KernelCrush said:
I thought the slurry mode was to calculate pitch rate into a batch. The same number can be used as a slurry pitch rate to a starter? Does that account for additional growth in the starter?
I didn't want to use this old slurry and would normally toss it but it was a real performer on first use. Reused it once last weekend but that's not ready to rack yet. Was really pushing it and lost. One look at it this morning and it was down the slop sink.
You are absolutely correct. That calculator tells you the amount of slurry to pitch into a batch. But, it tells you how many cells are in a certain volume of a certain age. A small amount of work with Yeastcalculator.com will tell you how much slurry you need to grow into a final volume for pitching. For example:
pitching into a 1.050 / 5.5 gallon beer requires 200 billion cells
My rule of thumb is no more than 20% dead cells, so that's 40 billion dead cells. If my slurry is a month old, it has 40% dead cells. In order to keep the dead cell count below 40 billion I can only use 100 billion cells of slurry (or less). In real life, I'd be conservative and probably cut that in half again. So, I'd start with 50 billion cells of slurry (which keeps the dead cell count around 20 billion). If I'm growing a starter anyway....why not?
I go over to MrMalty.com and use the slurry calculator. I put in my target beer and slurry harvest date. MrMalty says that I get 193 billion LIVE cells from 177 ml of slurry that is a month old. That's 193/177 = 1.09 billion cells per mL of slurry. In order to pitch 50 billion cells into my starter, I need to add 50 billion / 1.09 billion per mL = 46 mL of slurry.
Note that MrMalty estimates the viability at 50% (rather than my rough 40% estimate).
So, back to yeastcalculator.com. In the liquid yeast properties, set the initial cell count to 50 billion, and uncheck the calculate viability from date checkbox. Also set the viability to 100% because we already took care of that over at MrMalty.
In each of the "step" sections set your aeration method to whatever you use. I use a stirplate, and I've always used JZ's calculation method (I need to look into Kai's data....maybe I need to change). I have a 1L flask, so I set my starter volume to 800 ml.
For the first step, Yeastcalculator says that I from my initial 50 billion cells, I will end up with 141 billion cells total. So, I'm not to the 200 billion required, yet. I will need to decant the first step and add more wort to grow the remaining 50 billion cells.
So, I update the second step, as before. If I grow another full step (800 ml) then I end up with 247 billion cells---slightly over-pitching. By backing the volume down to 500 ml, I end up with 199 billion cells. Incidentally, when you decant you don't have to decant every last drop. You really only need to decant as much as you are going to add back. In this example, 500 ml in order to make room in the flask.
Also, if you have a larger flask (2+ liters) you wouldn't need the second step. A 1.7 liter single-step starter would grow all the needed cells to pitch 199 billion cells.
I used to do this a lot---back when I used to harvest and freeze my own yeast. I would grow a large quantity of yeast and then freeze 12.5 ml of slurry in another 12.5 ml of glycerin. Then I would grow these small yeast quantities up into full starters for pitching. Its a hassle that is only worth it for hard to find strains like the platinum strains or maybe something you harvest from a commercial bottle.