Sugar in Grapes

Appears in
Oxford Companion to Wine

By Jancis Robinson

Published 2006

  • About

sugar in grapes, the raison d’être of viticulture. The central role of sugar in the utility of grapes for wine, table grapes, drying grapes, and other viticultural products cannot be over-emphasized. sugars produce sweetness and ferment to produce ethanol, both of which are valued by humans. However, of all sugary plant produce, none yields a commodity as highly valued or widely produced as grape wine.

The free sugar that accumulates in grapes, glucose and fructose, is the result of translocation of sucrose photosynthesized in leaves and moved via phloem tubes into grape berries during ripening, where it is inverted (hydrolysed) by the enzyme invertase. The astonishing feature of grapes is that this accumulation occurs at the same time as water is accumulating in the berry, yet concentration is also increasing; in other words, sugar is increasing proportionately more than water. Other phloem-provided sucrose moves throughout the vine dispensing energy and the carbon skeletons for all organic molecules throughout the vine. Additionally, sugar is used for carbon storage, as starch in wood, and for the formation of glycosides in the storage of secondary metabolites in vacuoles of cells (see flavour precursors and flavour compounds).