November 19, 2012
A study by Blasco and coworkers confirms that beer foam is littered with corpses of dead yeast. Or at least with bits of their cell walls.
This has been known for awhile. But what these researchers did was to identify one of the key proteins in the cell wall important for maintaining a good head on beer.
The authors knew from previous studies that certain mannose binding proteins play an important role in beer foam. So they used primers that lined up with the 5’ and 3’ ends of one of the known foam-related genes from S. cerevisiae, AWA1, to look for similar genes in the brewing yeast S. pastorianus. This allowed them to PCR out the CFG1 gene.
To show that this protein was involved in the foaminess of beer, they next knocked the gene out of S. pastorianus and used this deletion strain to do some brewing. What they found was that while beer made with this strain had about the same amount of foam, it didn’t last as long. This strongly suggested that CFG1 was involved in maintaining a good head on a mug of beer, earning the gene its name: Carlsbergensis Foaming Gene.
As a final experiment, they added the gene back to a strain of S. cerevisiae, M12B, that makes beer without foam. When this strain expressed CFG1 and was used to brew up some beer, that beer was foamless no more. This suggests that CFG1 may be important for foam formation as well as stabilization.
What is probably happening is that during fermentation, yeast cells are autolysing, releasing their cell wall proteins into the beer. Since Cfg1p is hydrophilic on one end and hydrophobic on the other, it forms very stable bubbles. And foam is simply a bunch of stable bubbles.
Hopefully scientists can use this information to tweak the amount of foam a given beer yields. Then a drinker can choose lots or little foam, long lasting or short lived foam, or any combination he or she wants.
Root beer foams for a different reason
by D. Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics
Categories: Research Spotlight