Putting ALDC to the Test: Lager Edition

Many brewers now opt to use ALDC to help control diacetyl in beer. It can either be added manually or it can be delivered via yeast that has been enabled to make it. Though brewers are capable of optimizing processes and ingredients for reducing diacetyl without an ALDC assist, there are circumstances where using it could have a strong advantage. Like when brewing with strains that are known to be high diacetyl producers, to safeguard lagers and other styles that reveal diacetyl-related faults more easily, and in heavily dry-hopped styles (see diacetyl and dry hop creep). ALDC can often reduce diacetyl more thoroughly simply because of how it works.

Yeast already has a mechanism for cleaning up diacetyl in beer. So, what exactly does ALDC do differently? And how effective is it? We designed a brewing experiment to show diacetyl formation with and without ALDC to review those concepts — a timely, seasonal lager seemed like the ideal vehicle.

Using gas chromatography (GC), we measured the amounts of diacetyl that accumulated in three versions of the same Festbier. It was brewed from a single batch of wort split three ways and each fermented with a version of German Lager I. The variable was whether there was ALDC enzyme (and how it was delivered). One fermentation was entirely without ALDC, one used exogenous ALDC (enzyme added manually) at the start of fermentation, and the third used a version of German Lager I that is enabled to produce ALDC on its own, called German Lager I DKO. The resulting data illustrates how typical diacetyl levels are affected by the use of ALDC.

Before we go over the graph in detail, let’s review how diacetyl forms, to understand what exactly ALDC is (and isn’t) doing in the process. The ALDC enzyme works differently from yeast’s own processes for clearing diacetyl. It targets excess alpha-acetolactate before it becomes diacetyl.

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

ALDC stands for α‑acetolactate decarboxylase. Its function is encoded in its name: an enzyme (“ase”) that targets α‑acetolactate and breaks it down via its carboxyl group. ALDC breaks down α‑acetolactate to acetoin before it can become diacetyl, thereby preventing diacetyl from ever forming.

Acetoin has a much, much higher perception threshold than diacetyl. That means there would have to be a whole lot more of it present than there is in beer to have an impact on flavor and aroma. So, ALDC’s elimination of excess α‑acetolactate and its conversion into acetoin effectively neutralizes the negative impact of diacetyl.

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 naturally oxidizes into diacetyl. And it needs to stay present in effective concentrations throughout fermentation. 

Once any α‑acetolactate becomes diacetyl, ALDC is no longer an effective solution. Therefore, if brewers are adding ALDC manually, timing and dose rates are very important. 

Different manufacturers have slightly different usage recommendations. Differences are partly due to how the ALDC was ​“farmed.”

Yeast has a process of its own for clearing up diacetyl, without ALDC. Perhaps confusingly, it also results in acetoin, but through a totally different route; yeast imports and breaks down diacetyl as it comes in contact with it in wort. It breaks it down using a naturally occuring 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 its rate 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 this process slow, it is sometimes ineffective at reducing diacetyl levels to entirely below sensory threshold.

Yeast cells’ function slow 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 to diacetyl (there’s a rate to that, too). 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.

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

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.

Now that you’ve got a firm understanding of what’s what. Let’s take a look at how it shows up in our data.

Brewers traditionally have used techniques like a diacetyl rest to control diacetyl at the end of fermentation. In the graph above, the orange line represents the diacetyl present in a simple German Lager I fermentation. It serves as a baseline of how diacetyl typically accumulates — and is cleared — without the help of the ALDC enzyme. 

Early on in fermentation, generally off-flavors are forming and yeast is very actively clearing them up. Yeast cells are producing aroma compounds like esters as part of their metabolism, and they are also making α‑acetolactate.

In our experiment, from day zero to around day four (a.), the rate of α‑acetolactate production by yeast cells exceeds its rate of use, and the excess is becoming diacetyl in the wort. We can see that the rate of the formation of diacetyl is also pretty quickly exceeding yeast’s rate of clearance, and diacetyl concentrations are cumulatively rising.

