Understanding the development of phenolics in grapes throughout the growing season is essential for understanding all aspects of production. Whether your utilize phenolics analysis or not, the accompanying narrative is fundamental for understanding the development of every varietal and style. We start with a foundation of variables responsible for grape quality (soil type, climate, water availability, oxidation, sunlight, vine balance, genetics, and biodiversity) and then layer in the phenolics lens. This provides a more concrete framework to explore abstract concepts like ripeness and qualitative changes in fruit.

Veraison begins with coloration and ends with the complete transition from green to black berries. As a baseline, we begin berry measurement at the start veraison to track color development in the skins. Interestingly, the start of veraison also coincides with peak tannin concentration which will progressively decrease in astringency and extractability as oxidation lignifies the seed coat (Kennedy et al., 2000). We continue measuring anthocyanins accumulation throughout ripening and until harvest to get the best estimation of phenolic potential for each lot. Over multiple vintages, berry phenolic data establishes a baseline for quality and serves as a reference point for evaluating changes in a vineyard from farming practices to climate variation. This is especially important for growers and winemakers seeking assurance for the quality of their crop at key benchmarks within the season. For example, between the start of veraison and the end of ripening is a window of time for grapes to accumulate anthocyanins and other metabolites. While veraison is a critical time for quality development, grapes are also protected by the momentum of their growth. Firm berries protect the grapes from dehydration and an active vine metabolism promotes recovery after stressful events. Once peak anthocyanins is reached, the fruit is much more susceptible to quality loss, thus providing a protective mechanism for both parties when choosing how to grow and when to harvest your fruit.

The ripening window is a progression of metabolism where grapes soften, acids metabolize, sugars accumulate, and flavors and their precursors form (transitioning from red to black fruit characteristics for red grapes). The onset of ripeness begins with the second phase of berry growth (Kennedy, 2002). Depending on the site, anthocyanin accumulation can occur upwards of 70 days post the onset of veraison. The total quantity of anthocyanins and tannins heavily depends on the cultivar, climate, water availability, and a myriad of site characteristics. Ripeness is challenging to define in the context of grape flavor beyond the absence of pyrazines. Choosing when to harvest is one of the most consequential decisions to determine wine quality, so rather than focus on flavors which are highly subjective in nature, we refer to phenolic ripeness which has a clear endpoint.

For context, tracking anthocyanin accumulation as a measure of phenolic ripeness was a concept originally proposed by Glories and Augustin in 1993 and focused on anthocyanin concentration/extractability and tannin concentration/size (Glories, 1993). The CASV method, developed by the French Chamber of Agriculture, further developed and popularized this method targeting the week after peak anthocyanin accumulation as being ideal for harvest. Of all the possible parameters, we focus on peak extractable anthocyanins concentration because of their unstable and finite nature. Pairing this analysis with berry sugar loading (Glu/Fru), we can actively monitor the vine’s metabolism as a metric of ripeness. The rise and fall of both parameters tend to coincide together, thus making the end of accumulation a definitive endpoint for phenolic ripeness. Layering in flavors and the assessment of other physical attributes into your harvest decision is key for maximizing quality year to year.

Winemakers equipped with extractable anthocyanin and berry sugar loading data can make informed decisions about harvesting relative to phenolic ripeness. The slowing of vine metabolism makes grapes particularly susceptible to quality loss from environmental stress. Winemakers can build flexibility into their operations by harvesting earlier and employing precise oxidative treatments in the cellar. While we can appreciate the natural beauty of refining mouthfeel through field oxidation, the associated risk with climate change is compromising to phenolic balance and grape quality. Increasing weather volatility requires we build flexibility into our craft, and harvesting closer to ripeness secures greater quality for the winemaker to be able to shape in the cellar. In addition to Brix, pH, and titratable acidity, we also evaluate the development of extractable anthocyanins, fresh berry weight, and berry sugar loading to enhance our understanding of grape maturity. Of course, winegrowers will also incorporate berry flavor according to their taste preference and discretion. Tracking vine metabolism is incredibly powerful because while the grape berries continue to mature beyond this peak, the primary mechanism beyond ripeness is oxidation, not metabolism.

