Revision as of 22:50, 16 March 2007

work in progress, feel free to correct spelling and grammar errors though. BTW, there is a lot of stuff that even I haven't proof read yet. (Kaiser)

Lagers are fermented with a bottom fermenting lager yeast (Saccaromyces carlsbergenensis). These yeasts are able to ferment at lower temperatures than the top fermenting ale yeasts (Saccromyces Cervesiae). The result of this low temperature fermenation and maturation is a beer with a cleaner flavor profile (less esters, less higher alcohols) than its top fermenting counterpart.

The extended cold storage (lagering) makes these beers more shelf stable than ales which eplains why most of the world's beers are of the lager variety.

Because lagers ferment at lower temperatures than ales and yeas metabolism works slower at these colder temperatures, they take longer to ferment and also require more attention than ale fermentations. This article is aimed at the novice as well as the advanced lager brewer. The first section is a how-to for your first lager fermentation. The following sections delve deeper into the subject of lager fermentation and discuss the pros and cons of different fermentation techniques and also give some background information on lager fermentation in commercial breweries.

Your First Lager Fermentation

Though many experienced brewers may read this and note that this is not the absolutely best way to ferment lagers, it is the most foolproof and that's what you are looking for for your first lager fermentation. You need the first batch to be a success so you get hooked on lagers and their smooth taste. Then you will start digging deeper into the subject and find a fermentation schedule that works best for you and your set-up.

One day before brew day pitch a 2 qt (2 L) well aerated starter with an Activator Pack (Wyeast) or vial (White Labs) of the lager yeast of your choice. Both companies offer really great yeast strains. If you are looking for a versatile lager yeast go with the German Lager (WLP830 or Wyeast 2124; This might be the most widely used lager strain in German beers) or whatever your recipe calls for. Keep this starter at room temperature 68 - 70 *F (18 - 19 *C) and let it start fermenting. It may throw off some sulphur notes (rotten egg smell) which is common for lager yeasts.

Brew an average gravity lager OG: 1.044 - 1.056 (11 - 12 *P). These beers will not result in toxic alcohol levels for the yeast. Once brewed, chill the wort to a temperature below 70 *F (20 *C). The mid 60's should work best for this fermentation schedule. If you are not able to get the wort that cold with your chiller and your tap water, you can use a pump to recirculate ice water though the chiller. Because this pump doesn't have to be food grade, a simple submersible utility pump will do.

When transferring the wort into the fermenter, make sure to leave most of the hot break and hops in the kettle. This can be achieved with Whirlpooling or straining. The latter can be problematic since the fine break material tends to clog the strainer. The removal of hot break, some cold break and hops is recommended because the beer would be sitting on this trub for a long time (4 weeks). After transfer into the fermenter the wort needs to be aerated very well. A healthy lager fermentation requires more oxygen than an equivalent ale fermenting in order to reduce the stress on the yeast. The required oxygen level of 8 ppm (mg/L) is best achieved though 1 to 1.5 mins of pure O₂ or 20-30 mins sterile air though a 2 micron stainless steel stone.

Pitch the whole starter into the primary fermenter. Now wait until you see fermentation activity (low kraeusen or bubbles in the airlock) until you move the fermenter to an area (basement or fridge) where you have a constant 48 - 52 *F (9 - 11 *C). Now let the primary fermentation take its course for a few (3-4 weeks) until there is no airlock activity left.

After that rack the beer to a lagering vessel. It can be another carboy or a soda keg with shortened dip tube. The beer is then moved to an area where the ambient temperature is between 32 and 38 *F (0 - 3 *C) where it will remain for another 4 weeks.

Now you can either rack to a serving keg and force carbonate, in case you didn't do the force carbonation during lagering, or bottle. If you plan to bottle condition the beer you may want to add fresh yeast with the priming sugar, because the yeast present in the beer may not perform as well anymore. After all, it is about 7-6 weeks old. A quarter to half a pack of dry yeast properly hydrated should is the easiest way at this point. It also doesn't matter if ale or lager yeast is used since the flavor profile of the beer has already been determined by the yeast used for the primary fermentation. If you don'd add fresch yeast you need to be more patient with the conditioning of the beer.

