Same lesson, with the figures in motion.
Forced Diacetyl Test (ready to cold crash?)
A quick test to tell whether the beer has cleaned up diacetyl (butter/butterscotch) and its odourless precursor — before you cold crash. Heating a sample forces the precursor (α-acetolactate) to convert into detectable diacetyl; you then compare against a room-temperature control. The test simulates what would happen in the package if you stopped now. Method and interpretation table from Yeast (Chris White & Jamil Zainasheff).
Materials
- 2 clean glasses
- Aluminium foil
- Water bath — a pot or the pressure cooker used open (not pressurised)
- Thermometer
- Ice bath / cold water bath
- Beer sample (near the end of fermentation, after the diacetyl rest)
Before you start
- Run the test near the end of fermentation — after the diacetyl rest and before cold crashing.
- Use clean glasses — stray aromas confuse the smell test.
The control (room-temp) sample is your reference. It's the DIFFERENCE from the heated sample that reveals the precursor.
Step 1 — Prepare the samples
- Pour equal amounts of beer into two glasses and cover with aluminium foil. Label one HEATED and one CONTROL.
- Leave the control glass at room temperature.
Step 2 — Heat
- Heat the water bath to ~60–70 °C.
- Place the heated glass in the bath so the beer sits below the water level. Hold ~15–20 min (keep the bath in range).
Hot water and glass cause burns. Use the pressure cooker OPEN as a water bath — never pressurise it with beer inside.
Step 3 — Cool
- Put the heated glass in an ice/cold water bath and cool it to the same temperature as the control (~20–24 °C).
Cool to the same temperature before smelling — a warmer sample smells stronger and can fool you into thinking there's diacetyl.
Step 4 — Smell test & interpretation
- Swirl and smell both glasses (taste too, if you like). Diacetyl = butter, butterscotch, movie popcorn.
- Compare with the table and decide your next move:
Control Heated Conclusion Neg. Neg. Done — ready to cold crash. Neg. Pos. Precursor present — more time with the yeast (extend the rest, retest). Pos. Pos. Lots of diacetyl — much more time, or possible contamination (bacteria). - The Diacetyl Rest — read more →