Author: Jake Huolihan

The fermentation process produces numerous compounds that can lend undesirable flavors and aromas to finished beer. The good news is that most of these compounds are naturally scrubbed out by yeast during a healthy fermentation. However, when fermentation goes awry, these compounds are what lead to common off-flavors, one of the most-cited being diacetyl (aka C4H6O2), a vicinal diketone that is said to impart an intense buttery flavor to beer.

Toward the end of the fermentation process, particularly with lagers and other cold-fermented styles, brewers will often raise the temperature to increase yeast activity, the goal being to encourage clean-up of diacetyl and other off-flavors. The typical advice is to raise the temperature of the fermenting beer to around 68°F/20°C and hold it there for a few days before proceeding.

Due in large part to my interest in quicker turnarounds, I routinely raise the temperature of my beers a few days into fermentation, and I chalk up he fact I’ve never had an issue with diacetyl to this practice. Curious as to what impact, if any, raising the temperature for a diacetyl rest has, I decided to test it out for myself.

| PURPOSE |

To evaluate the differences between beers that either went through a diacetyl rest at the end of fermentation or not.

| METHODS |

Seeing as English ale is often associated with diacetyl, I brewed a British Brown Ale for this xBmt and opted for a slightly lower than recommended fermentation temperature.

Churn

Recipe Details Batch Size Boil Time IBU SRM Est. OG Est. FG ABV 5.5 gal 60 min 30.2 IBUs 19.9 SRM 1.049 1.015 4.5 % Actuals 1.049 1.02 3.8 % Fermentables Name Amount % IdaPils (Cargill) 4.75 lbs 41.99 Pale Ale Malt (Rahr) 4.75 lbs 41.99 Brown Malt 1.5 lbs 13.26 Chocolate Malt 5 oz 2.76 Hops Name Amount Time Use Form Alpha % Hallertau Magnum 8 g 60 min Boil Pellet 14 Pekko 11 g 30 min Boil Pellet 15 Yeast Name Lab Attenuation Temperature Pub (A09) Imperial Yeast 72% 64°F - 70°F Notes Water Profile: Ca 91 | Mg 0 | Na 8 | SO4 50 | Cl 100 Download Download this recipe's BeerXML file

The night before brewing, I weighed out the grains and collected the water in my kettle, adjusting it to my desired profile. After setting my controller to heat the water first thing the next morning, I milled the grain.

With the water appropriately heated, I stirred in the grist then checked to ensure it was at my target mash temperature.

The mash was left to rest for 60 minutes with intermittent stirring.

With the mash rest complete, I transferred the sweet wort from the MLT to the kettle.

While lautering, I weighed out the kettle hop additions.

The wort was then boiled for 60 minutes with hops added as stated in the recipe.

Once the boil was complete, I quickly chilled the wort.

A refractometer reading showed the wort was right at my planned OG.

Identical volumes of wort were racked to separate sanitized Brew Buckets.

The filled vessels were connected to my glycol rig and left to finish chilling to my desired fermentation temperature of 60°F/16°C, which took about 15 minutes. At this point, I split a starter of Imperial Yeast A09 Pub evenly between each batch.

With fermentation activity starting slow after 3 days, I ramped the temperature of one batch to 68°F/20°C for a diacetyl rest while the other remained at 60°F/16°C. Signs of fermentation were absent in both beers 2 days later, so I took hydrometer measurements indicating each beer hit the same FG.

With a second set of hydrometer measurements taken 2 days later showing no change, I proceeded with packaging.

The filled kegs were placed in my keezer and burst carbonated before I reduced the gas to serving pressure. After a couple weeks of conditioning while I was traveling for work, the beers were carbonated and ready to serve to participants.

| RESULTS |

A total of 19 people of varying levels of experience participated in this xBmt. Each participant was served 1 sample of the beer that did not experience a diacetyl rest and 2 samples of the beer that did receive a diacetyl rest in different colored opaque cups then asked to identify the unique sample. While 10 tasters (p<0.05) would have had to accurately identify the unique sample in order to reach statistical significance, only 7 (p=0.46) did, indicating participants in this xBmt were unable to reliably distinguish a British Brown Ale that went through a diacetyl rest from one that did not go through this process.

My Impressions: I attempted 3 semi-blind triangle tests and was forced to guess every time, as the beers tasted identical to me. While I was unable to identify the odd-beer-out, I was pleased with the outcome of the beers!

| DISCUSSION |

Diacetyl is real, and there’s a known threshold where most people are able to perceive it. It’s highly likely that anyone who’s ticked a lot of commercial beer or drank a wide variety of homebrew has come across buttery diacetyl a time or two. One of the most common methods used by brewers for reducing the risk of diacetyl involves raising the temperature of the beer toward the end of fermentation. However, when served samples of British Brown Ale made with and without a diacetyl rest, tasters in this xBmt were unable to reliably tell them apart, suggesting the method had minimal impact on this style.

It’s possible the beer whose temperature was not raised during fermentation did have more diacetyl, which that being the case, would suggest the level was under the perceptible threshold. And despite Imperial’s warning that a diacetyl rest is strongly recommended with A09 Pub yeast, other strains may very well produce higher amounts that that a diacetyl rest could help reduce.

There are a number of reasons brewers might choose to raise the temperature of their beer toward the end of fermentation, and while this particular xBmt didn’t seem to show it had the expected impact on diacetyl, I have no plans to cease my use of the practice. In addition to being easy insurance in the case diacetyl does happen to be present, it also allows me to turn around beer faster, which I’m definitely a fan of.

If you have any thoughts about this xBmt, please do not hesitate to share in the comments section below!

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