In the first of a series of articles, our wine specialist, Maurizio Broggi, discusses the importance of choosing the optimal harvest time, and the influence of climate and grape varieties.

Harvest is perhaps the most frenetic and busiest period of the year for winemakers, but the last few days that precede harvest may be even more stressful. The decision of when to pick the grapes is arguably the single most important decision in the winemaking process and the greatest contributor to the style and the quality of a wine. The timing of harvest determines the ripeness of the grapes, ultimately affecting the chemical composition of the fruit and thus the starting point of the winemaking process.

Timing is crucial as the level of maturity of the fruit changes rapidly in a matter of few days and sometimes even hours. Picking too early may result in acidic, low alcohol, less flavorsome wines, but picking too late may well be equally as negative, resulting in unbalanced, alcoholic, low acid, jammy wines. One of the major decisions for the winemaker is therefore finding the sweet spot between these two extremes based on the stylistic and qualitative goal the winemaker wants to achieve.

Depending on the style, grape variety, and climatic conditions, harvest typically takes place between August and October in the northern hemisphere, and February and April in the southern hemisphere. In cooler regions, particularly for mid-to-late ripening grape varieties such as Riesling, the harvest may fall into November in the northern hemisphere and May in the southern hemisphere. Frozen grapes for German Eiswein can still be on the vines as late as December or even February of the following year.

Climate change has pushed forward the time of picking in most wine regions of the world, which is now on average ten days to two weeks earlier. In a traditionally cool climate like Champagne, the hot, dry summer of 2022 resulted in an unusually early start of harvest in August, rather than in early September as is more common.

“An August harvest used to be a once-in-a-lifetime experience in Champagne in the past,” says Charles Philipponnat, president of the renowned Champagne house Philipponnat. Picking at the famous single vineyard ‘Clos des Goisses’, began on August 24th last year.

Grapes are quite unique as a fruit. Remarkably, they naturally contain the basic chemical framework of sugars and acidity to make a balanced alcoholic beverage. Many other fruits require the addition of either sugar or acid to make a balanced fermented drink. Grady Wann, winemaker and director of the Winemaking Certificate Program at UC Davis, defines the grape berry as a “self-contained winemaking machine”.

Indeed, grapes not only have the ideal proportion of sugars and acids, but they also host yeasts on their skins. At harvest, water in the grape berry accounts for about 70 to 80% of the berry weight. Sugars may represent up to 25% or more, organic acids about 1%, and phenolic compounds between 0.2 and 0.5% of berry weight. Sugars are undoubtedly the key component as they will be converted into alcohol by yeasts during fermentation. The role of acidity in grapes is equally important. Acidity contributes to the wine’s tartness and sourness, which are essential to provide balance and freshness to the wine. Additionally, acidity greatly impacts wine’s color, aging potential, and microbial stability.

During summer, grapes go through a stage known as veraison, which marks the beginning of grape ripening. Berries soften and change in color from green to red-black for black grapes, and to yellowish for white grapes. Sugars begin to accumulate, and acidity starts decreasing. Since sugar determines the wine’s potential alcohol, it is not surprising that historically, sugar concentration was synonymous with grape maturity. Grapes were picked once a certain level of sugar concentration was attained. Even today, sugar level is one of the main contractual parameters in the sale of grapes between growers and wine producers.

If sugar concentration is often considered the primary factor that defines grape ripeness, the decreasing level of acidity is, without doubt, the second most critical indicator to assess grape maturity. In general terms, winemakers looking for optimal ripeness seek the ideal combination of sugar concentration and acidity. However, this optimal combination is very much dependent on climate, grape variety, and style; what constitutes the desired ripeness in Champagne would be considered underripe in Grand Cru vineyards in Burgundy.

In cool-to-moderate climate regions, the combination of sugar and acidity is often a reliable indicator of grape maturity. In warmer regions, however, it may not be enough. Winemakers in the New World have realized that sugar ripeness is seldom sufficient to predict the optimum timing of harvest to ensure high-quality wines, particularly in the case of red wines. It was noted that sugar ripeness is not synchronized with the maturity of flavors and phenolics. This observation led to the introduction of a new concept of grape maturity known as physiological or phenolic maturity. While the traditional concept of grape maturity considers sugar and acidity as the key basic indicators of ripeness, the aim of physiological ripeness is to evaluate the maturity of color pigments (anthocyanins), phenolics (tannins), and flavors. Physiological ripeness does not replace the conventional assessment of grape maturity based on sugar and acids, instead, it complements it, and it is adopted as an additional tool to help winemakers estimate the optimum harvest date to ensure the picking of high-quality grapes.

In conclusion, the careful balance of understanding the nuances of sugar and physiological ripeness, factoring in the impact of global climate change, and making crucial decisions about timing, all contribute to the intricate art of grape harvesting.

To be continued – in the second part of this series Maurizio Broggi looks at how winemakers measure ripeness.

Want to read more? Take a look at some of our other blogs:

• • How winemakers measure grape ripeness
  • The harvest dilemma – navigating environmental and practical constraints
  • The basics of biodynamic winemaking