The following sections will explain lager fermentation in more detail and show procedures that can improve the quality of the finished beer even further.

The conventional fermentation in a German lager brewery

The majority of the information given in this section is taken from a German brewing text book "Abriss der Bierbrauerei" (Overview of beer brewing) by Ludwig Narziss, one of Germany's leading Experts on brewing.

After the whilpool the wort is cooled to close to 32 *F (0 *C) to maximize the cold break. It is then warmed up to pitching temperature which can be between 41 *F (5 *C) and 46 *F (8 *C). The majority of the cold break (~ 60%) is removed through either sedimentation tanks, flotation tanks, centrifuges or filtration. Filtration is the only means of complete cold break removal. Once the cold break is removed the wort is aerated with sterile air to achieve a wort oxygen content of 7-8 ppm (mg/l). In case of cold break removal through flotation, the aeration of the wort is during the flotation process.

Yeast is pitched at about 500 ml thick yeast slurry per 100 l 12 *P (1.048 SG) wort (this equals about 100ml or 3oz yeast slurry per 5 gal). Once the yeast is well distributed this equals about 15 x 10⁶ cells per ml. When it comes to pitching and primary fermentation temperatures cold and warm lager fermentation exists. The cold fermentation uses a pitching temperature of 41 *F (5 *C) and a maximum fermentation temperature of 48 *F (9 *C) and the warm fermentation uses a pitching temperature of 46 *F (8 *C) and a maximum fermentation temperature of 50 - 54 *F (10 - 12 *C). This should not be confused with warm vs. cold pitching. There is no warm pitching in German lager fermentation.

Once the yeast is pitched it takes about 24 hours for the low Kraeusen to develop. High Kraeusen starts on the 3rd day, when the maxium temperature is reached and lasts until the 5th day. At this time yeast growth slows down and the yeast starts to flocculate. This is when the beer is slowly cooled at a rate of 0.7 - 0.7 *C in order to avoid shocking the yeast. At this time the primary fermantation is cosidered done, but the beer has only attenuated to about 40% - 60%.

When the beer is racked to the secondary/lagering tanks it has a temperature of about 39 - 41 *F (3.5 - 5 *C). The remaining fermentable extract is 1.2 - 1.4 % by weight (about 5 - 6 gravity points). Many breweries mix beer from different batches in the lagering tanks to compensate for fluctuations in color, bitterness, attenuation and other parameters. The addition of 20-50% beer fermented with low flocculating yeast is beneficial for achieving a higher attenuation since this yeast will work longer than better flocculating strains of yeast.

During the secondary fermentation (a.k.a lagering) the tanks are closed and the pressure build-up is controlled by a pressure sensitive bleeder valvle. This system, called Spundungsapparat, ensures the proper carbonation of the beer during lagering. The German Purity Law prohibits the use of non-fermentation CO2 for beer carbonization. It is also more economical for a brewry to use the CO2 produced during fermentation.

Key for a good lagering is control of the yeast contents and temperature profile such that the fermentation slowly continues during the whole time the beer is lagered. Only this allows for the processes to happen that are commonly referred to as maturation: reduction of diacetyl, acedealdehyde, higher alcohols etc. The lagering takes between 4 weeks and 6 months. At the end of lagering the beer has the desired attenuation, which is generally a little higher than the limit of attenuation. For light colored beers this attenuation is about 2-4% and for dark beers as much as 6% above the limit of attenuation. Light Export style beers can have an attenuation as close as 0.5% below the limit of attenuation. A difference between actual and limit of attenuation means that there are fermentable sugars left in the beer which are a vital part of the flavor profile, but larger percentages of these sugars result in in a less shelf stable beer.

