Techniques - Reusing Yeast

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