Post ripening and weather permitting, winemakers will often hang their fruit on the vine to improve mouthfeel and concentrate flavors. When employed well, this technique can harmonize flavor and mouthfeel in the vineyard by softening tannins, lowering reductive strength, and requiring less intervention in the cellar. What might take years in wine’s matrix can be achieved in weeks via field oxidation. This is because sunlight and relatively higher temperatures facilitate faster oxidation of phenolics in the vineyard relative to wine. The compromise in this case is a loss of anthocyanins, the potential oxidation of fruit flavors, and decreased aging potential.

Grape quality during field oxidation is particularly fragile because the fruit is no longer safeguarded by the vine’s active metabolism, which is why we suggest winemakers diversify their vilification strategy. By controlling temperature and oxidation in the cellar, winemakers can harmonize flavor and mouthfeel without risking their fruit. Anthocyanins are more readily available to polymerize and soften tannins along with applications of macro and micro oxidation.

It is important to understand that extractable anthocyanins are finite and unstable while tannins are stable. Tannins change during ripening, but they generally do not decay. Rather, decreasing tannin concentrations during ripening are due to increasing berry weights and decreased extractability from seed lignification (Bogs et al., 2005; Valero et al., n.d.). Tannin activity, or the stickiness of tannins, decreases more rapidly in the vineyard oxidation and is accelerated with exthus modulating astringency and refining texture. It is important to consider the role oxidation plays in Near phenolic ripeness, grape tissues soften, berries dehydrate, and metabolism slows. For Bordeaux cultivars, this period tends to coincide with 24 °Brix, but various sites have also shown peak accumulation past 30 °Brix. 

From their peak, extractable anthocyanin concentrations steadily begin to fall. The amount they fall is subject to each vineyard, but it is common to see as much as 50% loss within three weeks post peak extractable anthocyanin accumulation. The rate of decay increases with temperature making the harvest decision a strong stylistic determiner. From a phenolic perspective, field oxidation lowers color potential, decreases tannin activity, and decreases phenolic reactivity. Bound anthocyanins do form in the vineyard (<50 ppm), but little in proportion to the amount formed during the first 100 days post-crush. Great wines of the world tend to have flavor ripeness that coincides with phenolic ripeness. These grapes tend to be abundant in flavor which integrates into structure as colloids form via bound anthocyanin polymerization.

References

Kennedy, J. A., Matthews, M. A., & Waterhouse, A. L. (2000). Changes in grape seed polyphenols during fruit ripening. Phytochemistry, 55(1), 77–85. https://doi.org/10.1016/S0031-9422(00)00196-5

Bogs, J., Downey, M. O., Harvey, J. S., Ashton, A. R., Tanner, G. J., & Robinson, S. P. (2005). Proanthocyanidin Synthesis and Expression of Genes Encoding Leucoanthocyanidin Reductase and Anthocyanidin Reductase in Developing Grape Berries and Grapevine Leaves. Plant Physiology, 139(2), 652–663. https://doi.org/10.1104/pp.105.064238

Glories, Y. (1993). Maturité phénolique du raisin, conséquences technologiques: Application aux millésimes 1991 et 1992. Journee Technique Du C. I. V. B. : Actes Du Colloque, 56–61. https://cir.nii.ac.jp/crid/1570291224331367808

Kennedy, J. (2002). Understanding grape berry development. Practical Winery and Vineyard, 24.

The Future of Winemaking: Honoring the vision of Professor Roger Boulton. (2022, November 7). https://livestream.com/accounts/11451219/rogerboulton

Valero, E., Sánchez-Ferrer, A., Varón, R., & García-Carmona, F. (n.d.). Evolution of grape polyphenol oxidase activity and phenolic content during maturation and vinification. Retrieved August 22, 2023, from https://core.ac.uk/reader/235692900