Yeast activity - cake vs suspsension

[hopsnhawks]

posted this question in the Beginners forum but thought I should put it here as well:

new to brew and am unfamiliar on how yeast behave in the carboy

trying to justify my transfer into a secondary carboy after fermentation but lots of folks insist there is no need, that 3 weeks in the primary is essentially the same as splitting time between a primary and secondary

is it thought that yeast present in the yeast cake, which I would assume are dead/dormant, continue to clean up whatever they can? if this is the case than wouldn't the yeast only be able to work on the liquid that is in direct contact with the cake?

it would seem that the job of clean up may actually be left to the yeast that remain in suspension and are still active

in that case moving to a secondary wouldn't hinder any clean up as you would pull all the circulating yeast with you and you would protect your beer from any autolysis that occurs over the course of +3 weeks

definitely not trying to start any brewhaha about which technique works best, just trying to wrap my head around how this all works, thanks

 

[Yooper]

it would seem that the job of clean up may actually be left to the yeast that remain in suspension and are still active

in that case moving to a secondary wouldn't hinder any clean up as you would pull all the circulating yeast with you and you would protect your beer from any autolysis that occurs over the course of +3 weeks

definitely not trying to start any brewhaha about which technique works best, just trying to wrap my head around how this all works, thanks

Yes, the "clean up" is a process at the tail end of active fermentation. The yeast first start with the simplest sugars, and then as the more easily digested sugars are gone, they digest their less preferred foods (such as maltiose). When that is gone, they actually go back and digest other things, including some of their own waste products (like diacetyl). That usually happens within 24 hours or so of reading FG.

Moving to a clearing vessel ("secondary) won't hinder the process normally but in some cases racking over to the clearing vessel before fermentation is finished can stall fermentation. That's not all that common, though. It does happen.

The other important thing to note is that nothing "magical" happens with racking, either. It's not like autolysis would only strike those first yeast that fall out in the primary, but not in the clearing vessel.

Autolysis is actually uncommon at the homebrew level, because it's related to pressure and temperature as well as time. A 5 gallon batch of beer in a carboy or bucket just doesn't put as much pressure on the yeast as a 10 barrel conical would in a commercial brewery.

 

[pwkblue]

I am not a true yeast expert, but your statement is true: the yeast in suspension do all the work, the yeast in the trub are for the most part dormant (but still alive)

 

[Yooper]

I am not a true yeast expert, but your statement is true: the yeast in suspension do all the work, the yeast in the trub are for the most part dormant (but still alive)

There are some highly flocculant yeast strains where the yeast might fall out before finishing, and in those few cases the fermenter can be swirled to get them active again, but in most cases the trub is comprised of spent yeast, some protein, hops debris, etc.

 

[Hermit]

Autolysis is actually uncommon at the homebrew level, because it's related to pressure and temperature as well as time. A 5 gallon batch of beer in a carboy or bucket just doesn't put as much pressure on the yeast as a 10 barrel conical would in a commercial brewery.

Semi-related. In BLAM one monk mentions their move to wider, shallower conicals though I think their primary gains is surface area for the yeast. Yeast that has flocked out to the bottom isn't usually/necessarily dead. The clump just got heavier. This settled yeast may well still be active but it won't do much if covered so taking your beer off the yeast cake too soon may not be such a good idea. You will also be surprised how much yeast you get in the bottom of your secondary if you do rack it off the cake.

 

[Kaiser]

The amount of yeast in suspension will depend on the strain, residual fermentation activity and time.

But the yeast in the cake still has pretty good access to the beer. It’s not just the top layer that is in contact with the wort. Nutrients (things to be cleaned up) do diffuse into the cake. But for the most part only the yeast closer to the top will the bulk of the work.

The beech wood aging process that Anheuser Bush employs for some of its beers is a way to speed up beer maturation by creating a larger surface onto yeast can adhere and have access to the beer. They take beech wood strips and put them into the lagering vessel. Then they add beer and Kraeusen. To my knowledge they could do without it but marketing won’t let them. But beech wood aging started as a way to produce a cleaner beer.

When I rack from the primary to a secondary fermenter, which is a keg in my case, I always make sure to pick up some yeast from the bottom.

Kai

 

[bobbrews]

Diacetyl and 2,3-pentanedione are the two ketones that are typically grouped and reported as the vicinal diketone (VDK) content of beer, which is the primary flavor in differentiating aged beer from green beer. The beer does not need to be resting on the yeast cake for off-flavor clean up of ketones (since there will be enough yeast left in suspension to take care of that task).

Fermentation is not a clean and efficient process. There are plenty of waste byproducts leaching into your wort/beer by the yeast during the process of alcohol creation. Acetaldehyde levels are also higher than desirable during and directly after primary fermentation. The conditioning time required for the yeast to fully reabsorb these VDKs and byproducts is still up for debate. It took them 7-10 days to expel everything, so I do not believe cleanup will only occur for 24 hours. Clean up may be faster if the beer is resting directly on the cake, but not by too much.

I typically give my ales 3-4 weeks total before bottling and I definitely notice increased quality as opposed to the lesser, 2 weeks from grain to glass. Nevertheless, I would avoid racking off the cake until at least 10 days. You want to be absolutely sure that primary fermentation is complete, and there is absolutely no krausen, before you decide to rack.

 

[WoodlandBrew]

Lots of good responses, but just wanted to put a little different spin on it.

There is actually a fair amount of active yeast down in the cake. Some strains, like WLP004 and WLP002, do quite a bit of work from the bottom. If you have a transparent fermentor watch the yeast at the bottom you'll see occasional bubbles of CO2 coming out of the cake. They look like fireworks as they streak up and yeast falls off the bubble. This is fermentation of sugar to CO2 occurring deep inside the yeast cake.

If the yeast are still consuming or absorbing byproducts then it is best to leave it on the cake. The more yeast you have during clean up the more space there is for all the byproducts to go.

If it is a high ABV beer you probably don't want it sitting on the cake for more than a few weeks. The alcohol will start to kill the yeast.

Some pretty plots here:
http://woodlandbrew.blogspot.com/2013/01/abv-effects-on-yeast.html