In the 1990s a new word-term that refers to ‘Augustus’ (Latin for taste) was derived to denote a specific period during grape ripening. This is the time period in which the varietal flavours and colour intensify - and differs from sugar and acid formation. The stage of ripening when; aromas and flavours become apparent.
This previously non-specific, distinct stage in the vines cycle is part of the late ripening period when the water-flow (xylem) in the berry is suppressed and has started to shrink. From the stored sugars in the berry are now increasingly glycoside (primary flavours or aromas) are formed. This occurs in parallel with the maturation of phenolic compounds such as tannins and anthocyanins. This state is possible at the optimum physiological maturity of the berry.

 

Following fruit set, the grape berries are green and hard to the touch. They have very little sugar and are high in organic acids. They begin to grow to about half their final size when they enter the stage of veraison. This stage signals the beginning of the ripening process and normally takes place around 40-50 days after fruit set. During this stage the colours of the grape take form red-black or yellow-green depending on the varietal. This colour changing is due to the chlorophyll in the berry skin being replaced by anthocyanins (red wine varietals) and carotenoids (white varietals). In the process known as engustment, the berries start to grow dramatically as they accumulate glucose and fructose and acids begin to fall.
There are several factors which control the onset of veraison and ultimately engustment. This is because the vine is biologically programmed to channel all its energy and resources into the berries, which houses its seed-offspring, so that they may have a better chance of survival. Conversely, very vigorous vines with lots of leaf shading for photosynthesis and water supply will delay this process due to the vines energy being directed towards shoot growth. For the production of high quality wine, it is considered ideal to have an earlier veraison.
As ripeness constitutes a variety of factors, there are many methods that viticulturist and winemakers use to determine when the grapes are sufficiently ripe to harvest. The most common method of determining ripeness involves measuring the sugar, acid and pH levels of the grapes with the purpose of harvesting at point when each number reaches its most ideal range for the type of wine being produced (sparkling, still, fortified, rosé, dessert wine etc...). In recent years winemakers have shifted away from focusing purely on those numbers towards considering other factors including the ripeness of tannins, the development of flavour precursors and the potential for glycosides to develop. A combination of these factors apart from sugar, acid and pH are considered ‘physiological’ ripeness of the grape.
While it is difficult to objectively measure the qualities of physiological ripeness, researchers in the wine industry have been continuing to study methods to determine the presence of flavour precursors and glycosides in the ripening grapes. Recently methods to determine chlorophyll content in leaves non-destructively have been applied to measuring anthocyanin content.
Flavour precursors are flavourless compounds that occur naturally in grapes as the result of normal metabolic activity of the grape vine. They are more abundant in grapes than the phenolic compounds known as flavonoids, and include compounds such monoterpenes, which contributes to the floral aroma of Riesling and Muscat, and methoxypyrazine, which contributes to the green-pepper aroma associated with Sauvignon Blanc. When these components are ‘free’ they are known as ‘flavour compounds’ but when they combine with sugars in the grapes, they become glycosides or ‘flavour precursors’. The relationship between the presence of glycosides in wine grapes and the potential for quality in the resulting wine is not an exact science but this remains an area of continuing research and development.