Besides sugar and dried malt extract (DME), beer can also be carbonated with unfermented wort (a.k.a. Speise) or actively fermenting beer (Kräusen or Kräusen beer, sounds like croysen). This procedure is generally used by breweries which bottle condition their beers. The reason for this is simple: freshly fermenting beer is readily available in a brewery. It also has several advantaged over priming with corn sugar/DME:
The main disadvantage is added complexity.
If you want to prime with Speise or Kräusen, you are best off keeping some of the original wort from that batch. Some brewers just keep it in a sanitized jar in the fridge, but I like to store mine in the freezer. This way I don't have to worry about contamination during the next few weeks until I bottle. I also don't worry about sanitation when I collect the wort. As shown in the Whirlpooling article, I get it by filtering the hot-break/hop sludge, that is left in the kettle, through a paper towel.
When I need it, I will take the wort from the freezer defrost and then boil it for 10 min in a 2000ml Erlenmeyer flask. This will sanitize it. If you need to add more bitterness to the beer you can also add hops to this boil and boil longer. Chill the wort overnight or in an ice bath.
If you want to bottle with Kräusen, you need to inoculate the wort with yeast. You may add dry or liquid yeast. I simply take a sanitized racking cane to pull some yeast samples from the fermenter. To do this, close one end of the cane with your thumb and push it all the way to the bottom of the primary or secondary. Open it to suck up some of the yeast sediment. Moving it around while doing this also helps. Close it up again and move the cane to your starter vessel and dump the yeast in there. Repeat this a few times until you have enough yeast.
At this point you may also use a different yeast than the one used for fermenting the beer. This can provide a few advantages. Lager yeast, for example, may be used during the colder months to allow conditioning of an ale at basement temperatures. Better flocculating yeast can be used to bottle a beer which has been fermented with poorly flocculating yeast. In this case it is beneficial to drop out the initial yeast with a fining agent like gelatin. My Kaiser Alt is bottled this way.
Now wait until it starts fermenting.
Calculating the volume
The problem with calculating the necessary volume of Kräusen arises from the fact that it’s sugar content keeps changing while it is fermenting. As a result you’ll have to make a guess at the needed volume when you prepare it and calculate the actual volume on bottling day.
Before the necessary Kräusen volume can be calculated the amount of missing carbonation needs to be calculated. There are two different units of measurement which are commonly used for carbonation. g/l CO2 expresses the weight of the dissolved CO2 in one liter of beer whereas volumes of CO2 is a bit more complicated to explain. Volumes of CO2 expresses how much larger than the beer volume the CO2 volume would be if it was not dissolved in the beer but a gas at atmospheric pressure. Here is an example. 1 pint of beer with a carbonation of 2 volumes of CO2 means that the CO2 in that pint of beer would occupy 2 pints if it was not dissolved in the beer but captured in a non pressurized bag. The conversion between the two is simple: One volume of CO2 equals 2 g/l CO2.
The amount of CO2 that needs to be added to the beer depends on two factors: the current CO2 content of the beer and the desired carbonation. The CO2 content of “still”, I.e. unpressurized beer with a head space full of CO2, depends largely on its temperature. It can be found in Carbonation Tables in the column which corresponds to 0 pressure (either kPa or psi).
The desired carbonation depends on the style of beer and brewer’s preference. The aforementioned Carbonation Tables list carbonation guidelines for various styles. The missing carbonation, which needs to be supplied by the sugar in the Kräusen, is the difference between the current CO2 content and the desired CO2 content.
With the assumption that 1 g of sugar is fermented into equal parts of CO2 and ethanol, which is not exactly true but good enough for our calculations, the following formulas calculate the necessary Kräusen volume. This calculation makes the assumption that the Kräusen has been prepared with the same wort as the batch of beer to be primed:
For metric units:
for U.S. units:
One of the inputs to these formulas is the volume of the primed beer (Vpb) which includes the Kräusen volume that is the result of the calculation. This is done to keep the formula simple and the best way to handle this, is to make an initial guess at the Kräusen volume. 5% of the beer volume works well. If the calculated Kräusen volume is then much different than the initial guess the calculation can be repeated with the better Kräusen volume estimate. After that it should be accurate enough.
If you want to calculate the amount of Speise necessary simply use the original extract or starting gravity as AEk or GUk, respectively.
On occasion it can happen that the calculated volume of Kräusen is more than what you have available. Especially, since you need to leave some of the Kräusen behind in order to avoid adding the yeast sediment to the beer. In these cases simply calculate the carbonation you can get from the Kräusen volume available to you and add sugar for any missing carbonation.
These are the formulas that allow you to calculate the carbonation you are getting from the available Kräusen volume:
Once you know the carbonation you get from the Kräusen you can calculate the remaining carbonation which needs to come from additional sugar:
and calculate the weight of sugar with:
On bottling day you will need to do the following:
The bottles should carbonate within about a week at the fermentation temperature appropriate for the yeast (10-14 C / 50-58 F for lager and 18-21 C / 64-70F for ales). This quick conditioning time is one of the benefits of Kräusening. The beer may still benefit from additional aging though.
I may use the same method for carbonating my lagers in a corny keg before I start lagering. But in this case I just make sure that enough Kräusen is added to get above the desired CO2 level. I then monitor the pressure build-up and blow-off any excess CO2. This has the advantage of adding fresh and healthy yeast which may attenuate the beer better before the cold conditioning is done. After cold conditioning, I rack the beer off the sediment into a serving keg.
Is all this extra work worth the effort? Every brewer has to decide this for himself/herself. Though it seems more complicated, the additional steps don't add much overhead to bottling for me since most of the time is spent filling bottles anyway. I like that the beer will be ready within about a week after bottling. Using the spreadsheet and measuring the actual attenuation of the Kräusen on bottling day removed the worry of having to wait until the Kräusen beer reached just the right attenuation.