Ask Mr. Wizard

White Spots On Top of My Brew


Andreas Bettin - Toronto, Ontario asks,

I’m an extract brewer and I recently noticed a bunch of white spots on top of my brew (which was a wheat recipe) after I transferred it to the secondary. I thought it was mold forming but read that it’s probably “yeast rafts” or flocculated yeast clumps. The beer turned out nicely but I’m curious what causes this to happen? Are certain beer styles more susceptible to this phenomenon?


I am not completely sure about the nature of these little floaters. I think, however, that it is possibly either yeast, as you suggest, or protein clumps. Some yeast strains have a tendency to form a dense top crop. When these beers are racked some of the top cropping yeast is carried over and the appearance is at times strange. Weizen yeast is an example of an aggressive top-cropper. In the case of Brettanomyces, a pellicle is formed on the surface of the beer and also has an odd appearance.

This could very well be the early signs of a Brettanomyces pellicle forming on the surface of your beer. If this is the case it is an indication of a problem, unless you intentionally added this yeast type. Brettanomyces is one of those strains considered to be “wild” and can be a problem to control if you bring it into your brewery and do not have rigorous sanitation procedures.

The other thing that may be responsible for the unusual appearance in the fermenter is trub carry-over from the brewhouse. Wheat contains a significant amount of gluten proteins and when wheat malt is mashed the glutens tend to be retained in the mash and discarded with the spent grain. If you steeped some wheat malt it is possible that some of these gluten proteins made it into the fermenter and resulted in the odd appearance.

The good news about things like trub and yeast that collect on the top of fermenting beer is that you can rack the beer away from the solids and there is no detrimental effect on beer quality. If this beer begins to start smelling leathery, earthy and like a barnyard after a few months of storage the source was likely Brettanomyces. If that proves to be the case you should be careful handling the bottles since Brettanomyces is a super-attenuating strain that will slowly ferment dextrins in your beer that Saccharomyces species cannot. This can result in bottle grenades.

Response by Ashton Lewis.