Yeast pitch and starters

A pitch rate is cells per milliliter per degree Plato, which is a way of saying that a bigger batch and a stronger wort both want more yeast. We use the long-standing figures from White and Zainasheff’s Yeast: 0.75 million for an ale and 1.5 million for a lager. Lagers get double because they ferment cold and slowly, and a lager short of cells is the classic cause of a fermentation that stalls and a beer that tastes of it.

Viability

Liquid yeast loses viability as it sits, and we apply the widely used figure of about a fifth of the cells a month, compounding rather than falling in a straight line, so the curve never hits an arbitrary floor. White Labs packs are the exception: White Labs publish their own figures for the PurePitch pouch, about 79% viable at five months and 72% at six, and a White Labs pack (we know one by its name or its WLP number) decays on that gentler curve instead. Dry yeast loses far less, about a fifth a year. If you have not given a manufacture date, the pack is assumed fully viable and the dialog says so, which is optimistic and labeled as such.

Dry yeast is different

A cell-count target is the wrong instrument for dry yeast. It arrives with the glycogen and sterol reserves a liquid pitch has to build in your wort, and judged on cells a by-the-book lager dose reads as a heavy under-pitch: five sachets wanted where the maker asks for under two. So a dry pitch is checked against the maker’s own grams per hectoliter, and the check says so in grams.

Starters

Starters use Kai Troester’s growth model, measured on stirred starters under foil, with a flat band for a shaken or a still flask. The wort is ten to one water to dry malt extract by weight, because his measurements found five, seven and ten degrees Plato grow the same yeast and twenty grows visibly less. We plan one step where one fits and two where it does not, and refuse a third: three steps is a laboratory schedule, and the honest answer is usually another pack. He puts the model at plus or minus fifteen percent or worse, so a plan that only just clears the target should be read as not clearing it.

The beer these numbers come from

Reference pale ale — 20 L into the fermenter, a 60 minute boil.

Grist	Amount	Color
Crisp Maris Otter Pale Ale Malt	4.5 kg	2.8 °L
Thomas Fawcett Caramalt	0.35 kg	9.9 °L

Hops	Amount	Alpha	Addition
Magnum	15 g	13.5 %	60 min boil
Cascade	30 g	6.8 %	10 min boil
Citra	40 g	13 %	20 min stand at 80 °C

It comes out at
Original gravity	1.0507
Final gravity	1.0094
Alcohol	5.52 %
Bitterness	35.66 IBU
Color	5.14 SRM (10.12 EBC)
Into the fermenter	20 L

The numbers

Viable cells needed

185.8 billion cells, from pitch-rate-plato.

cells (bn) = rate (M cells/mL/°P) × volume (L) × °P
⇒ Viable cells needed = 185.8 billion cells

Input	Value	Unit
Volume	20	L
Original gravity	1.05	SG
Wort strength	12.39	°P
Target pitch rate	0.75	M cells/mL/°P
Cells available	69	billion cells
Cells from packs	69	billion cells
Shortfall	116.8	billion cells
Packs required	3	—
Viability	1	—
Achieved pitch rate	0.279	M cells/mL/°P

What it assumes:

• Pitch rate defaulted to 0.75 M cells/mL/°P for a ale strain.
• SafAle US-05: assumed 11.5 g × 6 bn cells/g = 69 bn per sachet.
• SafAle US-05: no manufacture date, assumed 100 % viable.
• Ale and lager rates differ by a factor of two because a lager ferments cold and slowly.
• The selection is short of the target: another pack, or a starter, closes the gap.

Sources:

• Yeast: The Practical Guide to Beer Fermentation — Chris White & Jamil Zainasheff, Brewers Publications, 2010
• Pitching Rate Calculator — Jamil Zainasheff (Mr Malty)

Starter, dry malt extract

100 g, from braukaiser-starter-growth.

new cells = growthRate(cells pitched / g extract, agitation) × g extract
⇒ Starter, dry malt extract = 100 g

Input	Value	Unit
Target cells	189	billion cells
Available cells	69	billion cells
Method	stir-plate	—
Steps	1	—
First step volume	1	L
First step inoculation rate	0.69	—
First step growth rate	1.4	—
Final cells	209	billion cells
Dry malt extract	100	g/L
Starter gravity	1.037	SG

What it assumes:

• The growth model was measured on stirred starters under foil; every other agitation method is a flat band, not a curve.
• Two steps at most. A third is a laboratory propagation schedule, so the planner refuses and says how many packs would have done instead.

Sources:

• Estimating yeast growth — Kai Troester (Braukaiser), 2012
• Step Up Your Starter (NHC 2013) — Kai Troester (Braukaiser), 2013
• Yeast: The Practical Guide to Beer Fermentation — Chris White & Jamil Zainasheff, Brewers Publications, 2010