Correct way to calculate a starter overbuild

[Sparger]

Discovered this error today. I use brewer's friend yeast starter calculator and overbuild my starters by 0.8L to bank that amount for my next starter (I have 0.8L jars).
My erroneous approach, say I had 100B cells (brand new pack) and needed 200B to pitch for a new beer (pitch yeast). The yeast calculator says I need approximately 0.7 L starter (72 g DME) to step up my 100B cells to 200B cells for the pitch yeast. So, to get my yeast for next time (my bank yeast) I'd add in another 0.8 L (1.5L total starter with more DME) so I could bank the additional 0.8 liters of yeast. A 1.5L starter (with 100B cells) gets you 316B cells. What I was doing wrong was thinking that 0.7L got me 200B pitch yeast (the first calc in the yeast calculator), and the remaining 0.8L would get me 116B for my bank yeast (316-200). This is wrong. You have to figure out the bank yeast and pitch yeast ratios to the total starter (here a total starter of 1.5L). For example, above the bank yeast percentage calculation is 0.8L/1.5L = 53%. The pitch yeast percentage is 47% (0.7L/1.5L). This means I'm banking 53% of 316B = 167B. My pitch yeast is 47% of 316, i.e., approximately 149B (i.e., 316-167). In other words, my erroneous method resulted in 25% under-pitching (I wanted 200B and thought I was pitching that much) but it was only 149B. In addition, my old bank yeast amount was off. I thought I had banked 116B, but the actual bank yeast was 167B.
The corrected amounts: The correct total starter I would need to get 200B pitch yeast (starting with 100B pack) is actually 1.8L (359B) cells. 0.8/1.8L = 44%, so my bank yeast would be 44%* 359B) or approximately 159B, the remaining pitch yeast would be 200B (i.e., 56% * 359 or 359-159). FYI - doing this calc. is easier on a spreadsheet.
Assumptions: 1.036 starter OG; Braukaiser stir plate; 1 packet fresh yeast made today at 100% viability).
I welcome any input if I'm incorrect here. I was wrong the first time, I could be wrong again.

 

[chickypad]

So if I understand, you made a 1.5 L starter, poured off 47% (0.7 L) and thought you would somehow get 63% of the yeast in there? You don't need a spreadsheet, just add the amount you want to bank to your total you need for the batch and aim for that total amount of cells in the calculator. Then just divide one of those numbers by the total cell count so you know how much to pitch and how much to bank.

 

[Sparger]

So if I understand, you made a 1.5 L starter, poured off 47% (0.7 L) and thought you would somehow get 63% of the yeast in there? You don't need a spreadsheet, just add the amount you want to bank to your total you need for the batch and aim for that total amount of cells in the calculator. Then just divide one of those numbers by the total cell count so you know how much to pitch and how much to bank.

I always used the old wrong method with lots of yeast (2.5L starters, 350B+, etc.) in my actual brewing. With bigger numbers it wasn't so obvious to me (pitching 2.5L, retaining 0.8L etc.) It hit me when I made a small starter (1.5L).

 

[MoonBrewer]

I have been using the calculator on Brew United site, with good results. It allows you to add your overbuild to determine the correct total starter volume. Then, before pitching you just pull out the overbuild portion:
(Overbuild cell count/Total cells) * Total Starter Volume

The rest is for pitching.

http://www.brewunited.com/yeast_calculator.php

 

[Sparger]

I have been using the calculator on Brew United site, with good results. It allows you to add your overbuild to determine the correct total starter volume. Then, before pitching you just pull out the overbuild portion:
(Overbuild cell count/Total cells) * Total Starter Volume

The rest is for pitching.

http://www.brewunited.com/yeast_calculator.php

thanks for sharing that

 

[Sparger]

I have been using the calculator on Brew United site, with good results. It allows you to add your overbuild to determine the correct total starter volume. Then, before pitching you just pull out the overbuild portion:
(Overbuild cell count/Total cells) * Total Starter Volume

The rest is for pitching.

http://www.brewunited.com/yeast_calculator.php

It looks like the overbuild option on that site asks how many overbuild cells do you want, not what your required overbuild volume should be (0.8 L).

 

[chickypad]

The volume is going to vary depending on age of the yeast, aeration method, etc. What you want to tell it is the amount of cells you want to bank and then store the volume that it calculates for you.

 

[Sparger]

The volume is going to vary depending on age of the yeast, aeration method, etc. What you want to tell it is the amount of cells you want to bank and then store the volume that it calculates for you.

I dont really care about the overbuild cell count. I just want to fill my overbuild jar of 0.8L. No more no less. I then just put a label on it of how many cells and the date. Upping the starter to get a certain overbuld count is easy. The tricky part is realizing that you have to think in percentages of your overbuild to correctly get your pitch amount and overbuild volume of 0.8L.

 

[chickypad]

Seems odd you care more about the volume than the cell count but I guess to each his own. If you're worried about the jar being filled you can always top up with sterile water. I usually don't bother though and haven't had problems storing overbuilds of 650-900 ml in 1 liter jars.

 

[Sparger]

Seems odd you care more about the volume than the cell count but I guess to each his own. If you're worried about the jar being filled you can always top up with sterile water. I usually don't bother though and haven't had problems storing overbuilds of 650-900 ml in 1 liter jars.

Since I store overbuilds in a single 0.8L jar, filling it to the top gets me a maximum overbuild. That usually gets me well over 100B (say 164B). For the next starter, I just input 1.64 “packets” into the yeast calc and get a new viable yeast count.

 

[Rob2010SS]

Since I store overbuilds in a single 0.8L jar, filling it to the top gets me a maximum overbuild. That usually gets me well over 100B (say 164B). For the next starter, I just input 1.64 “packets” into the yeast calc and get a new viable yeast count.

100B is really all you need for an over build on a starter. The brew United calculator that another poster shared with you, that calculator will tell you how much volume for your starter and how much volume for your overbuild. You dont NEED to fill the jar.

 

[Jtk78]

I do see where you where heading, and as you know already, .8L of a starter will never net you less cells than .7L (assuming it's mixed evenly).

I overbuild starters as well, but essentially use your new math just taking a different path to get there.