NOTE: In the 2015 BJCP Style Guidelines, Dusseldorf Altbier is renamed simply as Style 7B Altbier in a newly named Category 7 Amber Bitter European Beer which groups amber-colored, evenly balanced to bitter balanced beers of German or Austrian origin.
Dusseldorf Altbier is one of the few styles that can be traced back for thousands of years. It was probably made by the Germanic peoples as far back as 3000 years ago. Dusseldorf is a town in Germany that is far enough north that the brewers there can brew ales all year around. This is one of the reasons that lagers didn't get quite as popular as elsewhere in Germany, such as Bavaria, where their summer beers turned sour more often than not and they were forced to brew only in the winter months.
The brewers in the Rhineland did, however, utilize some of the new brewing techniques that made lagers so popular throughout the world. One was aging the ales at very cold temperatures to allow the yeast to reabsorb and clean up some of the off flavors of fermentation. This is what puts the Dusseldorf Altbier in the "Hybrid" category of beer styles. It's an ale that is cold conditioned at lager temperatures.
If you were to try and describe a Dusseldorf Altbier in relation to a British beer as Horst Dornbusch did in his book called Altbier: History, Brewing Techniques, Recipes (Classic Beer Style Series, 12), you could say its flavor is somewhere between a brown ale and a dark ale. But as a German beer, it has some distinct German and continental characteristics. Unlike the British beers, the Dusseldorf altbier has a typical German full-bodied flavor and it is also maltier and less bitter than the typical British ale. Many have some residual sugar since is fermented cool at around 55°F (13°c).
Since the brewers were able to brew ales year round and others in Germany weren't, the town of Dusseldorf embraced the "old" or alt style of brewing ales using top fermenting yeast. The name "alt" means old but is a new name applied to the ancient beer. It is only in the last two hundred years that the beer began being called altbier to distinguish it from the "new" lagers which had taken over. Before that it was simply called "bier".
Since Dusseldorf is in the Rhineland, it can get rather cool at times. To get the altbiers brewed there to attenuate fully, the brewers needed a yeast that could ferment at those cool temperatures and would still attenuate well.
The altbier yeast is somewhat of an anomaly in that it is an ale yeast that will ferment aggressively at very cool temperatures to produce a beer that is well attenuated. At these temperatures, it does not produce many of the fruity esters typical of most ale yeasts. It lets the soft, lightly malty, sweet, toasty aromas shine through, producing a beer that is very clean.
It's very important that you use an altbier yeast if you want to get these defining characteristics. Mash at a low temperature, around 149-150°F (65-66°C) to get a very fermentable wort. Ferment cool, at around 60°F (16°C) and allow for at least a month of cold lagering to mellow the beer and allow the yeast to consume the unwanted byproducts of fermentation. Since the beer ferments so well and attenuates fully, to get the touch of residual sweetness that is appropriate in this beer, you can use a Crystal or Caramel malt which will leave some sweetness and body in the finished beer. Caramunich is a good choice, but keep the percentage low, at around 5% so it doesn't seem too sweet. There is nothing you can do about residual brewing sugars left that are unfermentable. No amount of lagering will reduce them, so err on the low side for your crystal malt addition.
References: Information for this page was adapted from the 2008 and 2015 BJCP Style Guidelines, the page on Altbier from The German Beer Institute, The German Beer Portal for North America, and Altbier: History, Brewing Techniques, Recipes (Classic Beer Style Series, 12) by Horst D. Dornbusch.
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