Brewing lagers in a home brewing setting

The process decribed above is how the best lagers are brewed. But it is very difficult to handle for the home brewer. The reason: the yeast needs to be kept working throughout the lagering phase otherwise the result is an under attenuated (=sweet) beer that is rather undrinkable and requires further attention (mostly the addition of fresh yeast and raising its temperature). Because of that all home brewing instructions are based on a lager fermentation process that is referred to as accelerated fermentation and maturation in commercial brewing. Even many commercial lager breweries use this accelerated process due to time constraints for the production of lager beers. Anheuser Bush for instance produces Budweiser with only one week of primary fermentation and 3 weeks of secondary/lagering. The key to this is their Beechwood aging process. The porosity of the beechwood allowes for a greater contact area between the yeast, which flocculated onto the beechwood strips, and the beer resulting in a shorter maturation time.

Home brewing a lager (the advanced process)

The following sections go into more detail about the home brewing process for lagers and try to explain the different approaches to the process that are done by different brewers.

Pitching rate and yeast propagation

Proper pitching rate is important for brewing lagers, especially if cold pitching (see next section) is chosen. As noted previously the proper amount of yeast for a 12 *P (1.048 SG) wort is about 100 ml yeast sediment for 20l (5 Gal) of wort. Jamil Zainasheff from the Brewing Network has a nice pitching rate calculator on his web page. This tool allows you to calculate the amount needed for proper ale and lager fermentation based on the original gravity of the wort and the viability of the yeast.

The problem with pitching based on a pitching rate is always knowing how much yeast one actually has available. Without a Hemocytometer it is hard to count yeast cells. That's why determining yeast amount based on yeast sediment is more practical for the home brewer.

The pitchable liquid yeast products that are out there (White Labs vials and Wyeast Activator packs) are not a sufficient amount of yeast when pitching cold. This means you need to propagate yeast by growing them in wort. Essentially you are making a starter but you are expecting more than waking up the yeast; you are expecting yeast growth. That's why the starter needs to be fairly big and needs to be prepared a few days in advance. How big depends on your equipment and how early depends on the temperature at which you keep the starter.

Equipment: If you have a stir plate you should be fine with about 1.5 qts of 10 *P (1.044 SG) wort because the constant yeast suspension and aeration lead to a more efficient yeast growth. If you don't have a stir plate and need to shake the starter once in a while to keep as much of the yeast suspended as possible, you should make a 2-3qt starter in a one galon growler. These amounts assume that you want to grow yeast from a White Labs vial or a Wyeast activator pack. Experience will tell you what works best for you.

Temperature: As much as there is debate about warm pitching vs. cold pitching there is debate about the propagation temperature for the yeast. Microbiologists will tell you that yeast (lager and ale) should be propagated at 75 *F (25 *C) because that is the temperature at which they will grow best. Brewers however generally agree that yeast should be propagated at or slightly above primary fermentation temperature because the yeast should not get used to living at higher temperatures than the primary fermentation temperature. Some home brewers report that yeast grown at room temperature tend to loose their flocculation characteristic which makes it harder for the yeast to drop out at the end of fermentation. Growing yeast at primary fermentation temperatures also avoids shocking the yeast because it is temperature is already close to pitching temperature.

Reusing an existing yeast cake is the easiest way to get a proper pitch of yeast. But is it recommended that the wort is not simply racked onto the old cake but that the yeast is taken from the primary fermenter and pitched according to the necessary pitching rate for the beer to avoid over pitching. Keep in mind that yeast quickly uses its vitality after primary fermentation and that such yeast should be stored cold (close to 32 *F is best) and pitched within a week. After that the yeast performance starts to suffer significantly and it might be necessary to make another propagation step with that yeast to recharge its glycogen and sterol reserves.

Cold vs. Warm pitching

Among homebrewers there is is a debate regarding the proper pitching temperature for lagers. Some say that you need to pitch warm to allow for better initial growth of the yeast and others say that you need to pitch below the primary fermentation temperature.

To understand that both sides have valid arguments one has to understand where they are coming from. Warm pitching has been introduced by home brewers and yeast manufacturers because it allows for pitching a lager with a smaller pitching rate and leads to a shorter lag time which is less concerning for the first time lager brewer. That's why warm pitching was suggested in the "Your first lager fermentation" section above. To pitch warm, chill your wort until it has a temperature of 65 - 68 *F (15 - 18 *C) aerate it well and pitch the yeast. Now wait until you see signs of fermentation (low kraeusen or bubbles in the airlock) and move to an area where you maintain about 50 *F so that the wort can cool down while the yeast starts to take off.

