Kräusening is the practice of adding a small amount of vigorously fermenting lager beer to a larger amount of lager beer that has just finished fermenting. Usually, the volume of fermenting beer (called kräusen beer, because it is added at high kräusen) is equal to 10–17% of the main batch of green beer.
Related brewing practices — such as adding spiese (wheat ale wort or fermenting lager beer) to a green German weizen (an ale) prior to bottling or adding a small amount of fermenting ale to help along the fermentation of a high-gravity ale — are conceptually similar to kräusening, but are technically not called kräusening.
Traditionally, kräusening has been used for a couple reasons relating to beer quality.
First, the addition of fresh, active yeast into just-fermented beer helps “clean the beer up.” Diacetyl, acetaldehyde and perhaps other fermentation by-products associated with green beer flavors are quickly taken up by the new yeast.
Secondly, the addition of kräusen beer can help with attenuation, especially with big beers. In the case of very big beers, new, fresh yeast may be needed to finish the job started by the main yeast. For example, kräusening is one of the practices the brewers of Samichlaus (Eggenberg) use to get that 14% ABV lager to fully attenuate.
And finally, if a commercial brewery kräusens their beer, they probably also trap the carbon dioxide gas (CO2) given off by the renewed fermentation to carbonate their beer.
Additionally, there is a practical aspect to adding fermenting wort to green beer for commercial brewers. Let’s say you’re Joe Commercial Brewer. You arrive at work one day and check on the wort you pitched yesterday. It’s fermenting nicely. Great. You also have several other fermenters full of conditioning beer. Super. Now let’s say you’ve been busy and, when you check, you find that all of your fermenters are full. Bummer — now you can’t brew again until you empty one of them.
But wait, all your fermenters have beer in them, but they aren’t really full, are they? Around 20% of the space inside each tank is reserved for headspace over the brew. Now that primary fermentation has subsided in most of the tanks, this space is being wasted. However, if you pump dollops of yesterday’s beer into six to 10 tanks (assuming all your tanks are the same size), you can empty the fermenter you filled yesterday.
The receiving tanks will be almost full — there will be a smaller headspace to accommodate the less-vigorous renewed fermentation — and you’ve just expanded your brewery’s capacity. When I toured the Spoetzel Brewery (makers of Shiner Bock), topping up their tanks was one way they increased the output of their brewery.
Got Kräusen Beer?
Although a time-tested practice in commercial breweries, kräusening — and related practices involving blending small amounts of fermenting beer into green beer — is almost unheard of in home breweries. The reasons for this are practical. First of all, you don’t need to kräusen a lager to make it turn out well.
Secondly, most homebrewers do not brew the same beer over and over on a regular schedule. Thus, when the time for adding kräusen beer arrives, they would need to brew a small new batch of beer. For homebrewers interested in trying kräusening, there are a couple ways around this — saving wort and making “quicky kräusen.”
One way to have a supply of kräusen beer is to save a bit of wort and a bit of yeast on brewing day. If you clean and sanitize a 2-qt. (2-L) bottle to save some wort and clean and sanitize a smaller container — such as a White Labs yeast vial — to save some yeast from your yeast starter, you have all you need to make kräusen beer for a 5-gallon (19-L) batch of beer. Store the wort and yeast in your refrigerator and pull them out the day before you need your kräusen beer.
Aerate the wort, let it warm to fermentation temperature and pitch the saved yeast. The next day — when the wort is at high kräusen — pitch it into your green beer. (Note that you can either ferment 4.5 gallons (17 L) of wort and add the 2.0 qts. (1.9 L) of wort to make a full 5.0 gallons (19 L), or you make and ferment 5.0 gallons (19 L) of wort, then add an additional 2.0 qts. (1.9 L) of kräusen beer to make 5.5 gallons (21 L) total.)
The biggest reason most homebrewers don’t kräusen is that they don’t brew the same beer repeatedly on a suitable schedule. When one beer is finishing fermenting, most homebrewers do not begin brewing the same thing again. However, there’s no reason that your kräusen beer needs to be made from the same recipe as your green beer. If you plan to kräusen a batch, you can alter your recipe so that whipping up a batch of kräusen beer is very easy. Specifically, you can make your beer slightly darker and hoppier than your target beer, then add some pale kräusen beer. The pale kräusen beer can be made simply by boiling some light malt extract.
For example, let’s say you planned on brewing 5.5 gallons (21 L) of Northern German Pilsner with a starting gravity of 12 °Plato (specific gravity 1.048) and 37 IBUs. You have a 5.0-gallon (19-L) recipe for the Pilsner and plan to brew 5.0 gallons (19 L) of beer, then kräusen it with a half gallon (1.9 L) of kräusen beer. To do this, you would first need to convert your recipe to the full batch size, including the original beer and kräusen beer. In this case, you’d multiply all the ingredients in the 5.0-gallon (19-L) recipe by (5.5/5.0 =) 1.1 to yield a 5.5-gallon (21-L) recipe.
