[eluterio]

I posted this on Homebrewtalk and would like some input here as well.  Thanks

My intention are to use a yeast cake but need some more info to convince me why I cant dump fresh wort on this specific yeast cake. 

I want to brew a batch of low gravity beer starting at 1.030-1.035 and about 3.5-4 gallons.  Making the yeast a second generational yeast.  From here I want to drop 5.5 gallons of 1.50-1.055 wort onto the yeast cake without washing or rinsing.  Here is my dilemma:

1.030 of 4 gallons of wort needs 86 billion cells no need for a starter.
1.035  of 4 gallons of wort needs 100 billions cells potentially no starter.
I read somewhere on here that 1 ml of yeast slury contains about 4.5 billions yeast cells, homebrewtalk forum. 

With this said, how many ml are in the yeast cake on average? Also on this forum i have read ther is 4.5 billion yeast cells per 1ml of yeast slury.  If there are 100 ml you will have 450 billion cells that is way too much for a starting gravity beer that is 1050-1056.  Yet it works with bigger beers as noted below.
So i have recently brewed a lager which takes more then 200 billion cells. Both examples are 5.5 gallons batches and I use Brewersfriend.com yeast calculator to figure all my yeast starters.  for a 1.050 lager it requires 387 bill cells.  I used this yeast cake for a bigger beer with a starting gravity of 1.066. A 1.066 requires 504 bill cells.  Even with the calculation I noted before 1 ml = 450 billion cells.  If this is the case and there are 200-300 I would have a estimation of 900-1350 billion cells and would be considered OVER PITCHING.  The beer turned out just fine.

Again all these numbers ive pulled off of estimations from brewersfriend and homebrewtalk forums.   
Now my questions will I be successful if  I use the lower gravity beer to use as a starter and dumped another on top of the yeast cake at a higher starting gravity? I just need some more input any would be appreciated.   

[morticaixavier]

It might be fine. The main risk in my mind of vastly overpitching is acetaldehyde which I find can be left behind if you overpitch too much. I suspect because everything happens so quickly and resources are consumed that the yeast floc and go dormant before metabolizing all the acetaldehyde. This would give the beer a granny smith apple flavor. that being said, these are all estimates and without putting it under the scope you can't really know the density of your slurry. and without even more complicated sciency stuff you can't know the viability and vigor of those cells.

I personally would pitch between 6 and 12 ounces (as determined by a standard kitchen ladle) into a 1.100+ beer. that is generally less than half the total yeast cake after swirling it up into slurry. for a 1.050 I would only use one scoop with the ladle so about 6 ounces which is about 177 ml.

for lager I might well pitch on the whole cake from the smaller beer.

[Joe Sr.]

Estimating yeast cell counts in this fashion is inexact, but probably close enough to get the job done.

Yes, you will likely be overpitching if you use the entire yeast cake.  Yes, you can just go ahead and put your fresh wort on top of the yeast cake.  You can make perfectly good beer this way and be happy with it.  You may also get off-flavors or unexpected flavors from the overpitch.  Some people say more esters.

An easy way to deal with this would be to pour the yeast cake out into one or more sanitzed jars/containers and use the amount you think is appropriate.  1/3 is probably what you need. 1/2 if you want to be conservative.

This also would give you the benefit of using a clean fermenter that doesn't have all the mess of the previous fermentation.

I've used whole yeast cakes in the fashion you plan to do with good results.  The process of pouring out and using only a portion is so simple and effective that I haven't done the whole yeast cake thing in years.

[eluterio]

Thanks guys for giving me some more to really think about this.  With your experience have you noticed a difference of using a fresh smack pack or vile to a yeast slurry?  There are people who believe you really don't get the true esters until the 3rd repitch?  In trying to brew apa that I did months ago version 1 was on a yeast cake version 2 was fresh yeast with starter, version 2 was lacking everything was close to the same the hops and malt were from the same bag for version 1&2. 

[morticaixavier]

sounds like you overpitched batch 2. I think that all else being equal, a similar pitch from a fresh pack with a starter should perform equally well to slurry. but there are a lot of variables here and it's hard to make any clear cut statements.

[Joe Sr.]

I repitch repeatedly though typically I make a fresh starter each time from a stored slurry.

I've noticed performance changes over time with subsequent reputches but the biggest for me would be failure to drop clear. I've not really notified flavor changes attributable to the yeast.

I'm not sure what was lacking, but there is the theory that over pitching will produce more esters. IF that's what you were looking for perhaps batch 1 was the over pitch. It's much more likely with a full yeast came than with a starter.