Krausening is a process traditionally used by German brewers to add carbonation to their beers. It was primarily used on lagers because the yeast would often go dormant during the very cold lagering phase of fermentation, although it was also used on wheat beers as well.
They found that adding the freshly fermenting wort not only provided a reliable means of carbonating their beer, it also greatly improved the flavor. The fresh yeast immediately began cleaning up the natural byproducts of lager fermentations, such as diacetyl in beer and acetaldehyde in beer. Given enough lagering time the yeast would have eventually cleaned up these off-flavors, but the addition of fresh new yeast reduced the time required to condition the beer.
Another benefit of krausening is increased hop presence (when hopped wort is added.) As beer ferments, iso-alpha acids from hops tend to cling to the cell walls of yeast and as more and more yeast is produced, more and more hop presence is lost on the flocculating yeast. The aromatic component of the hops are usually scrubbed with the evolution of CO2 during fermentation. Thus, by the end of primary fermentation, you will have lost much of your bitterness and aromatics you so dearly crave in your lagers. By adding a freshly fermenting (and hopped) wort to your final beer, and then either bottling or kegging it, these new hop flavors and aromatics are captured in your beer.
For large breweries that filter their lagers, krausening is one way of improving the shelf-life of their finished products. Bottle conditioned beers can be aged for years and some as long as decades. These days, consumers are warming up to the fact that craft beers are sometimes bottle conditioned and in fact many of the "best" craft beers are bottle conditioned. For many craft beer enthusiasts, bottle conditioning is a sure sign of a quality beer.
One other use for krausening is to help high-gravity beers attenuate more. By adding a krausening solution to a high-gravity beer when its yeast is weak and dying, the beer's fermentation may start with renewed vigor and finish out dryer and cleaner than it would have otherwise, something to remember the next time your barleywine doesn't finish where you expect it to.
So what about homebrewers? The reason many homebrewers don't krausen their beer is that they seldom make the same beer over and over. By the time a lager is ready for bottling and conditioning, several weeks have passed. Unless you plan ahead and brew another batch right before bottling, you must come up with another plan. For commercial brewers it's easy, they just grab some of the newest batch of the beer needing to be krausened and add yeast. So what are our options as homebrewers?
Lets look at each in a little more detail. If you plan ahead, you can make your recipe a little larger, say 5.5 gallons instead of 5. Collect the extra wort in a sanitized jar with a lid, and a bit of the yeast in, say the used and sanitized White Labs vial, then store them both in the fridge until needed. Just pull them both out the day before you plan to add them to your lager, aerate or oxygenate the wort and pitch the yeast when both are at fermentation temperature. Then when the small beer is at high krausen, pitch it into your green beer. Simple huh?
And what about making a mini batch of beer? One method is to hydrate, hop and then boil some dried malt extract of the same OG as your lager. Then just store it in a sterile container until needed. If you are worried about the lighter "mini beer" diluting the color or IBUs of your lager, then just plan ahead and make your original beer slightly darker and hoppier than your target beer. To find out all the specifics on this method, check out the article written by Chris Colby entitled Kräusening: Techniques in the November 2006 issue of Brew Your Own Magazine.
For either method the critical questions are "how much kräusen beer do I need, and what condition should it be in?" Usually you will need about 20% of your original volume. But to be more accurate with your carbonation levels, adjustments will be necessary depending on the gravity of your wort. The 20% will only work well when kegging your lager (you will still need to release some head pressure before serving). From an article written by Carl Townsend in his Ask The Brewmaster archives which deals with the question "How do I krausen my beer?" Carl gives this simple rule of thumb: If you want a decent carbonation level in your lager, you need to boost the gravity of the main batch by 3 gravity points.
One problem with the calculations is that the wort needs to be at high krausen when it is added to your green beer and this means that you will have to take the gravity of the krausen wort and do the math the day you add it to the main batch of green beer (since it will have changed from the day you pitched the yeast-ie. yesterday).
You can either pitch some of the original starter that you have saves, pitch some fresh yeast, grow some from your yeast bank, or siphon some from the bottom of the green beer in its secondary container. Just pitch the yeast and give it a day or two to come up to full krausen before pitching.
So here is the procedure: On bottling day, check the specific gravity of your krausen beer with a hydrometer and record it. Since you know what the FINAL GRAVITY of the green beer is, you will know what the expected FG of your spiese (German word for the reserved wort) will be. (You may have to make some assumptions the first time you use a "mini batch of beer" made with DME, such as the expected FG based on the average attenuation of the yeast used).
So now we will use some ratios to work out how much krausen beer to add to the main batch of green beer. It may be simpler to give the same example that Carl Townsend used in his article. "Suppose your kräusen starts at a gravity of 45 (i.e. Sp.G. = 1.045). You would expect this to ferment out at 11 or so. Suppose on bottling day, the gravity is 29. That leaves 18 points to go. To get 3 additional points to the beer, you need 3/18 = 1/6 of the starting volume. For 5 gallons, you need 5/6 gallon of this starter. If you made up a gallon, you'll have a bit left over. Note that if you wait too long, you won't have enough!"
The rest of the procedure is simple. Just add the krausening beer to your bottling container or keg and rack (or siphon) the green beer on top. Then bottle as usual. For lagers, keep the bottled beer at a low room temperature for a couple of days then gradually lower the temperature to lager fermenting temperature, say 50°F (10°C) for about a week or two. Open a bottle to check the carbonation before dropping the temperature further to lagering temperatures. Open a bottle after three or four weeks have passed to check the progress. You will probably find that you have made a super smooth, well carbonated lager that will age gracefully for a very long time if needed.
For those with a penchant for numbers, there is a really good formula in the book "Vienna, Märzen, Octoberfest," by Dr. George Fix (Brewers Publications, 1991) on how to figure the amount of fermenting beer to add to your green beer when krausening. The formula is:
Vp/Vb=SGb/SGp X Cv/(2.44 X SGb X SGp X F - Cv) where