Re-using Yeast Cake...Recs & Opinions

How much lower?

 

If you are looking for a study per se, I don't know off hand. But commercial brewers (and home brewers) have been doing forced fermentation tests for a very long time. I dare say there have been millions (or at least hundreds of thousands) performed so far. If they didn't work, they wouldn't do it.

ETA: Where are you getting your predicted FG from?

 

Up to about 4 points.

 

Based on attenuation calculations as per the particular yeast specifications.

 

Maybe I should have been more specific....whose attenuation calculations and what specifications? Or how were the calculations done? What I'm getting at is this: to say that Strain A should attenuate to X% Apparent Attenuation isn't meaningful (or it's just wrong) without also knowing the relative fermentability of the wort.

 

Then we can tackle this a different way. When brewing the same recipe, when I don't do a diacetyl rest, I genrally get one FG. When I do a rest, I generally get a lower SG, up to 4 points lower. Again this is not scientific - just anecdotal. It would be great to see an actual study that compares the difference. Perhaps Brulosophy?

 

I wouldn't mind seeing a proper study, with adequate sample sizes, and replication, but Brulosophy doesn't do those. But if a good study showed something different than the very large number (thousands or millions) of fast ferment tests, I'd turn in my propeller hat. Maybe the reason it hasn't been formally studied is that the very large number of fast ferment test observations make it seem obvious to those who would otherwise be tempted to study it.

 

It's possible that nobody's even thought of the connection before. A difference of a few points might not be that noticeable without a comparable sample. I'm tempted to split my almost fermented batch in half some time, and do a diacetyl rest on one half and no rest on the other. Then check the FG of each.

 

I found this in support of my theory:

"Diacetyl is a compound generated by the yeast in most beers, and its level or concentration varies by yeast strain and beer style. Most styles should not have any, but a handful of styles allow for a tiny amount. (People's sensitivity to diacetyl varies wildly, so "tiny amount" is subjective.)

Most yeast strains will usually "clean up" the diacetyl if given sufficient time in primary to complete the job. So leaving the beer on the primary yeast cake thru the entire fermentation cycle is key. Additionally, if your yeast strain or beer style recommends a diacetyl rest, it means raising the ferm temp 2 or 3 degrees Fahrenheit towards the end of the active ferm, often in the five to eight day range.

At that stage, the yeast is tired, drunk, and finishing off the more complex sugars and also trying to clean up diacetyl and other typical by-products of the fermentation. Raising the temp slightly helps the yeast with the clean up. Raising the temp may also keep the yeast eating and achieve 1-2 more points of attenuation. "

From the Moderator at BeerSmith:
http://www.beersmith.com/forum/index.php?topic=1995.0

Also, indirectly, Chris White of White Labs in his book "Yeast", says:

"Here are some common methods brewers use to try to drive a beer to attenuate a bit more:"

(He then lists 5 methods, one of them is the following).

"Increase the temperature. Higher temperatures increase yeast activity. Within reasonable limits, this is one of the best ways to help yeast reach the target final gravity".

None of this is definitive, but at least indicative.

 

I think you may be missing the point of the fast ferment test. Every single one of those has a comparable sample. First, you finish fermenting the test sample at a higher temp and measure the FG. Then you let what's left of the main batch ferment at normal temp until it reaches the same FG. That's how you know it's finished. If it never reached that FG, there would be no reason for thousands/millions of fast ferment tests to have been performed. The test tells you how low the FG is going to go...exactly the same FG as the warmed sample.

I disagree with the moderator at BeerSmith, unless he/she has data to show. However, if he/she is referring to low temperature induced yeast dormancy at the end of the fermentation, then I would agree. Which leads to...

You left out the part just before this where the book talks about causes of under-attenuation, one of which was an internal fermentation temp that is allowed to drop low enough that the yeast go dormant. The part you quoted is the cure. I do agree that if fermenting at the low end of a yeast's effective operating range, and not using fermentation temperature control that maintains the wort/beer temp (not ambient) without any drop towards the end (when the fermentation itself is generating less heat), the yeast could go dormant. But that's an incomplete fermentation, not a complete fermentation that landed at a higher FG.

Sounds like a plan. Please make sure of the following:

- Everything, including fermenters, temperature controllers, heating/cooling devices, closures, and quantity of yeast is identical.

- The fermentation temp (of the actual wort/beer) is well within the effective operating range of the yeast strain. Do not let the average temperature fall by even one degree near the end (or ever, if possible).

- The non-diacetyl sample is allowed to reach FG. This will take longer than the D-Rested sample.

- Measure your results, then repeat the experiment several times.

 

Thank you - I wasn't aware that's how the test worked. Now I see your point.

It's unfortunate the moderator didn't provide sources.

But that could be the point itself. Without using a diacetyl rest could regularly result in under-attenuation, while using it gets additional attenuation, closer to the target attenuation for that type of yeast etc.

Great advice. To ensure more control it might be best to use the very same sample. In other words brew, say a 5-gallon batch, but just before it gets within a few points of target gravity, transfer each half into two separate 2-1/2 gallon carboys. Then only do a diacetyl rest on one of those carboys, and wait until both carboys have finished fermenting. Then at that point carefully measure the FG.

 

If I was regularly getting under-attenuation, I would look at the basic process rather than add a crutch (the D-Rest). But I can see how it could work to compensate for a problem.

I would not recommend introducing a transfer to split the batch. I would split it from the beginning. One reason I wouldn't add the transfer is that racking could encourage the yeast to flocculate or behave in some other non-typical way.

 

The main problem with this is that both batches might not have the exact same amount of active/healthy yeast. I have made many split batches before (from a ten gallon batch), stirred up the yeast slurry, added half to each, and almost always I get different yeast activity in each batch. One is almost always significantly finished fermenting before the other, even though everything else remained the same.