The aim and benefits of a carefully conducted secondary fermentation are as follows [LN] [GAHM]
- achievement of final attenuation
- natural carbonation
- sensory maturation of flavor compounds
- improvement of head retention
- excretion of unstable protein compounds
- improvement of colloidal stability due to the low temperatures
- the temperature is between 3…5°C
- the residual fermentable extract is between 1,0…1,5%
- the fermentation slowed to a daily rate of 0,3…0,4%
Beer is then said to have attained its “Schlauchreife”, which is the point in time where a transfer to secondary fermentation tanks (often called “Lager” tanks) occurs. It is worth noting that an adequate amount of suspended yeast in the region of 12…15 million cells per ml should be present. Some brewers add a secondary yeast strain at this stage to ensure successful secondary fermentation and attenuation. Whereas during main fermentation a medium to highly flocculating yeast strain is used, a low flocculating strain at a ratio of 2:1…3:1 of main yeast strain to secondary strain sometimes makes its way into the secondary tank.
The cellar temperature where the tanks are situated is generally kept at 2…3°C. One of the reasons to bring the temperature of the main ferment close to this temperature is to avoid shocking the yeast, as well as conditioning the yeast to gradually handle the low temperatures at which it is expected to continue working in due course. [GAHM]
Transfers have an effect on fermentation as well, As the beer travels through the lines the yeast is set in motion again, which causes the fermentation to continue more vigorously. However, too vigorous is not desirable either. The aim of secondary fermentation is to gently and gradually continue the fermentation at low temperatures over a time-span of 4…8 weeks (pending on OG). Too strong a fermentation at this stage will cause the remaining extract to be consumed too quickly and lead to sub-optimal results. It is for this reason that Lager vessels are filled from below [LN]. When using home brew kegs this is simply done by filling via the outlet posts through the dip tube.
It is a well-known fact that beer is highly susceptible to oxidation during the cold phase and the transfer stage is no exception to that rule. Since fermentation is not complete, any residual oxygen left in the secondary tank is consumed. However, it is necessary to fill the receiving vessel to its maximum capacity in order to avoid too much oxygen remaining and damage the product.
Once the transfer is complete, a pressure relief valve and manometer (“Spundapparat”) is connected and the pressure is set according to the desired degree of carbonation.
Typical CO2 content is between 0,48…0,52% for Bavarian Lagers. In order to achieve optimal carbonation the following values are recommended [GAHM].
In his first book [LN], Ludwig Narziss goes to great length with a pedantic example on the temperature settings for each phase.
- [GAHM] Gerolf Annemüller, Hans-J. Manger. Gärung und Reifung des Bieres (2. Überarbeitete Auflage).
- [LN] Ludwig Narziss, Abriss der Bierbrauerei (7. Auflage)