The sugars in wine grapes are what make winemaking possible. During the process of fermentation, sugars are broken down and converted by yeast into alcohol (ethanol) and carbon dioxide. Grapes accumulate sugars as they grow on the grapevine through the translocation of sucrose molecules that are produced by photosynthesis from the leaves. During ripening the sucrose molecules are hydrolyzed (separated) by the enzyme invertase into glucose and fructose. By the time of harvest, between 15 and 25% of the grape will be composed of simple sugars. Both glucose and fructose are six-carbon sugars but three-, four-, five- and seven-carbon sugars are also present in the grape. Not all sugars are fermentable with sugars like the five-carbon arabinose, rhamnose and xylose still being present in the wine after fermentation. Very high sugar content will effectively kill the yeast once a certain (high) alcohol content is reached. For these reasons, 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 "mouth-feel") sometimes encourages winemakers to add sugar (usually sucrose) during winemaking in a process known as chaptalization solely in order to boost the alcohol content - chaptalization does not increase the sweetness of a wine.

Kilju (Finnish pronunciation: [ˈkilju]), in English also known as sugar wine, is a Finnish home-made alcoholic beverage made from sugar, yeast, and water. Its alcohol content is usually 15–17% ABV.

Cultural aspects

Kilju is considered to be a low-quality beverage that is consumed mainly for the sake of its alcohol content. It is most often found at student parties and it has a strong association with the Finnish punk subculture, presumably because of its "Do It Yourself" and anti-authoritarian characteristics. In Finland, only licensed restaurants and the government monopoly, Alko, are permitted to sell alcoholic beverages that have an ABV content of more than 4.7%. Consequently, producing and drinking kilju may be considered a form of anti-authoritarian protest.

Due to its low cost and simple production process, kilju is mostly drunk by low-income people or heavy drinkers, such as students and alcoholics. The simple production process also enables underage persons to make it quite easily.