Industrial lager brewing only does cold pitching because the proper pitching rates and yeast health can be ensured. Because of the initially higher fermentation temperatures, warm pitching is associated with an increased level of ester, diacetyl and fusel alcohol production which are components that, at higher levels, are not desired in a lager. Diacetyl will be reduced by the yeast during the maturation of the beer but most of the esters and higher alcohols will carry over into the finished product.

That's why cold pitching is also recommended for the home brewing of lager beer as long as a proper pitch of healthy yeast is available. If this is not the case, cold pitching can lead to a very long lag time and sluggish fermentation. In order to pitch cold, chill the wort to 43 - 48 *F (6 - 9 *C) and re suspend the yeast with some wort. Make sure to break up all clumps. If you have a stir plate, stirring the wort and yeast for a few minutes will take care of that very nicely. Now pitch into the wort and place the fermenter in a space with a constant temperature of 46 - 50 *F (8 - 10 *C). Most brewers use a fridge or freezer chest with an external temperature control for this. Expect the fermentation to start within 16 - 36 hrs. For lager fermentation the lag time should not be to short. Because of the low wort (it is actually beer since yeast has already been pitched) temperature there is less risk of contamination of the beer since the metabolism of the bacteria and wild yeast is also reduced.

Forced Ferment Test

When brewing lagers I strongly recommend that you do a forced ferment test. Palmer [Palmer, 2006] was referring to brewing lagers as flying blind because you don't know when they are done fermenting. With a FFT you will be able to determine the FG you your lager well before the actual batch has finished fermenting. This is especially helpful for all grain brewers because the final gravity of the beer is influenced by mashing.

The good thing is, when you made a starter you already have everything needed to peform a forced ferment test. When you pitch the yeast leave a little in the starter vessel and add some more (about 4 - 8 oz) fresh wort that has already been aerated. Now cover the flask or growler again and give it a good shake to aerate it even more. Keep this at a warm place and shake it frequently. You will see the fermentation starting shortly because of the high pitching rate and warm temperatures. Keep shaking the FFT until no more CO2 is released from it. At this point, after 3 - 5 days, the yeast has consumed all the fermentable sugars and you should let it drop out. A gravity reading of the beer produced by the FFT gives you the limit of attenuation of the wort. This is not necessarily the FG that you will be getting from the finished beer since the main fermentation is not done at high temperatures with a higher than recommended pitching rate. The characteristic of the strain and other fermentation parameters will lead to a FG higher than the FG from the FFT. And this is desired since the residual fermentable sugars are a vital component of the beers flavor. The less there are (the closer the actual attenuation is to the limit of attenuation) the drier the beer will be.

Now that you know the FG that you can expect from this beer, you are no more flying blind when fermenting lagers and you can make better decisions based on a gravity reading of the fermenting beer.

Primary Fermentation

Because of the slower yeast metabolism at lower fermentation temperatures, lager fermentations take longer than ale fermentations of the same wort. While ale fermentations are generally done after 3 - 6 days and final gravity is reached at that time, lager fermentations can take one to 3 weeks and the final gravity may not be reached after the completed primary fermentation.

In the classical lager brewing method, as described above, the primary fermentation is over after about 7 - 10 days, but the attenuation of the beer is not yet at the attenuation level that is desired at bottling time. Good fermentation management allows the yeast to be actively fermenting even during the lagering (cold storage) phase.

This need to be kept in mind when brewers talk about the length of primary fermentation for their lagers: What was the attenuation when the beer was racked to a secondary and what was the attenuation of the beer when it was done?

