Veraison

Véraison is a French term but has been adopted into viticultural English. The official definition of véraison is "change of colour of the grape berries" and signifies the change from grape berry growth to ripening.
Red grapes turn from green to red (anthocyanin pigment formation), white grapes from green to yellow (carotene and xanthophyll pigment formation).
Fruit development occurs in two distinct growth cycles separated by a period of little or no growth as part of a double sigmoid curve pattern. The first is the green stage where rapid cell division takes place and high levels of malic and tartaric acid are produced. Then during the "rest" period embryonic development within the seed occurs. This is followed by rapid water intake and cell enlargement but little or no cell division with a resultant softening, accumulation
of sugars and metabolism of acids.

Water is the main component of a grape berry with between 70 and 85% present.
Two sugars, glucose and fructose account for around 90% of the soluble carbohydrate in grape juice.
There are also traces of alcohols such as ethanol and glycerol.
Along with tartaric and malic acids (90%) citric, lactic, succinic and acetic acids occur in very small amounts.
The two main groups of phenols are tannins and anthocyanins.
Mineral nutrients include K and N (amino acids, ammonium ions and proteins).
Small traces of compounds that account for the flavour and aroma of wine eg. terpenes and methoxypyrazines along with hundreds of yet unidentified compounds are also present.
Technical ripeness is usually defined as a correct balance of sugar, pH and acidity. This is very dependent on climate conditions and most times an imbalance of these three criteria occurs.
For example, warm climates produce grapes of a higher sugar level but lower acidity. Cool climate grapes the opposite.
Also to be considered are aromatic maturity (varietal and fermentation aroma precursors) and phenolic maturity (tannin concentrations) which are also climate related.
To catch these three maturity indices in sync is always the aim but hardly ever achieved.
One has to strike a happy balance.
In Australia we can always add acid during the winemaking process but adding sugar is not allowed. So sugar levels, aromatic and tannin maturity are closely monitored during ripening.
Sugar ripeness is determined by taking samples of grapes from a cross section of the vineyard as the ripening process progresses and testing the juice of a combined crush with either a hydrometer or by refractometer.
Sugar levels are commonly designated in degrees Baume or Brix.
1° Baume = 1.8° Brix
We use Baume here as it also indicates the potential alcohol level of the finished wine eg. 12° Baume = 12% alcohol (v/v).

pH meter - hydrometer - refractometer
pH is measured by a pH meter and indicates the hydrogen ion activity of the acid. This is an important number as it will determine the addition rate of various chemicals during the wine making process eg. sulphur dioxide.
Titratable acidity (TA) indicates the concentration of the acid present in g/L (tartaric acid).
Not all varieties of grapes ripen at the same time.
Here we expect the Pinot Noir and Tempranillo to be ready early February, the Semillon a little later that month and the Cabernet Sauvignon some time in April.