Welcome to the wonderful world of fortified winemaking! Making Port-style dessert wines is really fun.
One of the side benefits of making these types of wines is that fortified wines are typically more shelf-stable than other table wines because its high sugar and alcohol levels (typically 6–9% and 18–21% respectively) helps protect it both from oxidation and microbial spoilage shenanigans. This means you can open a bottle, sip a glass or two and simply cork it up and put it back on the shelf for a few days or weeks without too many ill-effects (to the wine, I mean!). For home winemakers and those new to winemaking, fortified wines can be more forgiving for a similar reason — because they are all about the power of the fruit, sugar and booze it means no one will notice as much if your acid level isn’t perfectly adjusted, if you over-oaked it or if you have too much or too little tannin. They are not exactly “finesse” wines the way, say a 13.8% Russian River Pinot Noir would be.
A simple procedure for a grape-based Port-style wine would include the following: inoculate 23–26 °Brix red grapes with a neutral-aroma yeast. When Brix of your normal fermentation reaches 10–12, begin “feeding” the fermentation alcohol to arrest the fermentation, mixing the must almost continually in order to get a good distribution of the added alcohol throughout your fermenter. Brandy (distilled wine, 80 proof/40% alcohol) is often added to grape Ports, while fruit-based Ports often rely on neutral grape spirits or other aromatically-neutral alcohol so the fruit flavors and aromas can shine through. I know many home producers use Everclear as it has a high alcohol content (151 Proof/75% alcohol in some states, 190 proof/95% alcohol in others) and no other flavor or aroma. For fruit Ports, I personally like to use neutral spirits to let the fruit shine through, and for grape-based Ports, I think using oak-aged spirits can bring a benefit — it’s up to you as the winemaker, however, to decide what works best in your Port.
Depending on your yeast strain’s alcohol tolerance, the yeast will begin to lose steam as the alcohol level rises beyond what they can tolerate, typically 14% alcohol and above. Essentially, the rising alcohol level will kill your yeast cells and you’ll be left with a wine that has un-fermented sugar between 6–9% and hopefully an alcohol content of 18–21%. I recommend aging the stopped, sweet and alcoholic wine in a neutral oak barrel (or in a carboy or keg with French oak chips if you don’t have a barrel) for at least six months before thinking about bottling. Some fruit Ports can be ready faster than grape-based wines, which often need quite a bit of time — at least 10 months — to mellow out. I like to give fortified wines at least 18 months bulk aging to let them throw deposits and precipitates. Because of the higher alcohol content, proteins and other material will tend to come out of solution over time.
As you might imagine, there are many variables that will dictate what the final alcohol level and residual sugar level will be. The ability of your yeast to resist alcohol, your initial Brix and the alcohol level of your added spirit will all contribute to the algebra. When I worked at Bonny Doon, I remember building spreadsheet models where I would plug all of that data in to predict my final sugar and alcohol content, as well as when it was predicted my fermentation would stop. Whew, you can geek out on that stuff!
If you’ve been keeping up with the math, here, you’ll understand that you can’t just feed your way to fortified. Even by using alcohol-tolerant yeast and step-wise sugar additions, your yeast will peter out at no higher than 15.5% alcohol . . . almost high enough to be fortified, but not quite. You’ll just have an alcoholic wine with some residual sugar. To really smell, age and drink like a Port-style dessert wine, you really want your alcohol to be a little higher.
For further reading, check out the column that Joe O’Neal wrote on fruit Port back in 2004: www.winemakermag.com/component/resource/article/313