Proper temperature control is crucial for a clean lager fermentation, but it is not necessary to regulate the temperature of the fermenting beer. Keeping the ambient temperature at a constant level is sufficient if this temperature is chosen such that the beer will not exceed a maximum fermentation temperature of 50 - 54 *F (10 - 12 *C) which is usually reached shortly after the high kraeusen stage. Though many lager yeast strains indicate that they ferment with lager characteristics even in the upper 50's (14 *C) I recommend that the primary fermentation temperature shouldn't exceed 54 *F (12 *C). Keeping it closer to 50 *F (10 *C) is even better. When regulating the ambient temperature rather than the beer temperature, the beer temperature will fall back to the ambient temperature once the primary fermentation slows down.

That's when the kraeusen starts to fall back and you should take a gravity reading of the beer to check its current attenuation as well as taste. Once you have brewed a few batches of lager beer, this attenuation will also given you an idea of the yeast's performance during the primary fermentation.

Maturation of the beer

Once the primary fermentation is considered done the final gravity has not been reached yet and fermentation byproducts like diacetyl and acedealdehyde need to be reduced by the yeast. This process is called maturation of the beer and in the converional fermentation approach for lagers, as outlined above, it happens during the long cold storage.

But most of the books and instructions on home brewing lagers don't suggest this fermentation practice because it is very easy to get it wrong. The probolem lies in being able to chill the fermenting beer without shocking the yeast into dormancy. The latter will result in a cloyingly sweet and underattenuated beer that may still have elavated levels of diacetyl because the yeast was not able to sufficiently resuce these compounds. The key to such a fermentation schedule are excellent yeast health and precise temperature control wich is easier achieved in a commercial brewery than in a home brewery.

To avoid this problem, the homebrewers usually use what is referred to as accellated maturation in commercial brewing, a process which allows for the nearly complete fermentation of the beer before it is placed into cold storage. To further understand the various maturation techniques that are out there lets have a look at the following diagrams which I found in a presentation about beer production on the web server of the Technical University of Vienna [TU Vienna]:

different lager fermentation schedules[TU Vienna]

(A) - shows the temperature (continuous line), extract (=gravity) (dash-dot-dash line) and diacetyl (dashed line) of a converional lager fermentation. As you can see the maximum fermentation temperature is held for only 4 days before the beer is slowly cooled to lagering temperatures over the course of the next 7 to 8 days. As you can see, though the extract is close to the FG of 2 *P, the diacetyl level is still considerable at the time of racking (large arrow on top). This diacetyl is reduced over the course of lagering.

(B) - shows a lager fermentation schedule that uses higher fermenation temperatures and pressure to accellerate the primary fermentation and maturation of the beer. The controlled use of pressure (indicated as dash-dot-dot-dash line in the diagram) allows the production of esters and higher alcohols to dampen, which is more intensive at higher fermentation temperatures [Narziss, 2005]. Though more diacetyl is produced because of the higher fermentation temperature it is also reduced qicker, allowing the beer to maturate within only 8 days. primary fermentation under pressure is for the home brewer only of academical interest since it requires primary fermenters that can withstand pressure and a means of controlling that pressure without an increase of quality of the beer.

(C) - This fermentation schedule comes closer to what a home brewer can do. The wort is pitched cold at 44 *F (6 *C) and rises to 48 *F (9 *C) over the next few days where it is kept until an attenuation of 40 - 50% is reached. The fermentation temperature is then raised to give the fermentation another boost which results in the reaching the beers final gravity and the reduction of diacetyl below the taste threshold. After that the beer is racked from the yeast and quickly chilled to lagering temperatures. Since no further yeast activity is necessary (target attenuation has been reached and diacetyl has been reduced) there is no need in a gradual cooling of the beer to avoid shocking the yeast. This works well if you have a dedicated lagering space in which you keep multiple batches and need to keep its temperature constant.

