Véraison: How the Grape Berry Gets Sweet

Véraison, is the onset of ripening in the grape berry. This is where the grapes enter the second rapid growth phase and is marked by the rapid increase in both the berry volume, soluble sugar accumulation and a rapid decrease of the organic acid contents. In other words, the grape is beginning to get sweeter, tastier and less acidic.
The transition from the lag phase to véraison is a critical development checkpoint in the grape berry and can last for 1 to 2 days.¹
At véraison, water import into the grape berry shifts from the xylem to the phloem. (Please checkout my blogpost: Grape Berry Circulatory System). Before véraison, the xylem is the main pipeline that delivers water, mineral nutrients, and growth regulators from the root system to the rest of the vine, including the developing grape berry. Water can move in and out of the berry through the xylem.
The phloem, is the vascular pathway that brings the photosynthates or the sugary solution made from photosynthesis by the leaf (the source), into the berry (the sink). The phloem can transport the photosynthates only one way, from the leaves to the berries.
This sugar concentration delivered by the phloem is mainly determined by the grape genotype but the environment, temperature and cultural management such as irrigation can have an impact on the final sugar concentration.
The phloem delivers sugar in the form of the dissacharide sucrose which is comprised of one molecule of glucose and one molecule of frustose. (Please checkout my blogpost: Sugar in Grape Berry and Wine---Sugar Transport and Accumulation in Berry). The uptake of sugar in the grape berry can be affected by various parameters including:²

• light
• water
• ion status
• wounding
• fungal and bacterial attacks
• hormones

Massive accumulation of glucose and fructose in the vacuoles of mesocarp cells in the grape berry occurs after véraison. This is accomplished through the action of an enzyme called an invertase that converts the dissacharide sucrose via hydrolysis, into it's components, glucose and fructose. Different invertase isoforms are localized in the cell wall, cytoplasm and vacuole.
Before ripening, the level of total soluble sugars in the phloem of the pedicel of grape berry is lower than 50 mM, whereas it reaches more than 500 mM at the beginning of ripening and 1,000 mM at the late ripening stage!
References:
1. Xiao-Yan Zhang, Xiu-Ling Wang, Xiao-Fang Wang, Guo-Hai Xia, Qiu-Hong Pan, Ren-Chun Fan, Fu-Qing Wu, Xiang-Chun Yu, and Da-Peng Zhang, "A Shift of Phloem Unloading from Symplasmic to Apoplasmic Pathway Is Involved in Developmental Onset of Ripening in Grape Berry", Plant Physiology, September 2006, Vol. 142, pp. 220–232.
2. António Manuel Jordão, F. Cosme, and Alice Vilela, From Sugar of Grape to Alcohol of Wine: Sensorial Impact of Alcohol in Wine, Beverages, 2015, 1, 292-310.