Brewer's Ed

How Does ALDC Stop Diacetyl from Forming?

Brewers are using ALDC to control diacetyl, sometimes intracellularly. Why? How does it work?

By Danielle Sommer

Jan 10, 2025

How Does Diacetyl Form?

Yeast uses compounds from its food to create α‑acetolactate, which is necessary for amino acid synthesis. However, yeast happens to make it at a greater rate than it can use for that purpose. As a result, excess diffuses into wort. Once it’s there, it reacts with oxygen to become diacetyl. The spontaneous oxidative decarboxylation” of α‑acetolactate turns it into what’s called a ketone. Diacetyl is one of two vicinal diketones” (VDKs) that brewers pay close attention to.

When yeast clears diacetyl

Yeast has a process of its own for clearing up diacetyl, without ALDC:

  • Yeast imports and breaks down diacetyl as it comes in contact with it in wort. 
  • Yeast breaks diacetyl down using a naturally occurring yeast enzyme called diacetyl reductase.
  • Yeast performs diacetyl clean up throughout the entirety of fermentation, but yeast’s success in cleaning up more than is being formed varies when yeast’s rate of excreting excess α‑acetolactate and/or diacetyl’s rate of forming changes during different stages of fermentation. This is what results in highs and lows in overall concentrations of diacetyl. 
  • Not only is yeast’s process for cleaning up diacetyl slow, but it is sometimes also not effective at reducing diacetyl levels to entirely below sensory threshold.

The diacetyl rest

Yeast cells’ function slows down when fermentable sugars are running out at the end of fermentation, so their rate of diacetyl clean up slows, too. That means there’s a point toward the end of fermentation when new diacetyl is still forming because of lingering α‑acetolactate that is just in the process of converting. Enter the diacetyl rest.

the diacetyl rest is a brief period of raised temperature that brewers initiate to give yeast a little boost at the end of fermentation. The higher temps speed up the last of yeast’s metabolic processes and also speed up the conversion of any remaining α‑acetolactate in wort into diacetyl, ensuring all of it can be cleaned up before the yeast packs up and goes, and that no more of it will turn into diacetyl in-package. Remember, only yeast can clear diacetyl once it exists. And once yeast drops out, whatever diacetyl stays behind is there for the long haul.

Disrupting Diacetyl

ALDC is an acronym for α‑acetolactate decarboxylase. Its function is encoded in its name. ALDC breaks α‑acetolactate down into acetoin before it can become diacetyl, thereby preventing diacetyl from ever forming. Unlike diacetyl, it would take a whole lot of acetoin to make any impact on a beer’s flavor and aroma. So, ALDC’s elimination of excess α‑acetolactate by converting it into acetoin effectively neutralizes the potential for that buttery off flavor to develop.

Because ALDC goes to work on α‑acetolactate and not diacetyl itself, it’s crucial that ALDC is present in wort in effective concentrations before any of excess α‑acetolactate turns into diacetyl; and it needs to stay present in effective concentrations while any α‑acetolactate is in wort. If any α‑acetolactate becomes diacetyl, ALDC is no longer the solution. 

If a brewer is using yeast that is enabled to produce ALDC, a strain with what Omega Yeast calls Diacetyl Knock Out (DKO) technology, α‑acetolactate is converted to acetoin before it even leaves the cell. That makes DKO more comprehensive and complete in cases where manually-added ALDC could be challenged by wort factors, like maybe pH considerations or an application error.

Can I Skip the Diacetyl Rest?

Does ALDC save time on the diacetyl rest?

Though ALDC prevents diacetyl formation, counterintuitively for some, it does not necessarily eliminate the need for a diacetyl rest. Hm, Why?

There are multiple things happening during post-fermentation maturation. In addition to diacetyl clearance, there’s acetaldehyde clearance, sulfur clearance, and yeast dropping out. ALDC addresses one of those factors. So, whether you’re saving time or not by using ALDC really depends on which one of those factors is your bottleneck. If it’s diacetyl, then you may be saving time — but it would still make sense to raise temperatures to allow for maturation in the broader sense.

An Experiment

Let’s look at some data

Take a look at this stuff in practice in a lager brew trial.

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