D - Here is another fermentation schedule better suited for the home brewer than (A). The wort is pitched cold and primary fermentation is done at 48 *F (9 *C). Once the beer is within 2 *P (8 gravity points) of the targeted FG (see forced ferment test) the beer is racked to a secondary. Because of the colder fermentation temperature the addition of kraeusen beer can be benefitial for a better diacetyl reduction as well as better attenuation of the beer. For an explanation of that process see below. If you use a soda keg as the secondary fermentation vessel (make sure you shorten its dip tube by about one to two inches) the secondary fermentation can be used to carbonate the beer. Besides having a few practical reasons (keeping the keg sealed and having carbonated beer once the lagering is complete) it takes the home brewing process closer to the way lagers are brewed in Germany. Since the German Purity Law for beer doesn't allow the use of non-fermentation CO₂ for the carbonation of beer, brewers need to carbonate their beer during the secondary fermentation and lagering phase. Once the beer has been racked to a carboy or keg, it is kept at primary fermentation temperature until the targeted attenuation has been reached and the diacetyl has been sufficiently reduced. After that the beer can quickly be chilled to lagering temperatures because there is no need to keep the yeast working.

E - This is similar to D, but the pitching and primary fermentation temperatures are higher which results in faster fermentation and maturation.

F - Is a fermentation schedule that used an explicit maturation rest at a higher temperature: also known as diacetyl rest. This schedule resembles best what authors like Noonan and Palmer suggest for a proper lager fermentation: Pitch cold, let it ferment around 50 *F (10 *C) and once the fermentation slowed down significantly and the gravity of the beer is close to its final gravity, raise the beers temperature to 65 - 68 *F (17-19 *C) for a diacetyl rest. This diacetyl rest has the effect of giving the slowing yeast a boost to finish the last sugars and reduce the diacetyl. As the previous examples for fermentation schedules showed, such a rest is not really necessary. But it can be helpful if the secondary fermentation/maturation is sluggish and/or diacetyl is noticed in the beer after primary fermentation, especially when fresh yeast in the form of Kraeusen is added. After this rest is complete the beer can be crashed to lagering temperatures.

These 6 examples, which were taken from a techical brewing and not a home brewing text, hopefully illustrated the various options for primary fermentation control that the home brewer has. The most practical is option D since it requires 2 fermentation spaces at constant temperatures: one at 46 - 50 *F (8 - 10 *C) for the primary fermentation and maturation and one at 32 - 36 *F (0 - 2 *C) for the cold conditioning/lagering. Since no ramping of temperature is necessary such a fermentation cellar set-up easily allows for the fermentation/lagering of multiple batches of lager at different stages.

when to rack the beer

When using accellerated maturation, as described above, the beer can be racked before or after its maturation is completed. Though it will be taken from the majority of the yeast, plenty of yeast will remain in suspension to finish the job. Racking shortly after finished primary fermentation also enables the home brewer to harvest very fresh yeast that can immediately be used in another batch.

Adding Kraeusen

Kraeusen beer is fermenting wort which is still in its high kraeusen stage. When added to a beer after primary fermentation, it provides fesh healthy yeast, which may be able to do a better job in attenuating and maturating the beer. Sometimes it can also be beneficial to add a different yeast with the addition of Kraeusen. A less flocculant strain for instance can enable a better attenuation while the main flavor profile was determined by the more flocculant strain that was used during primary fermentation. For a commercial brewer the addition of kraeusen is simple since they always have various batches of the same beer at different feremntation stages. Some of them may even use different yeast strains for the above mentioned benefit.

In order to use the kraeusen technique, the home brewer will have to make a large (1 - 2 qt) starter from left over wort pitched with some of the yeast from the primary, if the same strain is to be used which is most common, or pitch it with a new culture of yeast. Left over wort from brewday can easily be kept in soda bottles in the feezer if it is boiled and cooled before the yeast is pitched. Once way of harvesting some yeast from the primaty is to use a sanitized racking cane, use the thumb to keep one end closed and push it down the carboy. Then relase the tumb and the beer will rush into the cane and pull a lot of yeast with it. Close the cane off again, pull it out and dump its contents in the starter vessel. This should be repeated 10 times to get enough yeast into the starter. Now the yeast in the starter is allowed to start fermentation at primary fermentation temperatures. When the beer is then racked to the secondary, the Kaeusen is added without adding the layer of yeast sedimen that may have already collected on the bottom.

