Yeast (wine) in the context of Residual sugars

Dessert wines, sometimes called pudding wines in the United Kingdom, are sweet wines typically served with dessert.

There is no simple definition of a dessert wine. In the UK, a dessert wine is considered to be any sweet wine drunk with a meal, as opposed to the white fortified wines (fino and amontillado sherry) drunk before the meal and the red fortified wines (port and madeira) drunk after it. Thus, most fortified wines are regarded as distinct from dessert wines, but some of the less-strong fortified white wines, such as Pedro Ximénez sherry and Muscat de Beaumes-de-Venise, are regarded as honorary dessert wines. In the United States, by contrast, a dessert wine is legally defined as any wine over 14% alcohol by volume, which includes all fortified wines—and is taxed more highly as a result. This dates back to when the US wine industry only made dessert wines by fortification, but such a classification is outdated now that modern yeast and viticulture can produce dry wines over 15% without fortification (and German dessert wines can contain half that amount of alcohol).

Sugars in wine are at the heart of what makes winemaking possible. During the process of fermentation, sugars from wine grapes 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.

Chaptalization is the process of adding sugar to unfermented grape must in order to increase the alcohol content after fermentation. The technique is named after its developer, the French chemist Jean-Antoine-Claude Chaptal. This process is not intended to make the wine sweeter, but rather to provide more sugar for the yeast to ferment into alcohol.

Chaptalization has generated controversy and discontent in the French wine industry due to advantages that the process is perceived to give producers in poor-climate areas. In response to violent demonstrations by protesters in 1907, the French government began regulating the amount of sugar that can be added to wine.

A stuck fermentation is the term used in brewing beer or winemaking when the yeast becomes dormant before the fermentation has completed. Unlike an "arrested fermentation", where the winemaker intentionally stops fermentation (such as in the production of fortified wines), a stuck fermentation is an unintentional and unwanted occurrence that can lead to the wine being spoiled by bacteria and oxidation.

There are several potential causes of a stuck fermentation; the most common are an excessively high temperature killing off the yeast, or a must deficient in the nitrogen food source needed for the yeast to thrive. Once the fermentation is stuck, it is very difficult to restart due to a chemical compound released by dying yeast cells that inhibits the future growth of yeast cells in the batch. Winemakers often take several steps to limit the possibility of a stuck fermentation occurring, such as adding nitrogen to the must in the form of diammonium phosphate or using cultured yeast with a high temperature and alcohol tolerance. These steps will each have their own subtle or dramatic effect on the resulting flavors and quality of the wine.

Governo is a winemaking technique reportedly invented in Tuscany in the 14th century to help complete fermentation and stabilize the wine. The technique involves saving a batch of harvested grapes and allowing them to partially dry. If fermentation of the main batch starts to slow or appears to be nearing stuck fermentation, the half dried grapes are added to the must which then gives the yeast cells a new source of sugar to enliven the batch. From there, the must can be fermented dry or stopped with the wine having a higher level of residual sugar. The process was widely used in the Chianti zones until the advent of temperature controlled fermentation tanks. From Tuscany the technique spread to Marche and Umbria where it is sometimes used today. In the Marche the technique is most often used on wines made from the Verdicchio grape to counteract the grape's natural bitterness and to add some sweetness and frizzante qualities.