Fermentation

Yeast starter

200 g of extract, 2 litres, a night on the stir plate: your pack becomes an army of 380 billion cells.

Result

Fill in the values and run the calculation, or try the example.

This is the calculation Krausi uses. Create an account to save it in a recipe or a brew.

Why make a starter?

A pack of liquid yeast contains about 100 billion cells when fresh, far fewer after a few weeks. That is too few for a lager, a strong beer, or simply a 25 L brew.

A starter is a small fermentation prepared a day or two before brewing. The yeast multiplies in a light wort. You get a large, active population for the price of a little malt extract.

How is yeast growth calculated?

The starter is made with 100 g of dry malt extract per litre, a gravity of about 1.037. Growth depends on the ratio r between the starting cells and the amount of extract:

dry extract (g) = 100 × volume (L)
r = starting cells (billions) / dry extract (g)
growth = 1.4 billion per gram if r ≤ 1.4
       = 2.33 − 0.67 × r if r ≤ 3.5
       = 0 beyond that
final cells = starting cells + growth × dry extract

This is the model established by Braukaiser for a starter on a stir plate.

Worked example

Values entered

Cells pitched into the starter, in billions: 100
Starter volume in litres: 2

Result from the tool

Dry malt extract in g: 200 g
Growth in billion cells per g of extract: 1.4 g
Cells at the end of the starter, in billions: 380

What the tool does not do

The model assumes a stir plate: without stirring, growth is about half as much.
A starter too small for the amount of starting yeast produces almost nothing.
The viability of the starting yeast is not corrected: enter live cells.

Frequently asked questions

What gravity for a starter?

About 1.037, which is 100 g of dry malt extract per litre of water. Any denser and the wort stresses the yeast instead of multiplying it.

How long should a starter be left?

Between 18 and 36 hours on a stir plate, at room temperature. It is ready when activity has died down and the yeast starts to settle.

Should I decant the starter?

For a starter of more than a litre, yes: chill it overnight, pour off the clear liquid and keep only the yeast at the bottom. The starter wort, oxidised, does not taste good.

And with the assistant?

Krausi can chain the two calculations: the cells your recipe needs, then the starter that provides them.

I have a fresh pack of liquid yeast, 100 billion cells. What if I make a 2 L starter?

Dissolve 200 g of dry extract in 2 L of water: after a night on the stir plate, you will have about 380 billion cells. That is enough for 20 L of lager at 1.050.

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