Grape maturation: more than just a change of colour

While Portugal is now seeing some signs of Spring with longer and warmer sunny days, Australia is well through Summer and with the 40 plus degree days behind us Autumn is just around the corner. In a wine production country this means that the busiest time of the year has arrived and it is vintage time! For us winemakers the long vintage days are the most exciting time of the year, when we finally get the grapes in the winery and eventually manage to turn them into (good) wine. But it is also one of the most critical stages of the whole winemaking process when the decision of what and when to harvest needs to be taken. In this review we will focus on what happen during the late stage of the grape development (maturation) and its relation with the harvest timing.

I work in the Clare Valley (Australia) mainly with ‘international varieties’, such as Riesling, Pinot Gris, Shiraz or Cabernet Sauvignon, which all grow in the same region and pretty much under the same conditions, but that ripen at different time in the season (in some cases up to 1-2 months apart). By the time that you read this article we should have harvested all our Pinot Gris but the Cabernet will be hanging on the vines for a few more weeks, so how do we decide what and when to pick?

The biological cycle of the grapevine is a set of physiological and biochemical changes triggered by temperature, sun exposure, hormones, water availability, etc, that starts early in the Spring with bud burst and finishes late in the Autumn with leaves fall, just before the vine goes through a period of dormancy during Winter.

What happens during each stage of the grapevine life cycle can potentially influence the quality of the wine, but it is the late stage of the berry development or maturation (ripening) that deserves more attention from a winemaking point of view.

Maturation is a growth phase that lasts from 35 to 55 days that follows the herbaceous growth and véraison and is characterized by some of the most noticeable changes in the grapes: pronounced berry growth, sugar accumulation, decrease of acidity/raise of pH and accumulation/changes in phenolic compounds and aromatic. We are able to look at these changes and use that information as a precious tool to predict maturity dates and establish the picking dates.

In the ripening stage the berries accumulate sugar and lower their organic acid concentration, with dramatic changes in the profile of the phenolic compounds and aromatics.

Sugars

The sugar content of the grapes is an important physiological parameter to access maturity and harvest timing as it defines the potential alcohol of the wine. During the herbaceous growth its concentration is similar to the leaves, but from véraison onwards there is a massive transport and accumulation of these carbohydrates in the berries. The sugar synthesis occurs in the leaves as a product of photosynthesis and migrates to the berries in the form of sucrose where it is hydrolyzed to glucose and fructose. The last two are the main sugars in grapes and as the season progresses their concentration gets to a point where they become the predominant total soluble solids in the juice, reason why sugar accumulation/ripening traceability is often based on density measurements (density, Baumé or Oechsle). It is not commonly used, but the glucose/fructose ratio can be a maturity indicative as it markedly decreases during the grape development until it remains almost constant at maturity (about 1:1).

White grapes generally ripen at lower sugar levels than red grapes and consequently the alcohol content of white wines is lower than reds once fermentation is completed by yeast (conversion rate of approximately 17g/L of glucose/fructose for 1% alcohol).

Acids and pH

Tartaric and malic acid are the two major acids present in grapes and responsible for the biggest fraction of the total acidity. The tartaric is rapidly accumulated in the berries during the herbaceous growth and remains almost constant during maturation; on the other hand, malic acid concentration declines during the ripening period and the ratio tartaric/malic varies drastically. The rate as the malic acid is metabolized depends on the variety and climate, but it is one of the main reasons why cool climate regions tend to deliver fruit with higher total acidity comparing to warmer regions. As the total acidity drops and some cations accumulate in the berries, the pH raises. The pH plays a crucial role in the microbiological and chemical stability of juice and wine it is also taken into account. At maturity the pH of white grapes is normally under 3.3 and 3.6 for the reds, and the total acidity between 5.0-8.0g/L of tartaric acid equivalents.

If you add a few drops of vinegar or lemon juice to your salads (acidification), this seasoning will make it taste better. The same could be thought for the importance of the acidity in wine. When in the balance, the acidity is the backbone of the wine, bringing brightness and freshness and lifting up other flavors.

Phenolic compounds and aromatic substances

The synthesis and accumulation of anthocyanins in the skins is the most visible expression of grape maturation in red grapes; behind the scenes, another phenols play a crucial role on the phenolic ripeness: the tannins. Tannins are present both in the skin and seeds and as season progresses they become less extractable and less astringent, more ‘round’ and pleasant. A potential good wine starts with a good assessment of the phenolic ripeness of the grapes, as it impacts the structure, mouthfeel, astringency, aromatics and ageing potential of the wine.

It is also during the maturation stage that the aromatic potential of the grapes develops and accumulates, mainly in the skins. These molecules can be free volatile aromatic compounds or non-aromatic precursors that will later be released by yeast during fermentation. Sometimes it may be hard to access the aromatic potential of the grapes at a given time, but we know how it can change during maturation. A good base wine for sparkling is produced with grapes harvested earlier in the season, not only to retain a higher natural acidity/low alcohol, but also because the aromatic profile is more neutral. Riesling or Touriga Nacional harvested later in the season originate wines with more floral expression.

Generalized graphical representation of grape berry compositional changes during development and ripening (from Watson, 2003)

The maturity point of the grapes is directly related to the style of the finished wine and can potentially limit its quality. Monitoring the parameters mentioned above as the grapes ripening progresses is a fundamental procedure to define a desired maturity point and decide the harvest date. Variety, soil, vineyard practices and growing conditions all influence the life cycle of the grapevine, so defining an exact maturation point is in reality difficult and quite subjective. However, if we can get the maturation to a point where all the parameters are in balance to a desired style then we are potentially in a good position to make a good wine!

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Author: Joel Santos

Expert in winemaking and wine biotechnology. Winemaker and technical manager at Tim Adams Wines (Clare Valley, Australia). The only Portuguese winemaker selected for the 2nd Ningxia Winemakers Challenge (Ningxia, China), working at Xin Hui Bin winery. Joel has done multiple vintages in several continents and regions. Experience as a wine researcher, focusing on the production of sparkling wine.

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