RIPENING

Ripeness is challenging to define in the context of grape flavor. The onset of ripeness is well defined by the second phase of berry growth when sugars accumulate, and secondary metabolites synthesize. These include most flavor compounds as well as anthocyanins for red cultivars.(Kennedy, 2002) The end of ripeness, however, is a significant challenge for winegrowers to define as choosing when to harvest is one of our most consequential decisions. Rather than focus on flavors that are highly subjective in nature, we can defer 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 (mDP)(Glories, 1993). Of these parameters, we focus exclusively on peak extractable anthocyanin concentration because of their finite nature. By pairing this analysis with berry sugar loading, we can actively evaluate the accumulation of primary and secondary metabolites as a measure of ripeness. The rise and fall of both parameters tend to coincide together, thus making their peak concentrations a definitive endpoint to “phenolic ripeness.” This is demonstrated in Figure 1 below.   

Figure 1. WINEXRAY's representative phenolic profile of a 21-day macerated Bordeaux varietal fermentation (The Future of Winemaking: Honoring the Vision of Professor Roger Boulton, 2022).

The ripening window is a progression of metabolism where grapes soften, acids metabolize, sugars accumulate, and flavors develop (transitioning from red to black fruit characteristics for red grapes). Depending on the site, anthocyanin accumulation can occur upwards of 70 days post the onset of veraison^X. The total quantity of anthocyanins and tannins heavily depends on the cultivar, climate, water availability, and a myriad of site characteristics. Over multiple vintages, berry phenolic data establishes a baseline for quality and serves as a reference point for evaluating change in a vineyard. 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.

It is also important to note that extractable anthocyanins are finite while tannins are abundant and persistent. 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.). Furthermore, tannin activity decreases with vineyard oxidation thus modulating astringency and refining texture. Extractable anthocyanins, on the other hand, will accumulate during this period until they reach their peak. 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. 

Conclusion

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.

References

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