Maturation/lagering vessel

There are 2 kinds of vessels that are commonly used by home brewers for the maturation and lagering of lagers: carboys and soda kegs. Because of their oxygen permeability, buckets shoul not be used for long term storage of any beer and are thus not suitable for lagering. When the beer is racked after the fermentation and maturation has been completed, the carboy or keg should be purged with CO₂ to minimize the amount of oxygen that is left in there once the beer has been transfered beause the inactive yeast will not be able to bind this oxygen. Though more expensive, soda kegs make for the best lagering vessel for the home berwer:

• their narrower shape allow more of them to fit in a freezer chest or fridge thus allowing for a more efficient use of the lagering space
• The beer can be carbonated (force or natural) during the lagering phase. This may actually be necessary to keep the lid sealed
• With a shortened dip tube the beer can easily be transferred to a serving keg by using pressure or siphon whithout any contact to the ambient air.
• they don't break

Natural carbonation

Unlike commercial German brewers, home brewers are not required to carbonate their beers naturally. But using natural carbonation provides a few benefits that should not be overlooked:

• since the yeast is still fairly active when the beer is racked, most of the oxygen that is picked up during this process (splashing or O2 in the head space) will be taken up by the yeast before it can react with other compounds in the beer. This is benefical because due to the shortened dip tube the lagering keg cannot be purged with CO₂ as efficiently as serving kegs can. To efficiently purge a serving keg (full length dip tube) fill that keg completely with sanitizer solution and push it out with CO₂ and don't open the keg after that. Because there was no air in the keg when it was closed off, all the sanitzer has been replaced with CO₂. Obviously this doesn't work so well with a shortened dip tube since a considerable amount of sanitizer would remain in the keg.
• When the keg is pressurized by the CO₂ from the fermenting beer, there is no connection to a CO₂ tank necessary to keep the lid sealed and to take samples with a picnic tap
• The beer is already carbonated when the lagering is completed.

To use natural carbonation a soda keg or other pressure resistant vessel needs to be used. Carboys are not designed to withstand pressure. Since CO₂ needs to be produced for the natural carbonation a sufficient level of fermentable sugars needs to be present after the beer has been racked. This can be achieved by:

• racking the beer when about 1-2 % fermentable extract (4 - 8 gravity points) are left. At this time there is also enough yeast in suspension to ensure a good secondary fermentation. This pocess is called Gruenschlauchen (green racking) in German brewing
• adding fermentable sugars in the form of Speise (a.k.a gyle, which is unfermented wort), malt extract or sugar. But if the beer has aleady fermented to far the amount of yeast that is transfered during racking may not be sufficent for a good secondary fermentation
• adding Kraeusen allows the addition of fermentable extratc and fresh yeast. This is the preferred method to use when the beer is racked late and the amount of yeast in the racked beer is lower. It also allows topping off the keg with another 2 qts of beer when the primary fermentation was done in a 5 gal carboy and only 4.5 gal beer could be racked to the keg.

soda keg with pressure gauge and bleeder valve

But different from priming bottles, the amount of sugar needed for natural carbontion doesn't have to be calculated at all since the actual carbonation in the beer can easily be adjusted with Spundung. Spundung is the German technical term for controlled pressure release from a fermenter. For this a pressure gauge needs to be connected to the gas-in connect of the keg. Based on a carbonation table the current CO₂ content of the beer can be determined from the current head pressure (measured by the pressure gauge) and the current temperature of the beer. If the carbonation is to high, the excess pressure can be relieved with the bleeder valve. If the bleeder valve is build as an adjustable pressure sensitive blow-off valve the process is even easier since you can set the maximum pressure ( equal to maximum carbonation level at constant temperature) and let the system vent fermentaion CO₂ as needed.

lagering/cold conditioning

Sources

[Narziss, 2005] Prof. Dr. agr. Ludwig Narziss, Prof. Dr.-Ing. habil. Werner Back, Technische Universitaet Muenchen (Fakultaet fuer Brauwesen, Weihenstephan), Abriss der Bierbrauerei. WILEY-VCH Verlags GmbH Weinheim Germany, 2005

[Palmer, 2006] John J. Palmer, How to Brew, Brewers Publications, Boulder CO, 2006

[TU Vienna] www.vt.tuwien.ac.at/scripts/172942/172942_Brauerei_04.pdf