Sugars In Wine Making Summary
There are 2 main sugars in grapes, Glucose (Dextrose) and Fructose.
Fructose tastes 1.73 times sweeter than Glucose.
As grapes ripen, the level of Glucose remains relatively constant, while the level of Fructose increases.
At typical normal ripening, the levels of Glucose and Fructose in grapes are about equal.
Sucrose (table sugar) is a di-saccharide, consisting of equal parts Glucose and Frustose.
Late harvest wines will have higher proportion of Fructose.
Yeast will only ferment Sucrose after breaking it down, with invertase and acid, into Glucose and Fructose.
Yeast will ferment Glucose before it ferments Fructose.
In wines like Port, the addition of neutral grape spirits stuns the yeast and halts fermentation, leaving a wine with a higher proportion of fructose sugars and creating a sweet wine.
Fructose, along with glucose, is one of the principle sugars involved in the creation of wine. At time of harvest, there is usually an equal amount of glucose and fructose molecules in the grape; however, as the grape over ripens the level of fructose will become higher. In wine, fructose can taste nearly twice as sweet as glucose and is a key component in the creation of sweet dessert wines. During fermentation, glucose is consumed first by the yeast and converted into alcohol. A winemaker that chooses to halt fermentation (either by temperature control or the addition of brandy spirits in the process of fortification) will be left with a wine that is high in fructose and notable residual sugars. The technique of süssreserve, where unfermented grape must is added after the wine's fermentation is complete, this will result in a wine that tastes less sweet than a wine whose fermentation was halted. This is because the unfermented grape must will still have roughly equal parts of fructose and the less sweet tasting glucose. Similarly, the process of chaptalization where sucrose (which is one part glucose and one part fructose) is added will usually not increase the sweetness level of the wine. – Wiki - Sugars in Wine
Sucrose is ideal for chaptalization of wines to be fermented dry.
Two wines sweetened before bottling, one with Sucrose, one with Fructose (equal amounts) will have the same residual sugar (RS), but the Fructose sweetened wine will taste sweeter. (Note – after a few weeks, the acid in the wine will break down the Sucrose into its Glucose and Fructose components).
During fermentation, the point where the Glucose and the Fructose starts being consumed may stress yeast, causing, among other things, a "stuck" fermentation.
WRT sparkling wines, Sucrose is usually added for the second (bottle) fermentation and in the dosage to sweeten after disgorging. It may be more advantageous to use Glucose in the bottle fermentation, thus acclimating the yeast to it. Then, sweetening the dosage with Fructose, where what little yeast is left, would have difficulty fermenting it.