Unlocking the secrets to high-quality wine: a new method for monitoring grape quality

The quality of grapes plays a crucial role in the production of high-quality wine. Various factors contribute to the quality of the grapes, including sugar content and berry weight. Monitoring these parameters is essential to determine the optimal harvest time and assess the vine and grape status.

A research group led by Professor Deloire has proposed a method for monitoring the sugar content and grape volume during the harvest season. This method involves a meticulous sampling strategy based on sugar concentration values, expressed in degrees Brix (°Brix), rather than fixed dates. The °Brix values indicate the amount of sugar in a liquid, in this case, grape juice.

The protocol sets different grape maturity stages based on sugar concentration:

• 6-8 °Brix: Indicates the onset of veraison, with green and soft grapes.
• 12-15 °Brix: Corresponds to mid-veraison, with generally 80% soft berries and 50% colored berries in the clusters.
• 18-20 °Brix: Represents the theoretical plateau of sugar content in the berries, with 100% soft and red berries for red grape varieties.
• 22-24 °Brix: Is the post-plateau stage of sugar content in the grapes.

°Brix values above 24: Generally correspond to overly ripe grapes.

Monitoring these °Brix values requires weekly visits to the vineyard to measure some berries' °Brix values and determine whether more sampling is needed to reach maximum sugar content. This weekly sampling is necessary as the grape sugar accumulation rate depends on the variety, environment, and cultivation practices.

Sampling should start at the beginning of veraison, with average °Brix values of 6-8. Between 18 and 22 °Brix, it may be necessary to visit the vineyard twice a week, as the sugar content plateau can be reached quickly within 2 or 3 days from 18 °Brix.

For sampling, a sample of 200 to 400 berries or about 10-12 clusters is usually taken in each vineyard. This sample size can be adjusted in the case of heterogeneous or small vineyards. However, the number of sampled berries should not exceed 10% of the vine's maximum fruit load to avoid altering the development of the remaining berries.

It is essential to measure the fresh weight and °Brix of the berries simultaneously to calculate the sugar amount per berry (mg/berry) using the following formula:

Sugar (solutes) per berry (mg) = (fresh berry mass (g) x (°Brix/100)) x 1000

This protocol allows rapid decision-making in different aspects:

• Water supply management through irrigation.
• Anticipating the harvest when water loss or berry wilting begins, especially after the sugar accumulation plateau, to limit yield losses and fruit concentration.
• Anticipating harvest dates based on the desired aromatic profiles of the wines (fresh fruit/ripe fruit).
• Vine performance assessment. The reduction or inhibition of photosynthesis due to thermal and/or water stress is observed in real-time when there is a decrease in sugar accumulation and a reduction in berry volume.
• Assessment of the heterogeneity of maturation dynamics, which can have consequences on the aromatic profile of the wines.

Monitoring grape sugar content and weight is crucial for optimizing grape quality and, therefore, wine quality. This protocol proposed by Professor Deloire's research group provides an effective methodology for vineyard management decision-making and determining the optimal harvest time.