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The Porto wine has a high alcohol by volume degree, of around 20° , the sweetness is due to its production process in which the grape juice, at an early fermentation stage, is added "aguardente", a distilled of grape, preserving the taste of the grape fruit. It is also called "fortified wine", as Moscatel de Setúbal and Vinho da Mandeira.
Yeas ago, I went to a friends house in Brasil, to meet a few friends. I had brought to them a Porto wine bottle. His father showed up, and I offered him "would you like some Porto?", he replied "no thanks, it´s too adicting" :)
By the way, many people drink Porto in very small classes, that is not correct, it is much better to enjoy it in a regular red wine glass.