by Kido Hirotaka

All of you fine readers are probably holding in one hand a glass with your favorite beer.

Go ahead: give it a sniff. Close your eyes, draw close and inhale the aroma. What’s it like? Do you detect the malt aroma? The fresh hop? The fruity esters? The yeast and alcohol? Probably the sublime mix of them all stimulates your sense of smell and makes you happy, doesn’t it? Diacetyl, the subject of this column, may be a part of that aroma. Let’s talk about the chemical, shall we?

Diacetyl is so subtle in well-made beer that most people can’t detect it. It exists to some extent in all beers, but when the brewing process goes awry, it can impose itself so badly that the beer loses all balance of flavor. This is of course the case with any compound in beer. If subtle, it can be recognized for the flavor characteristic it imparts, but when too much, it can literally leave a bad taste in your mouth.

Diacetyl is a natural product born of the fermentation process from yeast and lactobacillus. In certain fermentable food products like butter, bread or yogurt, it acts as an aromatic agent that creates character. People often describe the aroma as sweet like butterscotch or, in some cases, pungent like pickles or beans.

Diacetyl often accompanies astringency, coating the tongue with a thick sweetness that lingers too long. Diacetyl emerges early in the fermentation process as the yeast activates, but with continued fermentation and then rest, the yeast reabsorbs the diacetyl that it produced, returning to an odorless, tasteless state. This yeast and the amount of diacetyl, then, generally determine the length of the rest. In making delicious beer, it’s very important that the proper temperature is maintained to the very end of the fermentation process so that the yeast can do its work and lower diacetyl levels.

However, some breweries, worried about contamination, lower the temperature of the young beer too soon toward the end. Sure, that can fend off bacterial infections, but it deactivates the yeast. The whole point of active yeast at this stage is to soak up all the diacetyl. With such high levels of the stuff left in the beer, you inevitably get that buttery sweet flavor.

There are some acceptable amounts of diacetyl depending on the style of the beer. In English-style ales, for example, slight diacetyl imparts subtle flavor to the balance, but in well-made German pilsners and European amber lagers, it is undetectable. Either way, if there’s too much, the malt, hop and ester flavors will be overwhelmed and you can’t call that good beer, can you? I know this is difficult stuff we’re talking about, but you get it, right?

In the end, beers made by brewers who understand the qualities of their yeast achieve the proper diacetyl levels for the intended style. Off-flavors are absent, and the other ingredients and yeast, as well as aromas and flavors from additional ingredients, work to create good balance and delicious beer. Isn’t that just like the beer you’re drinking now?

Kido Hirotaka is the cellarman at Popeye in Ryogoku, Tokyo

This article was published in Japan Beer Times # () and is among the limited content available online. Order your copy through our online shop or download the digital version from the iTunes store to access the full contents of this issue.