There are 4 stages in the development of grape berries:

1. The vegetative period, this extends from the formation of the berry until ripening starts. The berries are small, hard and green.
2. Veraison: this marks the beginning of the ripening period. The berries change colour to yellow for white varieties and to light then dark red for black varieties. The colour change is abrupt and the sugar content of the berry rapidly increases.
3. The maturation of the berry, or accumulation phase. This stage lasts for 40-60 days. The grapes swell, the concentration of sugar increases and the acid concentration decreases. At physiological maturity the berries have attained their maximum diameter and total sugar.
4. When the berries are left on the vine they become over-ripe. The fruit becomes shrivelled as it loses water and the juice becomes more concentrated. This stage is important in the production of sweet and reccioto wines.

The accumulation stage

The accumulation stage is characterised by:

• A softening of the berry caused by an increase in skin elasticity and a swelling of the pulp cells triggered at veraison. The berries gain large amounts of water and increase in weight and volume.
• An increase in glucose and fructose which are the major sugars in grapes.
• A decrease in acid concentration. Some malic acid is broken down by respiration, but its concentration is mostly reduced by dilution. Tartaric acid concentration does not vary greatly.
• An increase in levels of phenolics. Anthocyanins accumulate in the skins of black grapes giving them their characteristic colour. This development is influenced by exposure to sunlight. Tannin levels also increase.

The majority of the desirable flavour components accumulate later in ripening after the sugar increase slows down. Both the intensity and the descriptive characters of the grapes change as the grapes mature. They appear to reach a maximum then decline during the over-ripe stage.

The image above is reproduced with the permission of the publisher, Winetitles Pty Ltd from Viticulture Volume 2: Practices, BG Coombe & PR Dry editors, 2001.

The physiological maturity of grapes can be defined as when they have reached the maximum sugar content and size and the vascular bundles that link them to the rest of the plant cease functioning. However, grapes are not necessarily always picked at this level of maturity.For instance, the desired levels of maturity for Pinot Noir grapes destined for sparkling, rosé or still red wines would be very different.