The Total Soluble Solids (TSS) in wine grapes is normally 99% sugars (glucose and fructose). It is thus a good assessment of sweetness, and prices are often based upon the TSS content in harvested grapes. The level of sugar accumulation depends partly on the number of grapes – too many on any vine will dilute berry sugar content – but also on whether growth and maturity has been slowed or allowed to proceed unchecked.
As grapes develop, the ratio of glucose to fructose changes. At maturity, they are present in roughly equal quantities and when the berry becomes over-ripe, fructose is the dominant sugar.
Sugar content is measured and expressed as °Brix – or °Baumé, °Öchsle, °Babo. The °Brix value varies according to the type of grape and levels are set for the style of the wine produced by each winery.
In general, the recommended °Brix level at harvest is between 18-22° for quality wines in short season, cooler regions. In warmer, lower rainfall areas, 24°Brix is accepted as good quality. For fortified wines the °Brix may be above 30.
Over-ripe grapes – those with an increased sugar content due to evaporation of water from the berry – are used for sweeter dessert wines. These grapes are harvested when they are very ripe and contain more than 250g of sugar per litre of must.
Considering nutritional aspects, the higher the availability of nitrogen, the higher the sugar content of the grape and the potential alcohol yield after fermentation, as well as the residual sugars in the wine, as measured by °Brix. Up to certain levels, nitrogen-use increases Total Soluble Solids.
Foliar boron and zinc also help improve grape quality by increasing TSS content and reducing acidity.