What is a diacetyl rest when making lager beer?

The diacetyl rest is a crucial phase in the beer brewing process, particularly for brewers seeking to perfect the flavor profile of their lagers and, in some instances, ales. Diacetyl, a vicinal diketone (VDK), imparts a buttery or butterscotch flavor to beer, which, while desirable in some beer styles, is generally considered a flaw when present in excessive amounts.

Understanding the science behind diacetyl rest involves delving into the biochemistry of yeast metabolism during fermentation and how specific conditions can influence the reduction of unwanted diacetyl.

To understand diacetyl rest, it's essential to start with the yeast metabolism during fermentation. Yeast, primarily Saccharomyces cerevisiae in ale brewing and Saccharomyces pastorianus in lager brewing, metabolizes sugars extracted from malted grains into ethanol, carbon dioxide, and various by-products, one of which is diacetyl.

The formation of diacetyl occurs in two main steps:

Formation of α-acetolactate: During the initial stages of fermentation, yeast converts sugars into pyruvate via glycolysis. Pyruvate is then transformed into α-acetolactate by the enzyme acetolactate synthase (ALS). This reaction can be represented as follows:

1. $\text{Pyruvate}+\text{Pyruvate}\stackrel{\text{ALS}}{\to }�-\text{acetolactate}+{\text{CO}}_2$

Oxidative Decarboxylation of α-acetolactate: Once formed, α-acetolactate can spontaneously decarboxylate, especially in the presence of oxygen, to form diacetyl. However, this step does not require the action of yeast enzymes and can occur both inside and outside yeast cells. The chemical reaction is as follows:

2. $$�−acetolactate→decarboxylationDiacetyl+CO2

The Diacetyl Rest Process

Diacetyl rest involves temporarily raising the temperature of the fermenting beer towards the end of fermentation. For lagers, this often means increasing the temperature from around 10°C (50°F) to 15-20°C (59-68°F) for a period of 24-48 hours or until diacetyl levels drop below the taste threshold. 

The purpose of this temperature increase is twofold:

Enhanced Yeast Activity

The elevated temperature boosts yeast metabolism, encouraging the cells to absorb and reduce diacetyl more efficiently. Yeast can convert diacetyl into less-flavor-active compounds like butanediol through a process known as reductive assimilation, following the reaction:

1. $\text{Diacetyl}+\text{NADH}+{\text{H}}^{+}\stackrel{\text{yeast enzymes}}{\to }\text{Butanediol}+{\text{NAD}}^{+}$

2. 
   

Acceleration of Diacetyl Reduction
Higher temperatures not only increase yeast activity but also accelerate the rate at which diacetyl is reduced to butanediol. The increased temperature reduces the time required for diacetyl to fall below sensory thresholds, thus shortening the overall fermentation time.

Significance of Diacetyl Rest

Implementing a diacetyl rest is particularly critical in lager brewing due to the lower fermentation temperatures, which can result in slower yeast metabolism and prolonged diacetyl reduction times. Without this rest, beers might retain a noticeable diacetyl character, detracting from the desired flavor profile. 

Although some ale styles are fermented at higher temperatures, which naturally encourage faster diacetyl reduction, a diacetyl rest can still be beneficial for high-gravity ales or strains with less aggressive diacetyl reduction capabilities.

Here's a step-by-step guide for homebrewers to effectively implement a diacetyl rest with the wort in a fermenting drum:

1. Monitor Fermentation Progress

• Primary Fermentation: Begin by allowing your beer to ferment at the recommended temperature for your specific yeast strain. For ales, this is typically around 18-22°C (64-72°F), and for lagers, around 8-12°C (46-54°F).
• Gravity Check: As primary fermentation nears completion (usually after 5-7 days for ales and 1-2 weeks for lagers), start taking daily gravity readings with a hydrometer or refractometer. You're approaching the right time for a diacetyl rest when the gravity is within a few points of the expected final gravity, indicating that the bulk of fermentation is complete, but yeast activity is still present.

2. Raise the Temperature

• Increase Gradually: Once you're close to the final gravity, gradually increase the temperature of the fermenting beer to the diacetyl rest range. For ales, you might already be in the appropriate temperature range, but for lagers, increase the temperature to between 15-20°C (59-68°F).
• Method: If your fermenting drum is in a temperature-controlled environment, adjust the thermostat accordingly. If not, you may need to move the drum to a warmer room or use a fermentation heating belt or wrap to gently increase the temperature.

Or use old sheets, works a treat!

3. Maintain Temperature

• Rest Period: Keep the beer at the diacetyl rest temperature for 24-48 hours. This period allows the yeast to metabolize any remaining diacetyl and related compounds, reducing them to flavor-neutral substances.
• No Stirring Needed: Unlike some wine-making processes, you do not need to stir the beer during the diacetyl rest. Disturbing the beer could introduce oxygen, leading to oxidation and off-flavors.

4. Test for Diacetyl

• Sensory Test: After the rest period, perform a sensory test to check for the presence of diacetyl. This can be as simple as tasting the beer to detect any buttery or butterscotch flavors. For a more sensitive test, take a small sample of the beer and warm it in a sealed container (like a microwave-safe bottle) in a warm water bath for 10-15 minutes, then cool it quickly and taste. Warming the sample can make diacetyl more perceptible.
• Professional Testing: For those seeking a more precise analysis, some brewing supply shops or labs offer diacetyl testing services.

5. Proceed to Cooling

• If Diacetyl is Absent or Below Threshold: Once the diacetyl is no longer perceptible or at acceptable levels, you can proceed to cool the beer down to lagering temperatures for lagers (typically around 0-4°C or 32-39°F) or to packaging temperatures for ales.
• Extended Rest if Necessary: If diacetyl is still present, you may extend the diacetyl rest for another 24-48 hours before testing again.

6. Lagering or Packaging

• Lagering: For lagers, after the diacetyl rest and subsequent cooling, you may lager the beer at low temperatures for several weeks to improve clarity and flavor stability.
• Packaging: For both ales and lagers, once diacetyl is managed, and the beer is conditioned, proceed to bottle or keg your beer as usual.

Implementing a diacetyl rest is a proactive approach to ensuring the quality and flavor profile of your homebrew. By closely monitoring fermentation progress and adjusting temperatures as needed, homebrewers can effectively reduce unwanted diacetyl, resulting in a cleaner, more enjoyable beer.