Brewer's Ed

Controlling Diacetyl with ALDC and Yeast Selection

A rundown on an enzyme that can help keep your beer clean.

By Shana Solarte

May 8, 2023

ALDC (alpha-acetolactate decarboxylase) is an enzyme that breaks down alpha-acetolactate, the precursor for diacetyl within the yeast cell and converts it into acetoin, a flavorless compound. You may be familiar with ALDC, as many brewers use it at the start of fermentation to help avoid the formation of diacetyl, which saves on tank time. However, ALDC is pH-sensitive and can be less effective as the pH level drops during fermentation (typically below 5), meaning that it might not be super helpful in the case of hop creep. ALDC is a proactive, rather than reactive, solution; it won’t help you clean up diacetyl once it’s already in the beer.

Aldc 2

ALDC in cell converts α‑acetolactate directly to acetoin; without ALDC α‑acetolactate is excreted and becomes diacetyl OR with exogenous ALDC, it becomes acetoin in the beer.

There is existing research on having ALDC expressed within the yeast cell. When ALDC is expressed in the cell it converts alpha-acetolactate to acetoin directly within the cell, therefore preventing alpha-acetolactate from building up and being excreted into the beer where it will turn into diacetyl. Another bonus to ALDC activity within the cell is that it will continue working even during later stages of fermentation, which can help mitigate hop creep.

Why use ALDC strains as opposed to adding the enzyme? Some brewers may see ALDC as lazy” brewing, but we see it as an added layer of quality assurance with the bonus of time and cost savings. Even with great yeast management, having ALDC expressed in the cell increases efficiency in the brewery by reducing tank time. Without the need for a lengthy diacetyl rest, your beer can be turned around faster with more confidence that diacetyl won’t be delaying your production schedule. Plus, breweries can save additional costs by not buying exogenous ALDC because the enzyme will be expressed in every batch, even when repitching. 

Should every strain receive the ALDC treatment? In short, no. Some strains are less prone to hop creep-induced diacetyl formation, like British V (OYL-011), but even these strains can benefit from ALDC in heavily hopped beers. There are a handful of strains that accumulate more diacetyl at the end of fermentation and are better candidates for this modification. Again, diacetyl can be avoided and kept to a bare minimum with healthy yeast and properly managed fermentation, but by including this ALDC activity at a cellular level brewers can have more predictable outcomes and save time on troubleshooting and problem solving.

Is diacetyl EVER okay? Here’s the thing: diacetyl can be pretty controversial! Some brewers have an incredibly low tolerance for diacetyl, going to great pains to remove every possible trace of it from their beer. Others don’t mind it so much, and may even prefer a touch of it in certain styles. Some traditional Czech lagers and English ales will have a low level of diacetyl, such that most drinkers wouldn’t necessarily consider the beers to be buttery, but having a pleasantly round mouthfeel and a hint of sweetness. Brewing these styles with zero tolerance for diacetyl would effectively change their flavor profiles and make them taste different from the classics.

Sometimes the presence of diacetyl can indicate bigger issues. Apart from stressed yeast or mismanaged fermentation, bacterial contamination is often another source of diacetyl. This can occur in the brewery when equipment like hoses, clamps, and even kegs are not being sanitized properly. Another common source of diacetyl is from dirty draft lines. Though ALDC-expressing yeast will not solve diacetyl issues that result from dirty draft lines (Pediococcus and/or Lactobacillus infections), these can be avoided with proper QC and draft line cleaning.

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