Friday, November 21, 2008

The Genetics of Yeast Flocculation

A friend sent me an interesting article from Popular Science on the genetics behind yeast flocculation:

Apparently the gene and the protein responsible for yeast flocculation (clumping and dropping) have been identified. The article explains the advantage yeast cells gain by clumping together (protection from environmental threats, including the ethanol their fermentation produces), and why it is a good example of kin selection (self sacrifice for the greater genetic good). The article also explains how a similar gene could have led down the path to the evolution of the first multicellular organism.

It is certainly a solid/interesting article, but it only looks at the benefits of flocculation, and does not mention how humans have impacted the evolution of this gene.

When yeast flocculate they pretty much stop fermenting, so if the cells drop early they will miss out on fermenting more sugar and possibly more reproduction. This is why yeast strains that are more attenuative tend to be the least flocculant. If this wasn't the case yeast cells would have evolved to become more and more flocculant, eventually becoming similar to a an acetobacter mother.

Brewers often select for the more flocculant yeast cells by repitching the cells that drop to the bottom of the fermenter after fermentation is complete. As a result most professional brewers have to start a fresh culture of yeast ever 6-10 batches because the yeast can become too flocculant resulting in lower attenuation. To prevent this (before sterile culturing was invented) brewers generally fermented in open tanks and would take yeast cells off the top of the fermenting beer during high krausen. Some breweries still practice this technique, particularly brewers who specialize in German Weissbier (as a result these strains tend to form large krausens and flocculate very slowly).

This is one of the wonderful things about brewing, no matter what you are interested in beer can tie in. Science, cooking, gardening, building, socializing, writing, travel, etc...


Epicanis ( ) said...

It annoys me when journalists do this: they write about peer-reviewed scientific research, but never link to the actual scientific papers involved.

I'm guessing this one is the one that prompted this article.

Interesting stuff...

("Green Beard Gene"????)

The Mad Fermentationist (Mike) said...

Thanks for tracking down the link.

Mike said...

They use an open fermenter at Ommegang. I thought this was mainly for yeast harvesting but it also adds to the wow factor for tourists.
So what your saying is that the best yeast are in the krausen? So its possible to re-pitch this yeast more often then the yeast that falls out? Less mutation I assume.

The Mad Fermentationist (Mike) said...

Yep, it is also much purer because it isn’t mixed in with the trub, and much more active because it is taken at the peek of fermentation. If you aren’t careful thought it is also a good way to infect your batch with wild microbes.

A big part of the fermenter for Belgian breweries is also ester formation. The shape and closure of the tank can cause yeast to behave in different ways. I believe dissolved CO2 reduces ester formation, so an open tank would lead to a fruitier beer than a closed tank. The open tanks also have a height:width ratio of close to 1:1 where cylindro conicals are normally closer to 2:1. Brew Like a Monk has a good deal on this, certainly worth taking a look at if this sort of thing interests you.

Mike said...

Wow, good info! Thanks Mike!

I've been thinking about moving over to buckets for my beers that are quick fermenters. Gives the advantage of yeast harvesting and you won't have to worry about oxygen permeability since your not leaving it in the bucket for a long time. Don't have to worry about shattered glass either. Though you can't watch your science experiment and you have to worry about keeping your bucket clean and sanitary.

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