What is diacetyl and why does it matter?

Diacetyl is a natural fermentation byproduct that tastes like butter or butterscotch. While yeast naturally reabsorbs it, lager yeasts at cold temperatures may not clean it up completely. A diacetyl rest raises the temperature to help yeast metabolise remaining diacetyl.

Do ales need a diacetyl rest?

Most ale yeasts ferment warm enough (18-22°C) to clean up diacetyl naturally. However, some English ale strains and any ale fermented below 16°C may benefit from a brief rest at the end of fermentation.

When exactly should I start the diacetyl rest?

Start when fermentation is about 65-75% complete (roughly 2-5 gravity points above expected FG). Starting too early risks off-flavours from vigorous fermentation at high temperatures. Starting too late means less active yeast to clean up diacetyl.

How fast should I raise the temperature?

Raise at 2-3°C per day to avoid shocking the yeast. Rapid temperature changes can cause the yeast to produce more off-flavours or drop out of suspension prematurely.

Can ales benefit from a diacetyl rest?

Most ale yeasts clean up diacetyl naturally at warmer temperatures. However, if you detect butterscotch flavours in a finished ale, a brief temperature raise at the end of fermentation for 2-3 days can help.

BrewCalcs

Diacetyl Rest Calculator

Calculate timing and temperature for a diacetyl rest in lagers and ales.

Fermentation Temperature

Temperature for this process

Original Gravity (OG)

Gravity reading before fermentation

Current Gravity

Your latest hydrometer reading

Expected Final Gravity (FG)

Gravity reading after fermentation

Yeast Type

Yeast strain or type

How We Calculate This

Diacetyl Rest Timing

The rest should begin when approximately 65-75% of total attenuation is reached. This is calculated as: Start Gravity = FG + (OG - FG) x 0.30. At this point, enough yeast is still active to metabolise diacetyl efficiently.

Temperature and Duration

For lagers, raise to at least 18°C (typically 5°C above fermentation temperature). Hold for 2-3 days. Temperature should be increased gradually at about 2°C per day to avoid yeast stress.

Frequently Asked Questions

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Last updated: 2025-02-20

All calculations are estimates. Always verify values and adjust based on your equipment and ingredients.