Add new wort to old yeast cake?

[Bobcatbrewing42]

I saw someone refer to adding new wort to an old yeast cake. I have wondered if I could siphon off my beer for kegging and then add a new batch on top the same day to save all of that cleaning and sanitizing. How about using some sterile water to wash away part of the yeast cake to avoid overpitching? I would assume that the new batch would need to be aggressively agitated.

 

[JLP]

I've never did this, but it is highly recommended to wash the yeast and then pitch. If you pitch on top of the yeast cake you risk issues from the trub carry over.

 

[mkeckjr]

I've never did this, but it is highly recommended to wash the yeast and then pitch. If you pitch on top of the yeast cake you risk issues from the trub carry over.

I think there is some debate on this topic. I quick search will bring up this thread, which I find one of the most informative threads on this entire site:

https://www.homebrewtalk.com/f163/cold-hard-truth-about-rinsing-yeast-boiled-water-451925/

There are some strong points brought up by the thread starter, and by others who debate the topic throughout the thread, about pros and cons of washing yeast. My very short summary of this thread is that yes, if you are truly washing the yeast, meaning using sterile (autoclaved) water, then yes, the practice has benefits. If you are doing the ad hoc version where you are just boiling the water for rinsing, however, then there is some risk in rinsing the yeast as boiled water does not equal sterile water.

I used to wash yeast, but now I just pitch part of the cake (well, I swirl the old cake, let it rest then pour that off into a container to get as much yeast and as little trub as possible) into a new batch. I think the points EarlyAmateurZymurgist makes in the thread were compelling enough to me to discontinue the washing practice, as it introduces more risk (in my opinion) that just using the yeast cake, which is relatively low-risk at the homebrew level.

 

[JLP]

In honesty, I haven't had time to read the thread; however, IMHO, your bound to have less of a clean fermentation by pitching directly on a yeast cake. With all of the trub left over from the previous beer, all of the hop matter left over, and then over pitching your yeast... I agree with the fact that the yeast cake is the safest from infection if your previous batch turns out great. Granted some beers will stand up to this abuse, the more delicate ones stand a chance to have issues. Now pouring only a partial amount of the previous trub is a different story all together. Your really separating the gunk from the best part, and repitching. This is not the same as pitching directly on top of the yeast cake with the stuff that you are seperating left behind.

 

[mkeckjr]

In honesty, I haven't had time to read the thread; however, IMHO, your bound to have less of a clean fermentation by pitching directly on a yeast cake. With all of the trub left over from the previous beer, all of the hop matter left over, and then over pitching your yeast... I agree with the fact that the yeast cake is the safest from infection if your previous batch turns out great. Granted some beers will stand up to this abuse, the more delicate ones stand a chance to have issues. Now pouring only a partial amount of the previous trub is a different story all together. Your really separating the gunk from the best part, and repitching. This is not the same as pitching directly on top of the yeast cake with the stuff that you are seperating left behind.

I am not arguing that pitching onto a full yeast cake isn't a significant overpitch, as a matter of fact it almost always is. I think this is what will result in a less-than-optimal fermentation. I do not see how a large amount of trub, however, will heavily affect the second beer. Maybe color, sure, but otherwise I haven't seen any scientific evidence that supports trub affecting quality on repitch. The quality of the second beer seems to be more highly correlated with yeast characteristics, such as how many cells you pitch into the second beer and their viability after the first beer. The only reason I try to swirl to minimize trub, instead of just pouring, is so that I can get a more accurate estimate of the yeast cell count on repitch.

What I was arguing was that the practice of "washing yeast" before repitching is not "highly recommended" as you originally stated. I would say that it's debated at best, and seems to be, given the evidence and arguments supplied in that thread, higher risk than just pulling some of the yeast cake you previously used, which is what I was suggesting.

 

[JLP]

Call it a poor choice of words. When I said highly recommend, I meant between pitching directly on a yeast cake vs yeast washing. With yeast washing being a debatable subject, like all things there are pros and cons. Plenty of books teach the technique of executing the act. I could just be ill informed, however; am unaware of any stating the opposite.

As far as the hot break, cold break and hops left over from the old beer, and compounded by being added to the new... There is enough bonafied research out there to understand the effects of too much of a good thing.

 

[eelpout]

I do this all the time.
I make a 3 gallon mild in the big fermenter. Rack that beer into mini keg and immediately rack the imperial stout wort onto the full cake.

Mind you, this causes a nuclear powered fermentation to start almost instantly. Keep Carboy near floor drain for the first day.

 

[broadbill]

I think pitching on the yeast cake has its purposes....fermenting out big beers, stepping up yeast #s for lagers without having to do a yeast starter. That sort of thing. Best practice is to minimize the time the first beer sits on the yeast cake, and get the second beer onto it as soon as you take the first on off.

I am no longer a fan of harvesting, washing or pitching slurry. Too many variables that can lead to poor yeast quality/healthy cells; also I don't think that yeast is generally in too good of condition after fermenting out a beer or significant gravity and sitting under 5 gallons of beer for a few weeks. My anecdotal evidence suggests that when I had some funky fermentations (overattenuation, thin body,etc.) I linked it back to pitching slurry.

I'd rather propagate yeast for future batches on the front end (make extra large starters, save back a portion for future starters) than muck around with well, the muck at a bottom of a fermenter. People seem to like harvesting, maybe for the large amount of material they get. I just think it isn't the type of stuff you want to be putting in your wort, when you go to great lengths to make sure everything else is high quality.

My 0.02