Pilsner, Doppelbock, Schwarzbier—there are so many classic styles of beer that many homebrewers never get a chance to brew. All of these styles and many more fit into the lager category of beer, and they can sometimes be intimidating for a homebrewer to try. In reality, brewing lagers isn't much more complicated than brewing ales, but it does require you to have a little more space and a dedicated beer refrigerator with temperature control. Because of these requirements, brewing lagers isn't for everyone, but when you have the ability to brew lagers, you open up a whole new world of homebrewing.
Yeast is the primary difference between an ale and a lager. Before adding the yeast, lagers are brewed exactly the same way as ales. After you add the yeast, the fermentation schedule is very different. Lager yeast varieties have the ability to work at much cooler temperatures than ale yeasts. They lack the fruity esters produced by warm fermentation and instead leave a crisp, clean flavor. Lager yeast work slowly, but they are very thorough. They consume a wider variety of sugars than ale yeasts do, which helps produce that characteristic dry finish.
Because lager yeast work at cooler temperatures, it's important to start with enough yeast so they can do their job without getting stressed and producing off flavors. Getting enough yeast usually requires making one or more yeast starters. Though a one-liter yeast starter might be big enough to brew a 5% ale, for the same strength lager, you might need two liters or more to get the job done.
In addition, lager yeast will also need a lot of oxygen to build up their energy and work through the cold. When I write a lager recipe, I generally provide instructions to "shake the wort vigorously" after it has been cooled. The vigorous shaking dissolves oxygen into the wort, which yeast need in order to reproduce. When preparing wort for a lager fermentation, shake the heck out of it for at least 5 minutes, take a break and then shake it again. The more oxygen you dissolve into the wort, the more you'll get that smooth lager flavor.
The first stage of a lager fermentation should take place between 45 and 55°F, and usually in the lower end of that range. If you are using a spare refrigerator or freezer for the cool fermentation, an external temperature controller does a fantastic job of keeping the temperature consistent for fermentation as well as the lagering stage. Cool the wort to the fermentation temperature before pitching the yeast, or you risk producing off-flavors early on.
Even though the yeast can perform at lower temperature, they tend to take their time doing it. Where an ale ferments in one or two weeks, a lager takes at least twice that long. You may not notice the airlock bubbling for a couple days after you brew, but it isn't anything to be worried about. Fermentation activity may not be visible for two to three days after pitching yeast, and the foam krausen will be smaller when it does eventually form. The lack of visible fermentation along with the smaller krausen on the surface of the beer is one reason why lager yeast is referred to as "bottom fermenting", while the active ale yeast is called "top fermenting".
With the cold fermentation, lager yeast can have a difficult time cleaning up some of the compounds that produce undesirable flavors. Most noticeably, the butter-flavored diacetyl will be produced early during fermentation, but it may not be reabsorbed by the yeast at the end. To make sure this unpleasant flavor doesn't carry into the final beer, homebrewers who make lagers will often perform what's known as a diacetyl rest. As primary fermentation starts slowing down after 3 to 5 weeks, remove the fermenting beer from refrigeration for a day. Increasing the temperature will encourage the yeast to get a little more active, and will naturally remove much of the residual diacetyl.
A diacetyl rest is not always necessary, but it's a good idea for homebrewers who are just getting into lagers. After you have brewed a few batches, you'll get a feel for whether you need to warm the temperatures to clean up some off flavors or whether you can proceed to the final stage of making a lager.
The word lager is actually from the German word "store", since the beer is typically stored in a cold room or refrigerator for several weeks after fermentation. For the final lagering stage, the beer should be transferred to a secondary fermentor and stored in a refrigerator that is between 32 and 40°F for 4 to 8 weeks. The lagering phase cleans up some additional unpleasant characteristics, specifically any sulfur or other bad aromatics that were produced during fermentation. The cold storage also helps clarify the beer and get the brightness people expect from a lager.
After the lagering period, carbonate and bottle as usual. It's usually recommended to mix in a little additional dry yeast at bottling time when you add the priming sugar, since the original yeast might be a little shocked from the cooler temperatures.
So if you already have a refrigerator where you keep your homebrew, there's no reason not to try lagering. It will open up many new homebrewing opportunities, add a variety of flavors to your repertoire and provide insights into the contribution of yeast to your beer.