Not all sugars are fermentable with sugars like the five-carbon arabinose, rhamnose and xylose still being present in the wine after fermentation. For this reason, no wine is ever fermented completely "dry" (meaning without any residual sugar). Sugar's role in dictating the final alcohol content of the wine (and such its resulting body and mouthfeel) will encourage winemakers to sometimes add sugar (usually sucrose) during winemaking in a process known as chaptalization in order to boost the alcohol content.[1]

Glucose

Glucose, along with fructose, is one of the primary sugars found in wine grapes. In wine, glucose taste less sweet than fructose. It is a six-carbon atom sugar derived from the breakdown of sucrose. At the beginning of the ripening stage there is usually more glucose than fructose present in the grape (as much as five times more) but the rapid development of fructose evens shifts the ratio out to where at harvest there is generally equal amounts. Grapes that are over ripe, such as some late harvest wines, may have more fructose than glucose. During fermentation, yeast break down and convert glucose first. The linking of glucose molecules with aglycone, in a process that creates glycosides, also plays a role in the resulting flavor of the wine due to their relation and interactions with phenolic compounds like anthocyanins and terpenoids.[2]

Fructose

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.[3]

Sucrose

In most wines, there will be very little sucrose, since it is not a natural constituent of grapes and sucrose added for the purpose of chaptalisation will be consumed in the fermentation. The exception to this rule is Champagne and other sparkling wines, to which an amount liqueur de expedition (typically sucrose dissolved in a still wine) is added after the second fermentation in bottle, a practice known as dosage. The amount of sugar will vary with the sweetness of the sparkling wine, but even most Brut champagnes have a small amount of sucrose added.

In wine tasting

In wine tasting, humans are least sensitive to the taste of sweetness (in contrast to sensitivity to bitterness or sourness) with the majority of the population being able to detect sugar or "sweetness" in wines between 1% and 2.5% residual sugar. Additionally, other components of wine such as acidity and tannins can mask the perception of sugar in the wine.[1]

References