Lagerkeller Korrekt

Bière de Garde - Ambrée and Peacesteiner Bavarian Dunkel (L to R) in the kegerator, converted for lagering.

I brewed a Bavarian Dunkel and a Bière de Garde Ambrée  last week as part of my (semi-) annual summer foray into lagering beers.  Both beers were made over the course of a 9 hour brew day that included brewing both beers (with an extended boil for the Ambrée), the bottling of my Golden Raisin Dubbel, and some side-by-side tasting of Kate the Great clones with Lewy.  I've learned a lot about making lagers correctly from books (particularly John Palmer's How to Brew), various sources online, and interviews (and from screwing up previous batches) so I figured I'd share some of my thought processes as applied to these beers.  The whole point of lagering is to create a clean, (often) maltier, crisp, smooth finished product, devoid of diacetyl and other off-flavors, and lacking the fruitiness associated with ale fermentations.  All of this is achieved by way of strict temperature control.  Here are the points/processes I have found important:

• A very large yeast starter ensures that primary fermentation begins properly.  I pitched roughly 1000ml of slurry into both the dunkel and the ambrée.  While making and building the starter, working at room temperature is acceptable, but prior to pitching the yeast, the starter should be at roughly the same temperature as the wort.  I cool my wort (after chilling with the counterflow chiller) down to the desired lager temperature in my converted kegerator, and place the yeast starter inside to ensure that they are the same temperature when pitching occurs.
• Lager yeasts prefer a cooler and strain-specific temperature.  All yeast would happily ferment at warmer than normal brewing temperatures.  We control temperatures to coax the flavors we desire from the yeast.  It's important to stay within the range of primary fermentation temperatures for the specific strain that you choose to work with.  If you allow the temperature to rise too high, then you may be dealing with off-flavors in the final product.  If you drop the temperature too low, then you may have a sluggish or even stuck primary fermentation due to premature flocculation/settling.  WLP830 - German Lager ferments best at temperatures between 50-55°F.  I read temperature using a vial that has wort in it left over from hydrometer testing.  I keep a thermometer in the vial and adjust temperature accordingly with a digital controller.  My beers are fermenting at 52°F.  
• A diacetyl rest is important.  It's easy to do, too.  Diacetyl is that butterscotch flavor/aroma that is only vaguely acceptable in a very few styles of beer (English Barleywine, Oud Bruin, English IPA, some Stouts come to mind) at very low levels, and is pretty much universally considered a flaw in lagers.  I know that different yeast strains are more likely to produce diacetyl, and that some may not warrant a diacetyl rest, but I figure it to be a simple safeguard.  All you have to do is raise the temperature to between 55-60° F for 48 hours at the end of primary fermentation.  This allows the yeast to become increasingly active and clean up any diacetyl that might be present prior to cold conditioning.
• Minimize the impact of autolysis by transferring after the diacetyl rest.  Dead, ruptured yeast cells release undesirable off-flavors into beer.  With a large lager pitch you have a yeast mass that has the potential to impart a significant number of those off-flavors into your precious beer.  Transferring the beer off the yeast cake prior to cold-conditioning helps to prevent this from happening.  Pitching healthy viable yeast helps to minimize autolysis, too.
• Cold condition your beer at 10°F less than primary fermentation.  If you go too much lower then you risk shocking the yeast, which have become accustomed to working in a specific temperature range.  It's also a good idea to drop the temperature gradually (i.e. a degree or two per day).  Doing so prevents the yeast from dropping to the bottom and going dormant.  You want the yeast to continue to clean up your beer (albeit more slowly) during the lager phase.  Depending on your lagering temperature, the conditioning phase could take between 4 and 8 weeks.  Colder conditioning temperatures require more time to finish.

I plan to condition these beers for about 6 weeks at around 42°F.  I'll be watching fermentation carefully to make sure that I complete the diacetyl rest/transfer right near the end of primary fermentation.  I'll be kegging these beers.  They'll be on tap when my kegerator comes back online.

***UPDATE***

The kräusen forming as each of the beers ferments is some of the most unique and beautiful I've seen in a while.  I don't know whether that's attributable to the lager yeast or not.  Regardless, it's interesting to look at.

Peacesteiner Bavarian Dunkel

Ambrée Bière de Garde