The rate
Pitching rates are quoted in million cells per millilitre per degree Plato, which conveniently gives billions of cells when you multiply by litres.
- 0.50 — low-gravity or session ale
- 0.75 — standard ale
- 1.00 — high-gravity ale
- 1.50 — standard lager
- 2.00 — high-gravity lager
Lagers need roughly double, because cold fermentation slows reproduction and you want the population in place before the yeast has to work.
Packs are not what they say for long
A fresh pack is taken here as 100 billion viable cells, which is about right on the day it is made. Viability falls by roughly 20% a month in the fridge, and faster if the pack has been warm. A six-month-old pack may be under half its stated count.
Check the manufacture date, count down, and then decide whether you need a starter. Under-pitching is the single most common cause of the faults homebrewers blame on other things — a stuck or slow start, high ester and fusel production, and a yeast character that does not match what the strain is supposed to do.
Over-pitching
Less talked about and less damaging, but real. Too many cells means very little growth phase, and some of the character brewers want — particularly in Belgian and hefeweizen strains — comes from yeast under mild stress during reproduction. Pitch a Belgian ale at lager rates and you can lose most of what makes it Belgian.
For clean American and British ales, erring high is fine. For expressive strains, hit the rate rather than exceeding it.