Brew day
Decoction volume
A quarter of the mash brought to the boil, and the next step is reached without heating the tun.
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.
Create my accountWhy calculate the decoction volume?
Decoction is the traditional method of German and Czech lagers. You pull part of the mash, boil it separately, then return it to the tun: the whole mash rises to the next step.
It was the way to raise the temperature before the thermometer was invented. It is still practised for the malty character it brings, and because it allows steps without a heated tun.
Everything depends on the volume pulled: too little and the step is not reached; too much and you overshoot.
How is the decoction volume calculated?
It is a heat balance between the boiled part and the rest of the mash:
decoction volume = mash volume × (T target − T current) / (T boiling − T current)To take 20 L from 52 to 64 °C, you need to pull 5 L. Pull the thick part, rich in grain: the enzymes stay in the liquid in the tun.
Worked example
Values entered
- Total mash volume in litres: 20
- Current mash temperature in °C: 52
- Target temperature in °C: 64
Result from the tool
- Decoction volume in litres
- 5 L
What the tool does not do
- Heat losses during the pull and the boil are not counted: pull 10 to 15% more.
- The boiling temperature is 100 °C by default: enter yours at altitude.
- The calculation assumes the mash in the tun holds its temperature during the decoction.
Frequently asked questions
What is decoction for?
To raise the mash from one step to the next, and to develop malty flavours by cooking part of the grain. It suits German lagers, bocks and wheat beers.
How long should the decoction boil?
Between 10 and 30 minutes depending on the intensity you want, stirring so the bottom does not scorch. A 10-minute rest at 65 to 70 °C before the boil lets the enzymes work.
And with the assistant?
On brew day, give Krausi the temperature of your mash: he calculates the volume to pull for the next step.
My 20 L mash is at 52 °C, I want to go up to 64 °C by decoction.
Pull 5 L of the thick part and bring it to the boil. Take 5.5 to 6 L instead to make up for heat losses. I'm noting the decoction in the brew.