There are many situations in which a homebrewer might need to propagate yeast using more than one step-up – for example, building up from a bottle of commercial beer, or reviving a smack pack that’s several months old. And while I love Mr. Malty’s Pitching Rate Calculator™, doing two-stage starters with it can be frustrating.
One simple alternative is to use Wyeast’s Pitch Rate and Growth Calculator. It does have some limitations: it works only in gallons, doesn’t feature a viability calculator (although viability can be input manually as partial smack packs), gives results in fairly useless intermediate units (millions/mL) as opposed to cell count or pitching rate (millions/mL-°P), and does not allow for decanting the starter. So in my opinion the best option is to trick the MrMalty calculator into doing it. Here’s how:
- Set your production date or viability, then play with the gravity and/or volume fields until the output matches the volume of your first stage.
- Note the number of cells that result.
- Turn off the automatic viability calculation and enter that number for the viability.
- Enter your actual gravity and volume, and the calculator will tell you the volume of the second stage.
- Verify that the volumes are realistic and adjust the first-stage volume if needed. Ideally, you want to at least double the volume at each step.
For example, let’s say I want to build a three-month-old pack of Wyeast 2206 into enough yeast for a 21 L (5.5 gallon) batch of 20°P (1.084) doppelbock. The required cell count is:
(21 L)(20°P)(1.5 billion/L-°P) = 630 billion. And the pack’s viability is:
0.75^3 = 0.422, or about 42 billion cells.
So I need to increase the cell count by about a factor of 15. This is clearly not possible with a single starter. So I’ll start with a 3 L first stage. Using “intermittent shaking” with a viability of 42%, volume of 5.5 gallons, and gravity of 1.024, the calculator predicts 189 billion cells total. I then set the viability to 189% and the gravity to 1.084, and the calculator tells me that the second stage will need to have a volume of about 6.7 L. Clearly this is one of those situations in which I should be brewing a 2-gallon batch of light lager as a “starter”.