Jack Schmidling, Aug 14, 1993

There has been a great deal of enthusiastic reporting on the use of aquarium air pumps to aerate wort prior to pitching yeast and many rather preposterous claims of shortened time to the onset of fermentation resulting from the use of same. As the author's experience on one batch did not support any such claims, a controlled experiment was designed to determine the validity of said claims.

The experiment described herein compares the air pump aerator with several other less exotic methods along with an un-aerated control batch.

The wort used was withheld from a batch of Pilsner style beer with a gravity of 1.050. It was re-boiled several days after the original batch was pitched so that it could be pitched with controlled amounts of kraeusen from the new batch. The re-boiling was to re-established an anaerobic environment along with re-sterilizing the wort.

Prior to pitching, the wort was divided into (4) 500 ml samples in sterile, one quart mason jars and aerated in various ways as follows:

The wort was cooled to a temperature of 70F before being aerated. All four samples were pitched with 50 ml of working kraeusen. This active wort was taken as a single sample and thoroughly mixed prior to dividing into 4 individual portions to assure a homogeneous and identical yeast in each test.

The yeast used was Pilsener Urquel recultured from a slant obtained from Paul Farnsworth. (*2)

After pitching, the four test batches were placed in a refrigerator at 40F and checked every four hours for signs of incipient fermentation.


No sign of fermentation was detected until the 72 hr check. At this time, a small island of bubbles was just visible in the center of all four samples.

To accelerate the conclusion of the experiment, the samples were removed from the refrigerator and allowed to rise to room temperature (75 F). They were monitored until the entire surface of the fermenters were covered with foam. This occurred about 9 hours later on all four samples.

ALE YEAST............

To take the experiment one step further, a similar but abbreviated experiment was conducted with ale yeast.

Two 500 ml samples of a similar wort were treated as follows:

Both samples were pitched with 0.4 grams of granulated EDME ale yeast, poured directly on top of the wort.

They were maintained at a temperature 75 F and monitored regularly until the onset of fermentation. This occurred at 4.5 hrs in both samples. The tops of both fermenters were covered with foam at 10 hrs total elapsed time.


The experiment seems to confirm the author's previous experience and points to the conclusion that the method of aeration used has no correlation with or effect on the time to onset of fermentation. Contrary to frequently stated anecdotal experience, the un-aerated control samples started fermenting as soon and with the same vigor as the variously aerated samples. This was true both in the case of cold temperature lager yeast and room temperature ale yeast.

This experiment was not intended to test any other aspects of the brewing process that may be affected by wort aeration. Much has been written on the subject and the present author's intent was only to study the effects of aeration on the onset of fermentation.