The winemaking process of Port wines begins just like that of traditional wines, but the fermentation is stopped halfway through by adding grape brandy. By interrupting the fermentation, the added alcohol preserves part of the natural sugars. This is why Port wines are both sweet and rich in alcohol.
At the very beginning of the 18th century, it became common practice to add grape brandy to the wine right after fermentation. This helped protect the wine from spoilage, improve its flavour, and allow it to withstand poor transport conditions during long journeys.