The importance of aromatic maturity

by Professor Kees van Leeuwen

Phenolic maturity is a much debated topic – and often a good excuse to delay harvest as long as possible – and has been successful in that it is now increasingly unusual to find wines with hard or green tannins. I find, however, increasingly wines which are not harvested at the right level of aromatic maturity, and I believe this is a subject that deserves much more attention, in particular in a context of climate change, and the stylistic push towards extreme ripeness in wines.

Introduction
Quality and typicity are much valued attributes of wine, and are among the main sources of consumer’s willingness to pay, resulting in added value in wine production.

Varietal choices, viticultural techniques, and winemaking procedures all contribute to crafting quality and typicity, as does the origin (meaning the place where the vines grow), which is referred to as the terroir effect. Typicity, as perceived by sensory assessment of wine, is the result of a complex interplay among the numerous molecular compounds present in wine, and is directly related to grape composition at the time of harvest.

Inevitably, this means that harvesting grapes at adequate maturity is key to the production of high-quality red wines. Viticulturists, oenologists, and wine makers define several types of maturity, including:
physiological maturity, technological maturity, phenolic maturity, and aromatic maturity.

Grape ripening is a dynamic process, from veraison (the onset of ripening) through harvest, during which berry composition dramatically changes, both with respect to primary (sugars, organic acids) as well as secondary metabolites (phenolic compounds, taste-active molecules, aroma precursors, and aromas). The level of maturity at which the grapes are harvested has a major impact on berry components and, as a result, on wine typicity, and is influenced by viticultural management choices and harvest date, but also by the specific combination of variety, soil type, and climatic conditions.

Breaking down the different types of maturity
Unlike other developmental stages of the vine, for example budburst, flowering, and veraison, maturity is not an easy phenological stage to distinctly define. Viticulturists and winemakers tend to search for the best possible compromise among the various types of maturity according to the style of wine they want to produce.

Physiological maturity
This is a biological concept. From a reproductive point of view, grapes are mature at veraison, when seeds have become viable for generating new vines

Technological maturity
This is defined as the point when sugar is reaching a plateau and acidity is low (in particular malic acid). The importance of pH reflects the fact that, although sugar and acidity are important parameters in grape ripeness, other compounds need to be taken into account to determine optimum date for grape harvesting, such as polysaccharides, phenolics, amino acids and aroma compounds.

Phenolic maturity
Phenolic maturity is considered optimal when the anthocyanin concentration in the skins reaches a maximum and tannin concentrations have decreased from veraison, both in skin and seeds. More important than their quantitative evolution, however, is their structural evolution, leading to textural sensations which are appreciated by tasters. Extractability of tannins and anthocyanins increases with the loosening of cell walls during grape ripening.

Aromatic maturity
This is, probably, the most important of the four in determining wine quality and typicity, including terroir expression (meaning the identifiable taste of wine in relation to its origin). Aromas are strong drivers of wine typicity, and can be classified according to the chemical family they belong to, or alternatively to the level of maturity they can be associated with. In relation to the level of maturity, wines can be perceived as green, herbal, spicy, floral or fruity.

Aromatic expression in wine can be driven, in order from low to high maturity, by green, herbal, spicy, floral, fresh fruit, ripe fruit, jammy fruit, dried fruit, candied, or cooked fruit aromas. Green and cooked fruit aromas are not desirable in red wines, while the levels of other aromatic nuances contribute to the typicity of the wine in relation to its place of origin.

Wines produced in cool climates, or on cool soils in temperate climates, are likely to express herbal or fresh fruit aromas, while wines produced under warm climates, or on warm soils in temperate climates, may express ripe fruit, jammy fruit, or candied fruit aromas.

Terroir impact on aromas
These can have multiple direct origins, like soil type, climate (in particular temperature), surrounding vegetation (like the presence of eucalyptus trees close to vineyards), or plant reactions to pests. Major quantifiable factors driving the terroir effect on grape and wine typicity, and aroma expression, include air and soil temperature, vine water status, solar radiation, and vine nitrogen status. Among these factors, air temperature appears to have the strongest impact on grape ripening and aromatic maturity, followed by soil temperature, vine water status, and radiation.

The perceived aromatic maturity increases with air and soil temperature. Green, herbal and spicy nuances are favoured by low temperatures, while ripe, dried, or cooked fruit nuances are found more often when grapes ripen under warm conditions.

Optimal terroir expression can be obtained when technological, phenolic, and aromatic maturity are reached at the same time, or within a short time frame. This is more likely to occur when the ripening takes place under mild temperatures, neither too cool, nor too hot.

With thanks to Professor van Leeuwen. You can read the full article in Oeno One. It details, among other things:
– Advances in the understanding of how aromatic maturity shapes terroir expression and how it can be manipulated by variety choices and management practices, under current and future climatic conditions.
– The compounds underpinning the aromas of wines obtained from grapes harvested at different stages of maturity.
– How maturity can be advanced or delayed by different canopy management practices or training systems.
– How timing of harvest impacts aromatic expression of the produced wine.
– Gaps in the literature are highlighted to guide future directions of research

All papers published in OENO One (https://oeno-one.eu/) as well as the more accessible technical journal “IVES Technical Reviews” (https://ives-technicalreviews.eu/) are free to read, open access.