This is the fermentation process that turns grape juice into wine.
VOA: special.2010.10.13
Today, this result is emulated by either heating the wine to between 45 and 50 degrees Celsius after fermentation in stainless steel vats, or placing it in casks that sit on the top floor of wine cellars to bake in the heat for two years.
Although all Port, produced in the Douro River Valley of Portugal, is made the same way (partially fermented wine is fortified with a neutral spirit, which stops fermentation and raises the alcohol level), there are two distinct types: ruby and tawny.