It is essential to pick grapes at the desired level of ripeness and very useful to know as far in advance as possible a date when harvest can be safely projected & planned for. As a useful guide, in a normal season, harvest will be from around 100 –120 days from flowering or around 6 –7 weeks from veraison, dependent on the wine style required.

In order to be more accurate, berry sugar and acid levels can be monitored, usually twice a week. These can then be plotted on a graph, which can help predict when the target levels will be reached. Additionally, because of the empirical ‘link’ between the rise in sugars and the decline in acidity, the ratio of sugar to titratable acidity may be plotted. Generally towards maturity the slope of this curve plateaus.

Sampling

In order to measure berry development accurately, it is essential to get a representative sample of grapes from the vineyard. Hamilton and Coombe (1992) suggest a minimum sample size of 200 berries per homogenous vineyard plot, using mostly bits of clusters. This must include top and bottom of clusters, exposed and shaded bunches and also different sides of the rows and different heights on the vine. Berries at the centre of compact bunches are less ripe, and some whole clusters should be taken also.

In the lab, the berries are separated, counted and weighed. Juice is extracted with a small manual press or crushed by hand in a plastic bag and passed through a sieve. Juice sugar and acid concentrations are then determined

Monitoring sugar levels

Sugar is the major soluble solid in the must and hence its concentration may be measured by measuring the density of the must. The ‘denser’ the must, the more the sugar content (assuming that sugar is the only dry solid extract; true at the range 90-95%).

Units for this measurement vary around the world; France uses Baumé and Relative Density, Australia uses Baumé and Brix, USA uses mostly Brix, Germany and the UK uses Oechsle.
The equipment used to monitor sugar is usually either a:

• Refractometer that measures the deviation occurring when a beam of light moves from air to a sugar solution.
• Hydrometer that measures relative density or specific gravity.

Monitoring acidity changes

Changes in the acidity of grapes can be measured by changes in the titratable acidity and/or the pH of the berries.

Titratable acidity

Titratable acidity (TA) is the concentration of acid measured by neutralising a must or wine with a strong alkali.
In Latin and Southern European countries, titratable acidity is measured in sulphuric acid units (as if all the acid was made up of sulphuric acid), whereas New World and Northern European countries (including the UK) use tartaric acid units.
In order to convert:

Sulphuric to tartaric acid multiply by 1.53

Tartaric to sulphuric acid multiply by 0.65

pH

The pH gives the concentration of free hydrogen ions ([H+]) in a solution. As this figure is often very small, it is converted into its negative logarithm:

pH = - log [H+]

For instance, the concentration of hydrogen ions in wine is usually around 0.001 g/l, which is equal to a pH of 3.

The pH scale covers the range 0 to 14. Values from 0 to7 are acidic, 7 is neutral and 7 to 14 are basic solutions. The lower the number on the pH scale the higher the hydrogen ion concentration and the more acidic the solution. The lower the pH value of a juice or a wine the less chance there is of spoilage reactions occurring as bacteria find it more difficult to survive, and sulphur dioxide, the principal preservative used in wine, is more effective. The pH also has a strong influence on a red wine colour.

The pH value of grape juice and wine is normally in the range pH 2.8 to pH 4.2. As the pH scale is logarithmic, small changes in the pH represent large changes in the hydrogen ion concentration. For example there are twice as many hydrogen ions in a solution with a pH of 3.1 than there are in a solution with a pH of 3.4.

As results are built up (and compared with other vintages) an estimate of harvest date may ultimately be made.