Being a cheap bugger, I am a big fan of (re)pitching/reusing yeast. It's something I've practiced since the time I began brewing. I've tried just dumping new wort onto a yeast cake from a previous batch; however, after reading
this thread on Homebrewtalk.com, going forward I'll be utilizing a more measured approach. I would highly recommend giving the first post a read, it's very informative.
Referencing "Bob's" information (noted above), I've simplified the (re)pitching rates for my brew system as follows:
"Industry” pitching rule: 1 million cells x 1ml x 1˚ Plato"
Example: 25L @ 12 ˚ Plato (1.048) = 1,00,000 cells x 25,000ml x 12˚ Plato = 300 Billion Cells
Pitching Rates for 25L:
● 10˚ Plato (1.040) = 250ml slurry
● 12˚ Plato (1.048) = 300ml slurry
● 14˚ Plato (1.057) = 350ml slurry
● 16˚ Plato (1.065) = 400ml slurry
● 18˚ Plato (1.074) = 450ml slurry
● 20˚ Plato (1.083) = 500ml slurry
Note, online yeast calculators (YeastCalc, Mr Malty) actually recommend a slightly lower pitching rate for ales, 750k cells x 1ml x 1˚ Plato
To-date, I've only used dry yeast (or yeast harvested from bottle conditioned beers). I finally ordered some liquid yeast which I hope I can use as a sort of" house strain",
WLP007, White Labs Dry English Ale Yeast.
The yeast has been described as follows:
"Clean, highly flocculent, and highly attenuative yeast. This yeast is similar to WLP002 in flavor profile, but is 10% more attenuative. This eliminates the residual sweetness, and makes the yeast well suited for high gravity ales. It is also reaches terminal gravity quickly. 80% attenuation will be reached even with 10% ABV beers.
Attenuation: 70-80%
Flocculation: Medium to High
Optimum fermentation temperature: 65-70°F
Alcohol Tolerance: Medium-High "
Having finally dropped the goods on a liquid strain, I began researching various reusing, re-pitching, harvesting techniques to prolong my meager investment. Having purchased two vials, I hope to brew an entire years worth of beer from this yeast! I stumbled across
this thread which talks about "Splitting Starters".
Essentially, you make a large starter, more than what's required for the batch you intend to brew. You reserve a portion of this starter for a future starter, the remaining content is pitched to your batch. This process is repeated for each subsequent stater.
The problem I saw with this method is that to retain enough viable yeast for a future starter, I pegged this amount at 100B cells, you would not leave yourself with enough yeast for your current batch. I determined that in the context of a 2L starter, reserving 700ml (100B cells), the remaining 1300ml would not contain enough cells (in my 25L system) to properly meet the pitching rate defined above. To do so meant I would need to step-up this 1300ml with a second quantity of wort. Based on this theory, I summarized...
1 vial of White Labs liquid yeast = 100B cells
100B cells into a 2L starter = 310B cells (YeastCalc) = 155M cells/1ml
700ml x 155M = 108.5B Cells (reserve for future starter)
1,300ml x 155M = 201.5B Cells (pitch/step for current batch)
Step-up amounts (25L Batch Size):
● 0ml = 201.5B cells ( < 1.043)
● 500ml = 240B cells (1.044 to 1.051)
● 750ml = 289B cells 1.052 to 1.063)
● 1L = 330B cells (1.063 to 1.072)
● 1.5L = 397B cells (1.073 to 1.088)
● 2L = 453B cells ( 1.089+)
For example, in the event I would brew a 1.060 beer, I would build a 2L starter, split and then step-up the 1300ml portion with an additional 750ml of wort.
Going forward I plan to combine the two techniques. I will build and split starters for low gravity beers which require a minimal step-up. For larger gravity beers (ie 1.060+), I will harvest slurry after primary fermentation and re-pitch. The best of both worlds!
FYI, Yeast cell counts and step amount were calculated using:
YeastCalc
