The practice of adding ordinary
sugar to
grape juice during its
fermentation into
wine, in order to increase the
alcohol content. It ranges from necessary to unnecessary depending on the climate the
vines are grown in, and it similarly ranges from legal to illegal, since it may be regarded as a kind of cheat or
adulteration. However the fermentation product of
sucrose is the same as that produced naturally by the grapes, so it is not really an adulteration with anything foreign.
The purpose is not to sweeten the wine, since all the added sugar is intended to be converted to alcohol.
It is illegal in Australia and South Africa, in California, and in the warm southern countries of the EU. In all these places climate by itself should allow the grapes to ripen to their full sugar content. It is permitted in Canada, in the rest of the USA (which however producs little wine outside California), and in the colder climates of the EU. Intermediate climes in Europe are allowed to practice chaptalisation in a bad year.
There is also a question of grade: the highest-quality German denomination, Qualitätswein mit Prädikat, is not allowed to undergo it.
The process has long been known, and was described as such in a treatise on viniculture by the polymath French statesman and scientist Jean-Antoine Chaptal, Napoleon's Minister of the Interior. It is also known as Brix adjustment.
Other adjustment substances are possible, such as grape-derived syrup.* These practices are collectively known in enrichment. In French and German they are more forthright in calling them improvement (amélioration, Verbesserung). A more generally permitted alternative is instead to adjust the acid balance by encouraging malolactic fermentation (q.v.).
http://www.grape.org.za/is7k.html for a discussion of the South African position.
http://www.tcfb.com/alcohol/sghydrometerchaptalis.htm for the chemical calculation.
* Ouroboros says the addition of white grape juice concentrate is allowed in California, and is used particularly with chardonnays in poor years.