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Big, deep, and rich, Belgian dark strong ales are perhaps the most complex beers Belgium has to offer. The best examples unleash dark fruit, chocolate, and a host of other flavors on waves of strong yet smooth-edged alcohol and malt. These are beers for slow sipping and contemplation, and they'll become more elegant as they unwind with age.
Belgian dark strong ale isn't really a "style" so much as it is a catchall category. It's a designation that (mostly American) brewers and drinkers use to group Belgian-style beers that are bigger than dubbels and darker than tripels, so there's plenty of room for variation and experimentation. Which means, this category is fertile ground for homebrewers.
Brewing Belgian dark strong ales isn't cheap—I generally expect to spend an extra $10 to $15 per 5 gallon batch once I've bought the extra grains and sugars. However, considering commercial domestic and imported versions start at about $6 per 12-ounce bottle where I live, it's a relative steal to make your own. Here are a few tips to help you get started.
This part of the recipe is deceptively simple. According to Stan Hieronymus in his excellent book Brew Like a Monk, many of the best Belgian dark strong ales (such as Westvleteren 12, St. Bernardus 12, and Rochefort 10) are built on a base that's almost entirely two-row Pilsner and/or pale malts. This foundation can be all one type of malt, or a blend to create layers.
The beer's depth and complexity is added with character malts, alcohol, and sugar. For character malts, my advice is to keep it fairly simple starting out, or the flavors could get muddled. Complexity doesn't mean having a beer that tastes like everything imaginable, so try to keep your focus. A small portion of Munich or aromatic malt can add bready or nutty malt character. Special B will provide a raisin quality and CaraMunich can add dried plum and cherry notes. Avoid dark roasted grains, unless you choose to add a very small amount during the sparge for color adjustment.
To round things out, I like to add flaked oats or wheat to enhance mouthfeel and head retention.
Essentially, these dark strong ales are a sugar showcase. Unlike plain cane or beet sugars, which leave little more than alcohol behind after fermentation, Belgian candi sugars and other darker sugars impart an array of flavors, aromas, and colors. I'm a fan of the candi syrups from Dark Candi Inc. Their D syrup is reminiscent of plum, raisins, and figs, while their D2 syrup adds flavors of milk chocolate, cherries, and burnt sugar. (If you have any leftovers after brew day, these are great for tuning up a cup of coffee.) I've also found Turbinado sugar to be a great choice; it can provide a rummy or honeyed quality in your beer.
In addition to their contributions to flavor/aroma, color, and obviously alcohol, the sugars also help dry the finished beer out or, as Belgian brewers would say, make it "more digestible," which is crucial for a beer of this size. I look to get up to 15% of the recipe's total fermentables from sugar additions. Sugars can be added during the boil or after the primary fermentation is complete if you're concerned about the yeast favoring the simpler sugars over the malt sugars.
Belgian dark strong ales typically put hops in the back seat, relying instead on alcohol to balance the malt. Stick with a bittering charge of a clean, high-alpha hop like Magnum or Warrior at the beginning of the boil and save your other hops for a different beer. You could consider a small addition of a noble hop variety like Hallertau or Saaz late in the boil if you're looking for a modest flavor or aroma contribution, but these beers really aren't about being hoppy.
To ferment a quality Belgian dark strong ale, the yeast has to be Belgian and it has to be healthy. In my experience, the character of Belgian yeasts isn't something that can be duplicated by other yeasts. For Belgian dark strong ales, I prefer either the Westmalle strain (Wyeast 3787 or White Labs 530), which is also used by Westvleteren, or the Rochefort strain (Wyeast 1762 or White Labs 540). The Westmalle strain is a beast that'll furiously plow through sugars, leaving a very well-attenuated beer. It can produce a complex set of esters and phenols, some of which will dissipate if given time to condition. I've found the Rocherfort strain is cleaner, leaving a malt- and alcohol-focused beer with a more delicate ester profile. You can also pitch multiple strains for added complexity, or consider adding Brettanomyces or bacteria in the secondary if you want to funk it up.
Make sure you're pitching an adequate amount of yeast and giving the wort plenty of oxygen prior to pitching to ensure a healthy fermentation. You'll need to make a yeast starter, which just requires a little advance planning, or buy multiple packs of yeast. I use Mr. Malty's Pitching Rate Calculator to determine how much yeast I'll need for a batch. To aerate the wort, rock the carboy back and forth for a few minutes prior to pitching the yeast or oxygenate for a minute or two with an oxygenation stone, if you have one.
Temperature control is crucial on a beer this big. If the fermentation gets too hot, you're likely to create harsher, solvent-like alcohols. I prefer to keep the temperature below 70°F, at least for the first few days.
And, finally, give the beer time to condition. Seriously, let it rest for at least a few months before you drink it. It would be fine to drink sooner than that, but your patience will be rewarded. If brewed and packaged properly, this beer will start to shine after six months or so, and then peak after a year or more. This is a great beer to brew this fall and enjoy next fall.