Merlot variability at harvest affects wine composition

Amanda Vondras, Ph.D  Student
Dr. Elizabeth Tomasino, Assistant Professor
Dr. Laurent Deluc, Assistant Professor

The grapevine has a certain capacity to buffer itself in a changing environment without disrupting normal developmental processes. How different cultivars of Vitis vinifera respond to changing environmental conditions and viticultural practices beyond their buffering capacity are interesting research questions. It is complicated to consider these effects during a multi-dimensional developmental process like ripening. One approach to describe grape berry ripening is to treat whole clusters as a unit, aggregating all the berries within a cluster together for measurements, resulting in data that represents the average contribution of genes or metabolites during berry ripening. This approach overlooks dimensions of the ripening process at the berry level.

Research that we have conducted in the Deluc Lab found different ripening rates of berries within the same cluster. There is inherent variability within the cluster, the vine, and between vines. However, this is reduced as berries approach maturity in some cultivars. In examining how vineyard practices and changing environments affect fruit composition, we may be able to consider ripening within the cluster and the potential impact on ripening uniformity toward harvest, which we believe to be an indicator of increased quality. Within the Deluc Lab, we are researching the variability of individual berries during ripening to determine if this provides a more accurate depiction of the ripening process. We are collaborating with Drs. James Osborne and Elizabeth Tomasino to further study the effects that persistent berry variability has on fruit and wine composition and perceived wine quality.

During mid-véraison, there is significant variability of the berries within the cluster. Berries differ in size, softness, sugar content, and color. As grapes develop color near harvest, it may appear that variability is reduced. It is not clear whether or not variability is gone by harvest as this has not been researched extensively. A study conducted by Long (1987) revealed that the quality and complexity of a wine was dependent upon the average berry composition. Cluster heterogeneity at maturity was found to increase green characteristics from less ripe berries or jam-like characteristics from over-mature berries in wines produced. Likewise, this diversity of ripening states of berries within a cluster influenced phenolic maturity and wine composition at commercial harvest (Kontoudakis et al. 2012). In theory, we believe uniform cluster composition to be desirable for winemaking (Keller 2010). However, few studies have defined metrics for a “uniform” cluster. This is no simple task given that there are thousands of metabolites that comprise a grape berry and potentially contribute to fruit and wine quality.

The Deluc and Tomasino Labs conducted a research project in 2012 at OSU’s Woodhall Vineyard to estimate the influence of berry variability on Merlot wine composition. At mid-véraison, 100 clusters were used to monitor the progression of berries that were either green or red at that time point. The pedicels of these two berry classes were tagged with paint. Each cluster was harvested six weeks after mid-véraison, and berries were sorted based on the initial tagging as the green or red groups. Non-tagged berries that represented the intermediate ripening stages between green and red berries were used as the control group. Each group of berries was fermented separately using micro-ferments.

Chemical analysis of wine esters showed significant differences between the red and green berry groups as well as differences to the control. The wine made from the green group contained lower concentrations of some esters, and wine made from the red group contained higher concentrations of different esters. Differences in esters correspond to red- and black-berry aromas in Merlot (Pineau et al. 2009). Wine sensory analysis also resulted in significant differences with control wines having more intense floral, jam, and spice aromas, and greater in-mouth fruit density. Wines from the green berry class had more intense herbal and green aromas, and wines of the red berry class had more intense dark fruit, red fruit, and spice aromas. When wines were assessed for quality using a scale of 1 (low) to 3 (high), control and green berry wine were ranked as higher quality than the red berry wine. We concluded that berry variability present in Merlot at harvest affects the sensory characteristics and chemical composition of the wine. Further experiments to quantify non-volatile compounds (anthocyanins, tannins, and other phenolic compounds) will be performed on these wines using the OSU Mass Spectrometry Facility to complement our sensory and volatile chemical data.

To better understand the mechanisms of grape ripening, we are faced with a myriad of questions about the source, regulation, and mediation of asynchronous ripening. Although we assume that homogeneity of berries is best and that a heterogeneous crop (more variably ripe berries) would result in poorer wines, the interpretation of what level of variability is acceptable for optimum wine quality is unknown. There are many avenues to pursue in this research, as cultural practices and environmental factors may exacerbate or reduce the amount of variability during berry development. Furthermore, the amount of berry variability within the cluster at harvest may differ among cultivars.

Literature Cited

Keller, M. 2010. Managing grapevines to optimize fruit development in a challenging environment: A climate change primer for viticulturists. Aust. J. Grape Wine Res. 16:56-69.

Kontoudakis, N., M. Esteruelas, F. Fort, J.M. Canals, V. De Freitas, and F. Zamora. 2011. Influence of the heterogeneity of grape phenolic maturity on wine composition and quality. Food Chem. 124:767-774.

Long, Z.R. 1987. Manipulation of grape flavour in the vineyard: California, North Coast region. In Proceedings of the Sixth Australian Wine Industry Technical Conference, Adelaide, July 1986. T.H. Lee (ed.), pp. 82-88. Australian Industrial Publishers, Adelaide.

Pineau, B., J.C. Barbe, C.V. Leeuwen, and D. Dubourdiea. 2009. Examples of perceptive interactions involved in specific “Red-” and “Black-berry”aromas in red wines. J. Agric. Food. Chem. 57:3702-3708.

Selvaraj, Y., D.K. Pal, R. Singh, and T.K. Roy. 1995. Biochemistry of uneven ripening in Gulabi grape. J. Food Biochem. 18:325-340.


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