French winemakers use voltammetry to refine grape harvest timing and enhance wine aroma profiles

In recent years, winemakers in France have been exploring new methods to determine the optimal harvest date for grapes, aiming to produce wines with specific aromatic profiles. One of the latest techniques under study is linear sweep voltammetry, a method that measures changes in the chemical composition of grape must during ripening. This approach has been tested by a team of French enologists from Vinventions and Vignerons des Coteaux Romanais, particularly in the Loire Valley, with a focus on Sauvignon Blanc grapes.

The research began in 2015 and involves weekly monitoring of grape maturity using voltammetric analysis from the end of veraison—the onset of ripening—until harvest. The process consists of collecting samples of 200 berries from both sides of vineyard rows, pressing them by hand, and analyzing the resulting juice with a potentiostat. The device applies a voltage sweep and records the current response, which reflects the presence and concentration of phenolic compounds in the grape pulp.

Traditional methods for determining harvest dates rely on measuring sugar content, pH, total acidity, and berry weight. However, these parameters do not provide direct information about the aromatic potential of the grapes or the future wine. Sensory evaluation of berries can offer some insight but is labor-intensive and impractical for large vineyards.

The voltammetric method centers on an index called Maturox, calculated from the area under the current-potential curve between 400 and 1000 mV. This index decreases as grapes accumulate sugar and their volume increases—a phase known as active sugar loading. During this period, phenolic compounds synthesized before veraison become diluted as berry size grows, leading to a drop in the voltammetric signal. When sugar loading ceases and berry growth stabilizes or reverses due to dehydration, the Maturox index reaches a minimum or begins to rise again.

To better understand these changes, researchers conducted laboratory experiments using green tea solutions as models for grape must. Green tea contains monomeric phenolic compounds similar to those found in grape pulp but lacks sugars and alcohol. By diluting tea solutions and adding varying amounts of glucose and fructose, scientists simulated different stages of berry development: initial low-sugar must, active sugar loading with increased dilution and sugar content, and post-loading dehydration with higher sugar concentration.

The results showed that both dilution (mimicking berry growth) and increased sugar concentration contribute additively to lowering the voltammetric signal. The lowest signal corresponded to conditions simulating the end of active sugar loading—a key point for determining harvest timing. Further analysis confirmed that neither sugars nor acids alone produced significant voltammetric responses; rather, it was their interaction with phenolic compounds that drove changes in the Maturox index.

Field data from Mourvèdre grape parcels supported these findings. Weekly measurements revealed a consistent pattern: as berries matured, the Maturox index dropped until reaching a minimum at the end of sugar loading, then stabilized or increased slightly as dehydration set in. This minimum point aligned closely with stabilization in sugar per berry—a critical marker for winemakers seeking to control wine style.

In practical terms, this technique allows growers to pinpoint when their grapes stop accumulating sugar actively. This moment is crucial because it marks a transition in berry chemistry that influences not only sweetness but also aromatic development. For Sauvignon Blanc in particular, subsequent research (detailed in Part 2 of the study) demonstrated that harvesting at different intervals after this minimum Maturox point yields wines with distinct aromatic profiles: fermentative or vegetal notes when picked soon after, varietal thiol (boxwood) aromas at intermediate times, and citrus-like thiol aromas when harvested later.

The adoption of linear sweep voltammetry offers several advantages for vineyard management. It requires only small juice samples and provides rapid feedback on berry maturity without complex sample preparation or expensive reagents. The method is also adaptable for physiological studies at the single-berry level.

This research highlights how advances in analytical chemistry are helping winemakers make more informed decisions about harvest timing. By linking measurable changes in grape chemistry to sensory outcomes in wine, tools like voltammetry are becoming valuable assets for producers aiming to meet consumer demand for consistent and distinctive wine styles. As climate variability continues to challenge traditional viticulture practices across Europe and beyond, such innovations may play an increasingly important role in maintaining quality and character in wines from regions like the Loire Valley.