Kräusening is the traditional method that German brewers use to carbonate their beer. Traditionally kräusening is done with lager beers in order to overcome the problem of yeast going dormant during lagering. With ales, kräusening is not really necessary (the exception is wheat beers, which should be kräusened for style reasons).
Kräusening is the introduction of actively fermenting wort at the stage of “high kräusen” to a finished beer. Generally, the carbonation period should be significantly reduced over other methods of natural carbonation and kräusening helps in reducing diacetyl and aldehydes, which are quickly taken up by the new yeast.
If you’re currently carbonating with priming sugar or carbonation drops, you will notice that the flavor of the beer is different if you change to kräusening. I personally believe it gives the beer a cleaner flavor. Almost everything I brew is kräusened.
It’s a fairly simple process. When you’re done brewing and chilling, save some of your gyle (wort before yeast is pitched into it) in a gallon jug. Refrigerate that jug, and go about your usual routine with your brew. Don’t worry, some of its going back in at bottling time, so you’re not losing that entire volume.
Presumably you made a yeast starter for this brew, save a small amount (anything from 25ml or higher is sufficient) of that starter in a sterile jar before you pitch it into your wort. Refrigerate that along with the jug of gyle.
Just prior to bottling (how long depends on your method for making starters), take out your saved yeast starter and your gyle. Let them warm back up to room temperature. Take an O.G. reading from your gyle. Take an F.G. reading from your finished beer. Since this is the gyle that made this batch of beer, and the yeast, the F.G. of the beer gives you a reasonable assumption of where the F.G. of this starter will be.
This is where you get to make a choice. Various sources will tell you that in order to get reasonable carbonation; you need to add 3 points of gravity back into your beer. I’ve found that I’m happy with the results I get from adding 2 points, but you can adjust this to your style if more or less is required.
So, let’s say your gyle starts out at an O.G. of 1.060. Your finished beer ended up with an F.G. of 1.014. You can reasonably expect that this gyle will also get down to 1.014. Prepare the starter and let it begin. The actual volume you’re going to use is calculated on bottling day. When your starter shows signs of high kräusen, it’s ready to use. Take a gravity reading of the starter. Let’s say it’s at 1.040 now, and you expect it to get to 1.014. That means it’s still got 26 points left to go. To get 3 points of gravity added back to your beer at bottling time, you need:
3 points/26 points to go = 3/26 of the starting volume. For a 5 gallon batch (5 gallons of finished beer), you need 3/26 of 5 gallons = roughly 2.25 quarts that you’re going to add back at bottling time. If you wanted 2 points of carbonation, 2/26 = roughly 1.5 quarts back.
You don’t need to use the gyle from this batch; you can use any gyle that isn’t going to conflict with the taste of your brew. You just need to have an idea of what its O.G. is, and what its F.G. is going to be.
A simpler but similar method that I have used extensively is NOT kräusening, but carbonates very well. I believe it’s called Spiesegabe.
After brewing and chilling, save some of your gyle in a gallon jug and refrigerate till bottling day (the reserve gyle is called spiese in German). Prior to bottling, take an O.G. reading of the gyle. Basically using a formula from Charlie Papazian’s book (this is incorrectly called kräusening in the book):
gyle = (12 x gallons)/((S.G.-1) x 1000))
gyle = (12*5)/((1.040-1)*1000))
gyle = 60/40
gyle = 1.5 quarts
Take 1.5 quarts of gyle, boil it to sanitize it (yes, I know its been refrigerated all this time, try leaving it, sealed, out at room temperature for a couple of days, and watch it start fermenting without adding anything, boiling is not sterilization). Cool it, and mix it back into your finished beer prior to bottling. Note that I took this reference directly from the book, I’ve used it before, and it works fine, however since it does not take the final gravity into account in the calculation, your brews may wind up with a little more variation in carbonation than if you used the kräusening equations listed above.