Ask Mr. Wizard

The Importance Of A Diacetyl Rest


Donnie Maheu — Baton Rouge, Louisiana asks,

I am trying to determine the necessity and benefit of a diacetyl rest in lager fermentations. I have recipes from three different homebrew suppliers; one doesn’t suggest a rest, one recommends a few days, and another recommends one week. I am brewing small (2–2.5 gallons/7.6–9.5 L) at a time. Can you shed some light on this subject?


Diacetyl rests or colloquially known as d-rests, whether brewing lagers or ales, are good insurance policies to help ward off diacetyl. Many recipes focus on wort production and provide little in the way of specific guidance when it comes to fermentation and aging. This is especially true when it comes to nuanced methods like addressing “d.” Let’s take a dip in the fermenter and look at the diacetyl story.During metabolism, yeasts are busy converting sugars and amino acids into building blocks for new yeast cells and that cellular fuel called ATP (adenosine triphophate). Although fermentation (conversion of glucose into carbon dioxide, ethanol, and 2 molecules of ATP) is far less efficient than respiration (conversion of glucose into carbon dioxide, water, and 38 molecules of ATP), yeast cells generate sufficient energy through fermentation to build healthy and dense cell populations. Yeast cells synthesize all amino acids required for cell growth and metabolism, and along the way some of the intermediates related to amino acid synthesis escape the cell.

Diacetyl is a rather simple molecule that can provide a very distinct aroma to beer.

One of those compounds is called alpha acetolactate, a diacetyl precursor that is related to valine synthesis and valine metabolism. Skipping the biochemical details, alpha acetolactate is secreted into beer where it converts into diacetyl. Yeast can then reabsorb diacetyl, convert it into flavor-neutral compounds (butane diol and acetoin), and all is good. So why do some folks make a big deal about the diacetyl rest?

The key detail in this is the conversion of alpha acetolactate into diacetyl. The speed of this reaction varies with temperature and is accelerated by oxidants, such as oxygen and metal ions like iron. Consider a fresh bottle of beer containing alpha acetolactate. It’s a given that this diacetyl precursor is going to turn into diacetyl, but what’s not a given is how quickly the conversion will occur. If iron was picked up during beer filtration or a good gulp of air was sucked into beer during packaging, diacetyl may show up quickly. But if the beer is not filtered or bottled, a keg of cold beer containing diacetyl precursor may slowly blossom into a butter bomb over several weeks of storage. Heavily dry-hopped beers may also undergo a diacetyl spike associated with hop creep because yeast begin snacking on carbs, start producing valine, and secrete alpha acetolactate during hop creep. The takeaway is that there are several ways for diacetyl to show up in packaged beers and some paths are faster than others.

If you are cold fermenting lagers, not adding big doses of dry hops, and conduct a diacetyl rest by moving your fermenter to a room temperature environment (68 °F/20 °C) after peak fermentation is complete (usually after day five), two days is plenty. On the other hand, if you prefer to keep things simple by fermenting cold and conducting your “d-rest” cold, adding a week after primary is a good idea. And no d-rest at all is really rolling the dice. My suggestion is to use a d-rest for all styles if possible.

Response by Ashton Lewis.