Two-Stage Starter Calculations

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:

1. 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.
2. Note the number of cells that result.
3. Turn off the automatic viability calculation and enter that number for the viability.
4. Enter your actual gravity and volume, and the calculator will tell you the volume of the second stage.
5. 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”.