanced phenolic maturity, with good extractability of pigments and tannins. A lower ratio (<0.5) suggests the grapes may still be underripe, leading to less efficient extraction during winemaking.
Practical Applications In Winemaking
1.Harvest Decision-Making
The Glories Index helps winemakers determine the best harvest time for achieving the desired tannin and colour balance in wines. It complements traditional ripeness measures like sugar content (°Brix), titratable acidity (TA), and pH.
2.Tailored Winemaking
Grapes with high extractable anthocyanins may require gentler maceration techniques to avoid excessive tannins. Lower extractability might prompt adjustments, such as extended maceration or specific fermentation techniques, to optimize phenolic extraction.
3.Predicting Wine Style
High anthocyanin and tannin levels are ideal for structured, age-worthy wines. Lower levels suit lighter, fresher wine styles.
4.Vineyard Management Feedback
If the index indicates phenolic immaturity, viticultural adjustments (e.g., thinning, canopy management) can be implemented in future seasons.
BioSystems ‘Glories Index’ Optimised Analysis
The Glories method needs two different sample pretreatments. A unique sample was divided in two to quantify all the phenolic compounds.
After 4 hours of maceration and pretreatment:
- Sample 1 (pH 1) was run in a BioSystems analyser with an Anthocyanins kit to obtain Total Potential Anthocyanins.
- Sample 1 (pH 3.2) was run in a BioSystems analyser with an Anthocyanins kit to obtain Extractable Anthocyanins.
The calculation between them provides the EA % (ratio of Anthocyanins extractability).
(*) In order to quantify RPT (Total Polyphenolic Richness) run IPT analysis in Sample 1 (pH 3.2) in a Spectrometer or analyser that incorporates 280nm wavelength.
Then, make the calculation in a spreadsheet to obtain MP% (seed maturity).
Data Comparison – Y15 ‘Glories Adapted’ V ‘GLories Index’ Original
Dataset: 208 samples from two different harvests (2022-2023).
Slope factor applied in Y15: (2 * 1.4) = 2.8 – where (2) comes from the manual dilution involved in the previous pretreatment and (1.4) from the technique correction.
Conclusions
The results of the BioSystems method, that includes the slope correction are comparable to those obtained by the Glories method with the original extra steps and dilutions after the pretreatment.
This new technique is available on the BioSystems Y15 and SPICA analysers. To quantify RPT % through IPT analysis at 280nm you can work with a spectrophotometer or a SPICA analyser.
