It is well known that yeast propagation is best done with plenty of oxygen for the yeast. While this doesn't cause the yeast to start aerobic metobolization of sugars, the sugar concentration in worts brewers are using for yeast propagation inhibits aerobic metabolism, it is still beneficial for the yeast since it can create and store more sterols for healthy cell walls. In brewing fermentations sterol reserves are divided between mother and daughter cell when a daughter cell buds. This can lead to less than optimal sterol levels in yeast after the growth phase when adequate reserves are needed for sufficiently strong cell walls to withstand the increasing alcohol concentration.

During the early phase of yeast propagation on a stir plate, the starter beer will be saturated with air O₂ at a level of about 8 ppm. This is a result of the vortex that is formed and the constant mixing of the beer with air from the head space. As yeast growth progresses and CO₂ develops, the O2 concentration in the head space gradually declines and a Kraeusen forms, which also inhibits the contact between air and the beer.

Inspired by the design of the Carlsberg Flask^[1][2], a yeast propagator that can be completely sterilized before the yeast is added and which also allows for aeration before and during the propagation, I wanted to build something similar. These were the requirements:

• heat sanitation of the vessel and the wort
• keeping the yeast in suspension
• aeration during yeast propagation

What I ended up with is this simple device shown on the right. It uses a 5000 ml Erlenmeyer flask that can be purchased at Morebeer.com for only $40. The size 12 stopper is foodgrade (also available at Morebeer) and has two holes. One for a large glass tube to which a blow-off hose will be connected and another smaller tube to which a sterile air filter and an aquarium air-pump will be connected. The stock for the glass tubes was bought at cynmar.com. Bending the small tube over a flame allows for the attached aquarium air stone, which happens to fit perfectly, to be out of the center of the flask where a magnetic stir bar is stirring the wort. Initially the blow-off was supposed to be bent glass tube as well, but it is very difficult to bent a glass tube more than a few degrees without kinking it.

The wort and the propagator are sanitized on the stove top through boiling. Care must be taken such that the boil is not too vigorous and blows off through the blow-off tube. The glass parts may also be sanitized with a chemical sanitizer like StarSan, but sanitizing the device and the wort at the same time is the great advantage of using heat resistant glassware.

The yeast propagator in action. Air is pumped into the propagator at a slow rate while the beer is stirred to keep the yeast in suspension. Excess Kraeusen is blown-off. Due to the increased formation of Kraeusen, after all bubbles are created by the air, this form of propagation is only suited for bottom fermenting yeast which doesn't rise into the Kraeusen. Top fermenting yeast rises into the Kraeusen and too much would be blown off.

References

1. ↑ CPE Systems, Carlsberg Flask
2. ↑ Alfa Laval, SCANDI BREW® Carlsberg flask