Most yeasts available to the homebrewer -- even ale yeasts -- are not true top-cropping strains.  As fermentation draws to a close, the krausen (thick layer of foam) on top of the fermenting beer collapses, and the yeast settles to the bottom of the fermenter.

With a true top-cropping strain, the reverse happens: As fermentation slows down, the yeast all rises to the top, forming a large, floating yeast mass.  If you've never seen this happen before, it can be rather disturbing -- the first time it happened to me, I thought I had picked up a weird infection or something, because it looked so bizarre.

The first picture shows a batch of beer fermenting with Wyeast American Ale II (1272), a top-cropping strain.  The beer has been in the fermenter about 2 days, and fermentation is very active.  The airlock is bubbling about once a second, and things look normal.  You can see the ring of debris (trub and hop particles that made it into the fermenter) clinging to the glass, where the top of the krausen touches it.

The foam is light in color, with tiny bubbles.

The second picture shows the same batch of beer, 3 days after the first picture was taken (5 days after pitching).  Fermentation is winding down, and the airlock is only bubbling about twice a minute... but wait, what's going on with the krausen?  The foam is now more than 3 times the height it was when fermentation was most active; in fact, the head of krausen is now nearly as deep as the beer in the fermenter!  Also note the position of the original debris ring, relative to the current level of krausen.

The foam has a thicker consistency, and is starting to darken in color from all the yeast.

For yeasts which behave like this, it is a good idea to agitate the fermenter once or twice a day, when fermentation starts to slow.  If this is not done, the beer will take a very long time to finish fermenting out, since most of the yeast is up in the foam, instead of down in the beer doing its job!

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Copyright © 2000 by Michael Uchima, All Rights Reserved

(Posted to Web March 18, 2000; last updated March 20, 2000)