🍷 Grape sugar

Grape sugar is the sugar that builds up in the berries, mainly glucose and fructose. The leaves send sucrose to the berries, where it is split into glucose and fructose that collect in the cells of the flesh. After ripening begins, at veraison, the sugar in the berries rises quickly. ¹

How it builds up. Before veraison there is little sugar in the berries, and about twice as much glucose as fructose. Some 20 days after ripening begins, simple sugars reach about 1 mole per litre and glucose and fructose become equal. They are stored in the vacuole, where the enzyme invertase splits sucrose. ¹

How sugar becomes alcohol. During fermentation yeast turns sugar into almost equal masses of ethanol and carbon dioxide. The winemakers' rule of thumb is that 18 g of sugar per litre gives about 1% alcohol. In practice only about 90% of the theoretical alcohol forms, because some sugar feeds yeast growth and some alcohol evaporates. ²

How it is measured. Sugar is measured in degrees Brix with a refractometer: 1 °Brix equals 1 g of sucrose per 100 g of solution. ³ It is an estimate rather than an exact weight of sugar: in ripe berries sugars make up 90–95% of the dissolved solids, the rest being acids, pectins, salts, tannins and pigments. ²

Sampling. Sampling starts at least two weeks before the expected harvest, weekly after veraison. At least 200 berries are taken from each variety and block, from different clusters on both sides of the trellis. Sugar together with acidity helps choose the harvest date. ³

How much sugar is needed. Some cold-hardy varieties ripen to 25–27 °Brix, which gives wine with 15% alcohol or more. Others reach only 19–22 °Brix before off-flavours appear. ³

Sugar and price. In Georgia the price of grapes depends on their sugar content. In autumn 2026 the state company in Kakheti paid 1.50 lari per kilogram for Saperavi with at least 20% sugar, 1.00 lari for 16–20% and 0.50 lari below 16%; Rkatsiteli with at least 19% fetched 1.30 lari. ⁶

When sugar is short. In the EU natural alcoholic strength may be raised by at most 3% in zone A, 2% in zone B and 1.5% in zones C, by adding sucrose or concentrated must or by partly concentrating the must. Sucrose may only be added dry, and in Greece, Spain, Italy, Cyprus, Portugal and southern France it is banned, with exceptions. ⁵

Why it matters to you. Grape sugar sets a wine's strength: the riper the grapes, the higher the alcohol on the label. ² If not all the sugar ferments, residual sugar remains: a still wine with up to 4 g/l (up to 9 g/l with enough acidity) is still dry, and above 45 g/l it is sweet. ⁴