Mid fermentation (a.-c.), something begins to change. We see yeast’s rate of clearance exceed the rate of diacetyl formation for the first time (a.). After a sort of plateau (a.-b.), diacetyl concentrations then begin falling in earnest (b.-c.).

The dramatic success in clearing diacetyl here is probably because α‑acetolacate production by yeast cells is slowing, and there is therefore less precursor available to become diacetyl. That means diacetyl’s rate of formation is slowing at the same time yeast continues to be pretty effective at clearing it. 

Nearer the end of fermentation (c.-d.), however, there’s a rise in accumulation of diacetyl compared to yeast’s effort to clear it. This is in part because diacetyl is still in the process of forming from the remaining α‑acetolactate in wort, even though yeast has slowed in producing new α‑acetolactate; but now yeast’s rate of clearing has begun to slow, too.

Finally, increasing temperature during the diacetyl rest (c.) helps all remaining α‑acetolactate convert to diacetyl more quickly, raising diacetyl concentrations briefly (d.) and with that, also gives yeast a little extra oomph to clear it up, too. 

By day 14, diacetyl levels have tapered off to very low levels, though not entirely below threshold.

The dotted line represents the sensory threshold for diacetyl. Below that are lines representing the two ALDC-enzyme assisted fermentations. The aqua-color represents diacetyl concentrations in the fermentation where ALDC was added manually, while the purple-color shows diacetyl levels in the fermentation using the ALDC-expressing version of the strain (German Lager I DKO).

With both ALDC-assisted fermentations you can see that diacetyl formation never accumulates at all. The amounts that you do see represented in each line are more reflective of instrumental limits of detection, rather than perceivable quantities. That means diacetyl may be present, but at levels so low that they can only be detected by the GC.

A trained sensory panelist generally can begin to detect diacetyl in a pale ale starting around 40 – 50 ppb. That’s where we placed our threshold line. But in a lager, detection might be as low as 20 ppb. Here you can see that diacetyl levels are still above sensory threshold at day 14 in the fermentation lacking ALDC. Additional time would be needed to clear diacetyl in that fermentation.

ALDC or No?

Brewers can control diacetyl effectively with common brewing methods and processes developed for that purpose. ALDC is a tool that helps brewers control diacetyl by eliminating the precursor that would become diacetyl, and as we can see, it’s certainly an effective one. Whether used as a general safeguard, or just for particularly problematic beers is up to the brewer. Understanding how both diacetyl and ALDC work is very helpful for brewers looking to assess under what conditions they could use ALDC for the most benefit. Relying on yeast’s natural processes for clearing diacetyl can be less effective than using ALDC, especially for problematic fermentations, like when non-ideal re-pitching practices compromise yeast health, or when colder temperatures challenge yeast activity.

Add it manually or use yeast that make it?

When it comes to using ALDC, adding exogenous ALDC, and using ALDC via an ALDC-enabled yeast strain, both can be more effective than yeast’s naturally occurring method. However, using an ALDC-producing yeast strain has additional practical and functional advantages which makes it more likely to result in consistent success: 

It eliminates user-error pitfalls with timing, dosing, and other SOP considerations — like extra steps, and additional vulnerabilities for contamination. 

It stays in the cell, rather than diffusing throughout the wort, which makes it more resilient to inhibition in dry hopped beers and pH changes. Manufacturers of ALDC list optimal pH ranges starting somewhere around 4.0 – 5.0 and up to 7.0. Beer pH can occasionally dip below that, and when pH is too low, ALDC is less effective.

And an ALDC-producing strain can surprisingly also rival or exceed exogenous ALDC in cost savings when yeast is harvested and repitched (enzyme expression is stable within yeast across generations). 

ALDC-enabled yeast’s greatest limitation is that not all brewers can use it if they want to, since it is genetically engineered.

Stay tuned for the next experimental edition: how diacetyl forms in an ale fermentation that undergoes heavy dry hopping, and how ALDC affects fermentation.