Q. Should I put my beer in the kegerator right after it reaches terminal gravity or let it sit at room temperature for a while? If the latter, how long should I condition it for before cold crashing?
Hank Drew
Manchester, New Hampshire

Mr. Wizard says . . .

I spend a fair amount of time browsing homebrewing communities on social media. If you asked this question in one of those groups, a common response would be, “Yeah, dude, cold crash the keg, carb it, and drink it!” I can live with that answer, but a more accurate, even if less satisfying, answer is: It depends. Let’s peel back a few layers of this onion and see what factors matter.

Whether a beer benefits from additional time after reaching terminal gravity depends on several things. In my view, the most important are yeast strain, beer style, and the presence or absence of acetaldehyde and diacetyl. If you’re brewing a standard-gravity ale with a clean-fermenting yeast, such as one of the many Chico strains, and the beer is not dry hopped, anything beyond a short rest — about two days — is probably a waste of time.

That answer comes with two important caveats. First, if the beer had an original gravity above about 1.060 (15 °Plato), I want to know exactly what is meant by terminal gravity. Does it mean the gravity matches the recipe? Or does it mean you’ve measured the same gravity twice over a three- to four-day period? And if the gravity is stable, is it where you expected it to finish? Bigger beers often need a few extra days because fermentation and maturation simply take longer as original gravity increases.

The second caveat concerns dry hopping. When hops are added late in fermentation, hop amylases can break down dextrins into fermentable sugars. This so-called hop creep extends fermentation while increasing the likelihood of a diacetyl spike. The best way to avoid problems is to delay cooling until fermentation is truly complete and diacetyl has been reduced below its flavor threshold. I’ll point readers to the link at the end of this article to read more about hop creep and resist diving down that rabbit hole here.

Lagers deserve special mention. If you’re using a strain known for leaving buttery fingerprints, extra conditioning time is usually worthwhile because diacetyl reduction often lags behind the point where gravity stabilizes. The same is true for acetaldehyde. Near the end of fermentation, yeast reduce diacetyl and acetaldehyde using NADH as the reducing agent, regenerating NAD⁺ in the process.

In practice, you want to verify two things before cooling: That the beer has truly reached terminal gravity and that it is free of detectable diacetyl and acetaldehyde. Diacetyl is easily checked using a forcing test. Heat a sealed beer sample in a 140 °F (60 °C) water bath for 15–20 minutes, cool it to the same temperature as an unheated control sample, and compare the aromas. If the heated sample develops a buttery aroma, the beer needs more time before cooling. Stable gravity and a negative diacetyl forcing test are the two most useful process checks for deciding when to crash cool.

Too many brewers — including pro brewers — become frustrated because of a common logical fallacy. They insist that two batches were brewed the same way with the same ingredients, yet one batch developed a problem. The difficult part is usually identifying what actually changed so the problem can be prevented next time. In the case of post-fermentation conditioning, there are simply too many variables to control perfectly. The practical solution is to rely on objective process checks rather than the calendar.

I’m an old-school homebrewer and still prefer fermenting in a glass carboy because I can see what’s happening inside the fermenter. Before anyone asks, I ferment in a dark basement illuminated by LED lighting, so I’m not worried about light-struck beer.

I like carboys because I can watch yeast movement during and after fermentation, and that visual feedback is surprisingly useful. Since I’m rarely in a hurry, I follow what’s happening, pull samples to smell, taste, and measure, and then rack to a keg. Before transferring, I fill the keg with Star San and push it out with carbon dioxide, so I have no concerns about oxidation during racking. Depending on the beer style, I’ll often transfer the beer from one keg to a second before serving to leave the settled yeast behind. I’m not worried about yeast autolysis in the types of beers I brew, but I do like minimizing the amount of yeast that ends up in the glass.

Further Reading …

Large dry-hop additions that have become more prevalent over the past decade have led to surprises among commercial brewers and homebrewers alike. The phenomenon — leading to a lower final gravity, increased ethanol, and increased carbonation — is called hop creep and is due to diastase activity of hops. Learn more about hop creep and how to combat it.