fructose is, with glucose, one of the two principal sugars of the grape and sweet wines. It is a six-carbon atom sugar, or a hexose. Common table sugar, sucrose, is made up of one molecule of fructose and one of glucose.
The grapevine leaf in the presence of sunlight, water, and carbon dioxide makes sucrose by a complicated series of steps called collectively photosynthesis. The sucrose is transferred in the plant sap from the leaf to the grape berry. There the sucrose is split into fructose and glucose, the forms in which it is stored in the berry. The vine is unusual among fruiting plants in the extent to which it is capable of concentrating the two sugars fructose and glucose in its berries; sugars routinely represent 18–25% of grape juice weight, while 12% is the norm in apple and pear juice.
