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Question: How do I kräusen my beer?

Answer: Kräusening is the traditional method that German brewers use to carbonate their beer, most often lager beers. Basically, the process consists of adding freshly fermenting wort to beer that is ready to bottle. Kräusening overcomes the problem of yeast going dormant during the lagering phase of fermentation. It also helps clean up the flavor of the beer by reducing levels of diacetyl, acetaldehyde, and dries out the beer.

Most ales are fermented rapidly enough such that the yeast is still active at bottling time. Hence, kräusening is seldom necessary. An exception to this rule is with wheat beers. These are often kräusened, probably for traditional reasons rather than technical reasons. It would seem that ales that get some cold conditioning such as Kölsch or Altbier could benefit from kräusening, but this is seldom encountered.
In a traditional German brewery, kräusening is done by adding a bit of the most recent batch of beer to the batch that was made about six weeks ago. If you make the same style of beer every six weeks, this method works well. However, if you don't, you have two options:
1) Keep some of the wort from your last batch in the fridge in a sterile container,
2) Make up a small mini-batch of fresh beer.
In either event, the critical questions are how much kräusen beer do I need, and what condition should it be in?
Typically, 20% of the original volume is needed. However, to hit the desired level of carbonation, adjustments are necessary, depending on the gravity of your wort. A quick calculation shows that 20% of a typical wort would give way too much carbonation. If you are kegging, 20% works OK, but note that you'll need to bleed off some extra CO2 before serving your beer.

A simple rule of thumb is this: To get a reasonable carbonation level, you need to boost the gravity of the main batch by 3 points. You can work this out by simple ratios, but note the following problem: You'll want the yeast to be active when you bottle, so you need to work up the wort like a yeast starter solution. This will decrease the gravity you originally had, so you'll need to re-do the math.

As for the yeast, you can use a fresh pack of yeast, or you can use an autosiphon to suck up a bit of the yeast cake from the bottom of your secondary fermenter. Pitch your starter and give it a day or two to come up to full "kräusen".

Then, the trick is to wait until bottling day to do the math. Check the gravity of your kräusen with a hydrometer. Now use ratios to work out the volume to add, based on how far the kräusen has to go before it ferments out. Here is an example:

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!

So, mix the kräusen beer with the aged beer and bottle. For lager beers, keep the bottles at a low room temperature, for a couple of days, and then slowly drop the temperature to lager the beer. I recommend staying at 50°F a week or two. Pop open a bottle to verify carbonation before dropping the batch down to your final lagering temperature. After another 3-4 weeks, pop open another. You'll probably find that you've created a very nice smoother lager beer. While you're at it pop one open for me too.