Grape Development's “Translucent Stage” Key to Wine Quality

As summer arrives and temperatures surpass 15°C, grapevines begin to flourish. This marks the onset of the herbaceous stage, a pivotal period in the development of future grapes. Lasting between 45 and 65 days, this stage depends on the climate and grape variety. As this process concludes, the grapes enter a phase known as "veraison," during which they change color and begin to ripen. This transition signals the start of the grapes' physiological maturation, which lasts approximately 15 days.

However, even after veraison, the grapes are not yet ready for consumption or wine production. They must undergo another crucial stage before harvest, known as the "ripening period" or "translucent stage." This phase is vital for the final quality of the wine, encompassing the development of the grape from physiological to industrial maturity. The physical and chemical composition of the fruit is determined during this time, which directly influences the initial characteristics of the resulting wine.

Several factors influence the translucent stage, including environmental elements like temperature, orientation, climate, diseases, pests, and sunlight exposure. Vineyard management practices such as canopy management, training, and pruning also play a significant role, along with the grape variety and the desired wine style. This stage can last between 35 and 55 days, varying with grape types and environmental conditions. During this period, grapes continue to grow primarily through cell enlargement (a process called aurexis), accumulating substances that do not contribute to seed formation.

The skin of the grape grows less than the pulp, leading to an increasing tension in the skin until it becomes translucent, hence the term "translucent stage." This tension can sometimes cause the skin to crack, exacerbated by diseases affecting the skin. Cell enlargement is driven by the accumulation of substances in vacuoles, mainly water and sugars, along with mineral salts, amino acids, and phenolic compounds like tannins and anthocyanins. Concurrently, concentrations of amines and acids such as malic and tartaric acids decrease, resulting in an increase in pH.

At the end of the translucent stage, water loss through transpiration concentrates the sugars, signaling the arrival of "industrial maturity." This is the optimal time to harvest grapes for wine production, juice, or direct consumption. Physiologically, industrial maturity is reached when the harvest achieves maximum weight with the highest sugar concentration, or when the sugar-acid ratio is at its peak.

Despite the significance of this period, winemakers often use other criteria to determine grape maturity. For instance, maturity may be achieved when there is a specific balance between sugars and acids, the formation of varietal aromas (aromatic maturity), or the accumulation of polyphenols (phenolic maturity), a common method. Generally, warm, dry climates accelerate industrial maturity compared to phenolic maturity, while cooler, wetter climates with fewer sunlight hours tend to have the opposite effect.

Various "maturity indices" are employed to determine industrial maturity. These include general external maturity indices, sensory evaluation methods, and physical, chemical, and physiological maturity indices, as well as mathematical models of maturation. The quality parameters assessed during the industrial ripening of grapes before harvest not only indicate the grape's quality but also help determine its price. In contractual relationships between sellers and buyers, mathematical formulas can be developed to price the grapes based on these maturity indices.

In conclusion, understanding the stages of grape development is essential for producing high-quality wine. From the herbaceous stage to the translucent period and finally to industrial maturity, each phase plays a crucial role in shaping the characteristics of the final product. Winemakers and viticulturists must carefully monitor and manage these stages to ensure the best possible outcome for their wines.