Next, determine how much light malt extract it would take to brew your kräusen beer at your target gravity. In this case, 0.5 gallons (1.9 L) with light dried malt extract (DME) yielding 45 points/pound/gallon, would mean 8.5 oz. (0.24 kg) of DME. Finally, calculate the equivalent amount of pale malt you would need to yield that gravity of kräusen beer, about 1.0 lb. (0.45 kg).
Now, brew your 5 gallons (19 L) of beer, using the 5.5-gallon (21-L) recipe minus the amount of pale malt calculated above. (If you’re an extract brewer, just subtract the same amount of DME from your 5.5-gallon (21-L) recipe.) This will yield 5 gallons (19 L) of beer at 12 °Plato, but with 41 IBUs — a little higher than your final target. (If specialty malts are used, the color will be slightly darker than the target, too.) When primary fermentation ends, make your kräusen beer — in our example, also at 12 °Plato — and add it to the main batch of beer. Now your beer will be kräusened and at the right strength, bitterness and color.
One benefit of making quicky kräusen is you can add kräusen beer to multiple batches of beer with different recipes. This assumes that you planned ahead and made the recipe adjustments outlined above and all the batches used the same (or similar enough) yeast for primary fermentation. For example, let’s say you are brewing for a party and you make four batches of lager — all you have room for in your chest freezer. If you brew 5-gallon (19-L) batches and ferment in 6-gallon (23-L) carboys, you’ll have 4 gallons (15 L) of headspace that can be (at least mostly) filled with kräusen beer.
If you’re a lager brewer who wishes to kräusen, first prepare your kräusen beer. Either pitch your saved yeast to your saved wort or make a batch of “quicky kräusen” as described earlier. Be sure to aerate the kräusen wort well and let it arrive at high kräusen — the most vigorous stage of fermentation. You should keep the fermentation temperature of your kräusen beer near the fermentation temperature of your main batch. If you’re using a modified chest freezer, just place your kräusen beer vessel in the freezer with your main batch.
Transfer your fermenting kräusen beer either to a secondary fermenter or a Corny keg, then rack your green lager into the kräusen beer. In the case of a secondary fermenter, just place an airlock on the vessel and let the kräusen beer work at your previous fermentation temperature. In other words, there is no need to raise the temperature as you would with a diacetyl rest. If you have a cylindro-conical fermenter with a bottom dump, just dump the yeast from primary and add your kräusen beer to the fermenter.
The kräusen beer will absorb diacetyl and acetaldehyde and begin turning the green lager into a conditioned lager. It will also generate CO2, which can be trapped if you have the right equipment.
In order to trap CO2, you will have to rack your beer to a Cornelius keg and build yourself a spunding valve. (Do not try to trap CO2 in a glass carboy.) A spunding valve is a pressure relief valve attached to a fermenter or conditioning tank. The brewer selects a set point on the spunding valve and pressures below this point are retained in the tank. When the pressure climbs above this level, the tank is vented until the pressure dips below the set point. Brewers sometimes refer to using a spunding valve as “capping the tank.”
By retaining some CO2 pressure in the conditioning tank, the brewer naturally carbonates his beer with the CO2 produced during fermentation. He does not need to add priming sugar to the beer or add exogenous CO2 (under pressure) to force carbonate the beer. A small amount of sulfur, given off during lager fermentations, may also be retained when a spunding valve is used.
A homebrew spunding valve is easy to make. Marc Martin — former Primary Fermenter of the Austin ZEALOTS and current Replicator here at BYO — showed me how to build one. In a later issue of BYO, we’ll have an article that gives the full details, but I’ll sketch out the construction here for those who can probably figure it out for themselves when pointed in the right direction.
To build a spunding valve you need a “gas in” connector for a Cornelius keg, a relief valve, a pressure gauge and a “T” connector. Connect the relief valve (which can be found at Grainger or McMaster Carr) and the pressure gauge (the low pressure gauge on a CO2 regulator) and the “gas in” keg connector to the “T” connector. That’s it.
To use the device, simply seal a Corny keg containing green beer and kräusen beer. Attach the spunding valve to the “gas in” side — so it’s connected to the short tube that ends in the headspace, not the dip tube extending to the bottom of the keg — and wait for the pressure to come up. Once the pressure starts to rise, you can adjust the relief valve to retain more or less pressure. You can read the level of pressure (in PSI) from the gauge.
Whether you trap your CO2 in a Corny keg or kräusen in a secondary fermenter, let the kräusen beer work until the renewed fermentation subsides, then lower your beer to lagering temperatures and let it condition until ready.
Chris Colby, editor of BYO, likes to fill himself with lager beer.