The conventional fermentation in a German lager breweryThe 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 teachers and experts on brewing.
After the whirlpool 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 the other methods remove only about 2/3 of it. Once the cold break is removed the wort is aerated with sterile air to achieve a wort oxygen content of 8-10 ppm (mg/l). In case of cold break removal through flotation, the aeration of the wort is achieved 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 wort. 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 however not be confused with warm vs. cold pitching. There is no warm pitching in commercial 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 maximum 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 0.5 - 0.7 C in order to avoid shocking the yeast. At this time the primary fermentation is considered done, but the beer has only attenuated to about 40% - 60%.
A few days later, 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 to that 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 valve. 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 brewery 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. 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.
