Port-style wine underwent somewhat of an identity change several years ago. In 2009, the European Union changed the designation of any wine labeled as “port” (and various other incarnations of the word “port”) as being from Portugal only. Much like the strict “champagne” designation law, you must now label any wine made in the port style as “red dessert wine”, “white dessert wine”, or something of that nature if the grapes do not come from Portugal. The one exception is that previously approved labelings of “port” made anywhere in the world before this rule passed is grandfathered in, and may continue to use the pre-approved labeling. However, a wine does not have to be labeled port to be made in the port style.
To make a wine into port (we will use the more typical red grapes in this example), the winemaker starts with grapes that are a little higher in sugar content than usually picked for a table wine---about 26-30°B (degrees brix). The higher sugar content allows for more extracted color and jammier flavors, and leaves more sugar in the finished product.
Next, the winemaker starts fermentation as they would for any red grape. In this case, however, the winemaker does not want the yeast to survive to complete fermentation. For this reason, they may select a yeast that cannot tolerate very high alcohol content, or is specially cultured for making sweet wine. Personally, I also will deprive the yeast of some of the nutrients that I would normally make sure it has to finish fermentation.
Using a specialized formula, the winemaker figures out how much high proof brandy to add to stop fermentation, and when to add it. Once the brix gets down to a specific number based on the formula, a high proof brandy or neutral grape spirit is added at a specific volume to kill the yeast and therefore stop fermentation part-way through. Yeast cannot survive in a high alcoholic environment. Other ways to kill off yeast are exposing them to freezing temperatures or high amounts of sulfur dioxide, but alcohol is the most common.
Once fermentation has stopped (it takes about a day), the fortified wine is pressed off of the skins and ready to be put to barrel. Typically, neutral oak barrels are used. What you end up with is a rich, sweet, 6-15% residual sugar, 18-22% alcohol by volume dessert wine that can be quite intoxicating (in more ways than one) if everything is in balance. The wine can be bottled young and early, or aged for 10 years or more without spoiling. It just depends on the style the winemaker is looking for.
In the next installment of the Harvest Series, we will talk about oak selection.