You are so sweet

Residual sugar refers to the sugars left unfermented in a finished wine. It is measured by grams of sugar per litre (g/l). But how sugar morphs from grapes in the vineyard to the delicious glass of wine in your hand?

Grape Varietal

Every grape varietal has its own natural sugar backbone, genetically encoded since the dawn of existence. Grapes contain two primary carbohydrates, glucose and fructose, which together are known as sucrose. However, the amount of sugar in each grape strongly depends on the grape varietal.

Sun's Effect on Sugar in Wine

There are a number of things to consider when it comes to sunlight exposure. The amount of sunlight raining down on each leaf is roughly proportional to the amount of sugar that accumulates within these leaves and is later transported to the grape berries. Along these lines, a vineyard in a sunny spot with long daylight hours throughout the growing season would reach ripeness – and higher sugar levels – more quickly. Often, winemakers will prune and leaf trim, which can reduce the amount of photosynthesis and limit the amount of sugar accumulating in the grape berries. Less sugar means less alcohol, but adding insult to injury, these wines can often lack mouthfeel and finish. Carefully pruned vines, on the other hand, can yield wines that have very little sugar, are polyphenol-packed, and are rich and enjoyable on the palate.

Harvest Date

Winemakers carefully comb through their vineyards sampling grapes as they progress to maturity. Some winemakers have techy devices (hydrometers, pyncometers, brix meters, refractometers, and digital density meters). Others rely on the original testing device – their tongues, mouths, and sense of taste. They do this to determine whether or not there are enough concentrated sugars within the grape berries readying them for harvest. 

Fermentation

Once the grapes are harvested (and hopefully, hand-picked), they often go through additional sorting and crushing before being left in a vat to ferment. During fermentation, yeast float around in the vessel snacking on glucose and fructose, breaking down these sugars to their ultimate fate, ethanol (alcohol). A few rules of thumb on fermentation are:

Wines that are higher in alcohol are typically lower in residual sugar. This is because full fermentation has occurred and the yeast have successfully converted the sugar to alcohol.

Wines with lower alcohol are typically higher in residual sugar. If you do not have enough yeast or stop fermentation early, you are left with a wine with more sugar and less alcohol.

It is possible to have high-alcohol, high residual sugar wines, especially when grapes are intentionally harvested very late (think eiswine or trockenbeerenauslese). These are sweeter, dessert-style wines, and the fermentation process with them is a bit different. As the yeasts are eating away at the available sugar, the alcohol level rises – nothing abnormal about this. However, by the time the wine reaches an alcohol level of 15%, it becomes toxic to the yeast. This kills it off and leaves a handful of remaining sugar in the high alcohol, yet sweet drink. Hence, this is the reason why “Late-Harvest” wines are both sweet and high in alcohol.

Residual Sugar in Wine

Residual sugar is the amount of sugar left in wine at the end of fermentation. It greatly depends on the fermentation process, and how much of it is consumed by the fermenting yeast. As you can see, winemakers have a lot of choices to make when it comes to making wine.