Yeast starter and pitching rate
Whether you have enough yeast and what starter to make.
There must be enough yeast: underpitching makes fermentation start slowly, produces off-flavours and diacetyl, and a dense wort may stall halfway. The calculator works out how many cells the batch needs, how many a pack holds, and what size of starter to make up the shortfall.
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How many cells are needed
The pitch rate is measured in millions of cells per millilitre of wort per degree Plato. For an ale it is 0.75, for a lager 1.5: at a low temperature you need twice as much yeast. Dense wort above 15 °P calls for more still — about 1.0 for an ale and 2.0 for a lager.
Multiplying gives the required cell count: 20 litres of 12 °P wort at a rate of 0.75 is 180 billion cells.
N = k · V · P where k is the pitch rate, M cells/mL/°P; V is the volume, mL; P is the gravity, °P
What a starter adds
Growth depends not on the starter volume by itself but on the inoculation rate: the fewer cells per millilitre, the more each of them manages to multiply. The model of Chris White and Jamil Zainasheff describes this relationship as a curve: at an inoculation rate of about 25 million cells/mL the growth is roughly 1.7 cells per starting cell.
Agitation changes things noticeably: shaking adds 0.5 cells to the growth, a stir plate adds a full cell. That is why a stir plate grows considerably more yeast from the same starter volume.
G = 12.55 · x^(−0.459) − 1.0 where x is the inoculation rate, million cells/mL
Extract and starter volume
Starters are made with light malt extract at a gravity of 1.036–1.040: yeast grows readily in such a medium and the flavour of the finished beer does not change. Dry extract yields about 43 gravity points per pound per gallon — roughly 110 g per litre for 1.040.
The starter must be boiled for 10 minutes to make it sterile and cooled to fermentation temperature. Cover the vessel with foil rather than a stopper: the yeast needs oxygen.
What to keep in mind
- Viability falls over time: in a month of storage liquid yeast loses about a fifth of its cells. The number on the pack is an estimate, not a measurement.
- Dry yeast usually needs no starter: it is sold with a margin of cells and nutrients. A starter is needed for liquid yeast, old packs and dense wort.
- The optimal inoculation rate is roughly 25–100 million cells/mL. Below it the yeast multiplies sluggishly; above it the yeast wastes extract and grows worse.
- Before packaging the starter can be chilled and the liquid poured off: that way an extra two litres of diluted wort do not end up in the beer.