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.
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.