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

Original Gravity (OG)

Current Gravity

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Expected Final Gravity (FG)

Yeast 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.