When should the grape harvest begin?

The grape harvest is one of the most important operations in viticulture. Its success depends on the ideal point of ripeness, that is, when there is a balanced level between the sugars and the acids in the fruit. These two factors stabilise at the same time as the berries reach their maximum weight.

Evolution of Acidity, pH and Sugar during Ripening. Adapted from: Roediger (2006)

Factors that determine the moment of harvest

Besides the point of ripeness, there are many other factors that determine the ideal moment to harvest, such as:

• Characteristics and adaptation of the grape variety;
• Age of the vineyard;
• Purpose of the crop (fresh consumption or winemaking);
• Soil and climate conditions;
• Training system used;
• Cultural operations carried out over the annual cycle;
• Demands and characteristics of the target market.

Many growers and winemakers are concerned with the quality of the varieties and their behaviour in specific weather conditions, such as high summer temperatures and tropical nights, which cause the vines to lose part of their acids through respiration and to gain sugar levels above what is desired.

Only by bringing all the factors together is there a greater likelihood of producing grapes and wines of the very highest quality.

How to determine the harvest date

The harvest date can be determined through simple, empirical but subjective methods, which may include:

• Visual observation (for example, the colour of the berry skins);
• The ease with which the berries detach from the bunch;
• A sensation of sweetness in the mouth;
• The sticky feel of the must on the fingers;
• Ripe pips, brownish in tone.

Predicting the optimum harvest date should, however, make use of more precise and rigorous methods.

How to go about it

The grower should obtain representative samples from the plot by marking at least 50 vines per hectare, spread across several points.

To assess the concentration of sugars, two types of method are used: refractometry or densimetry. To determine total acidity, the method of titration with decinormal sodium hydroxide is used.

A refractometer, used to assess sugar concentration

Around 4 weeks before the expected harvest date, berries are picked from the marked vines (about 4 berries per vine), taking them from the initial and terminal part of the fruiting cane, from the upper and lower part of the bunches and, alternately, from one side of the row and the other. This should be repeated weekly and accompanied by analysis of the mass of the berries and of the must.

When two consecutive samplings show the same percentage of sugar, that is, the same probable alcohol content, as well as the same acidity, it is a sign that there has been no further evolution of the sugars and therefore that ripeness has been reached, and the harvest can begin.

Are you a grower? Have you started your harvest?

Tell us which methods you use to decide when to start picking your grapes!