Author: Jake Huolihan

There’s little argument in the brewing world that oxygen is the enemy of beer freshness, but while much of the attention is focused on exposure that occurs after fermentation, some believe vectors for oxidation exist even earlier in the process. In the case of hoppy IPA, there’s been increased focus on the ingress that occurs at dry hopping, a practice that typically involves opening the fermenter to add a charge of hops that have often spent some time in the ambient environment.

Brewers have recently come up with a range of clever tricks for reducing oxygen exposure at the point of dry hopping, some more complicated than others. One rather simple method involves flushing the dry hop charge with CO2 just before adding it to the beer, the idea being that the inert gas will displace most if not all oxygen present in the hops, thus reducing the risk of oxidation down the line.

The first I heard of the idea of flushing dry hops with CO2 prior to adding them to beer was from a prior xBmt by Greg Foster. Given the apparent sensitivity of certain hoppy styles, this seemed like a good way to ensure the beer was exposed to less oxygen. I don’t dry hop very often, so curious of the impact this method might have, I put it to the test on a recent batch of IPA.

| PURPOSE |

To evaluate the differences between an IPA where the dry hop addition was either flushed with CO2 prior to being added to the beer or added without being flushed with CO2.

| METHODS |

For this xBmt, I designed a fairly simple IPA with a moderate dry hop charge.

Riddle Me This

Recipe Details Batch Size Boil Time IBU SRM Est. OG Est. FG ABV 5.5 gal 60 min 134.7 IBUs 8.1 SRM 1.068 1.013 7.4 % Actuals 1.068 1.01 7.7 % Fermentables Name Amount % Pale Malt - 2 Row (Cargill) 12.5 lbs 83.33 Carahell 1 lbs 6.67 Crystal, Medium (Simpsons) 6 oz 2.5 Sugar, Table (Sucrose) 1.125 lbs 7.5 Hops Name Amount Time Use Form Alpha % Magnum 50 g 60 min Boil Pellet 12.2 Citra 30 g 45 min Boil Pellet 12 Mosaic (HBC 369) 30 g 30 min Boil Pellet 12.3 Citra 30 g 0 min Boil Pellet 12 Mosaic (HBC 369) 30 g 0 min Boil Pellet 12.3 HBC 438 22 g 0 min Boil Pellet 13.1 Citra 45 g 7 days Dry Hop Pellet 12 Cryo - Mosaic 45 g 7 days Dry Hop Pellet 22 HBC 438 45 g 7 days Dry Hop Pellet 13.1 Yeast Name Lab Attenuation Temperature Voyager (A05) Imperial Yeast 70% 64°F - 74°F Notes Water Profile: Ca 50 | Mg 0 | Na 8 | SO4 75 | Cl 36 Download Download this recipe's BeerXML file

I started my brew day by collected the full volume of water for an 11 gallon/42 liter batch then heating it up with my heat stick.

I then weighed out and 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 began transferring the sweet wort from the MLT to the kettle, during which 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 racked equal volumes to separate fermenters, running it through my plater chiller along the way.

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

The filled vessels were connected to my glycol rig and left to finish chilling to my desired fermentation temperature of 66°F/19°C, after which I pitch a single pouch of Imperial Yeast A05 Voyager into each.

The beers began showing signs of activity a few hours later and were left to ferment at the set temperature.

After a week of fermentation, signs of activity were beginning to slow, so I took initial hydrometer measurements showing both beers reached the same FG.

When a second set of hydrometer measurements taken 5 days later showing no change, I proceeded with adding the dry hops. For the non-flushed batch, I measured out the dry hops, placed them in a bag, and let sit in the open air for a few minutes before adding them to the beer. For the flushed batch, after placing the same amount of dry hops in a baggie, I flushed it with CO2 for 30 seconds, squeezed all of the air out, then repeated this 3 more times before adding the hops to the beer.

Once the dry hops were added, I purged the fermenter that received the flushed dry hops with CO2 while leaving the other batch alone. In order to allow any impact of the oxygen exposure to occur, I left the beers alone for 12 days before cold crashing and racking to kegs.

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 21 people of varying levels of experience participated in this xBmt. Each participant was served 1 sample of the beer where the dry hops were flushed with CO2 and 2 samples of the beer non-flushed dry hops in different colored opaque cups then asked to identify the unique sample. While 12 tasters (p<0.05) would have had to accurately identify the unique sample in order to reach statistical significance, only 7 (p=0.58) did, indicating participants in this xBmt were unable to reliably distinguish an IPA made with dry hops that were flushed with CO2 prior to being added to the beer from the same IPA where the dry hops were added without being flushed with CO2.

My Impressions: Out of the 5 semi-blind triangle tests I attempted, I happened to guess right 3 times. I perceived the beers as being identical in every respect, neither possessing the telltale signs of cold-side oxidation.

| DISCUSSION |

There’s not a brewer today who doesn’t acknowledge the negative impact oxygen can have on beer, particularly when the exposure occurs on the cold-side. This being the case, it’s understandable brewers would take steps to reduce oxygen exposure, especially when it comes to styles like IPA, as oxidation can turn a bright hoppy beer into a sickly sweet mess in no time. One vector of cold-side oxidation is the dry hopping process, as it typically requires adding hops that have been exposed to the air, hence some have taken to flushing their dry hops with CO2 prior to adding them to the beer. However, the fact tasters in this xBmt were unable to distinguish an IPA where the dry hops were flushed with CO2 from one where the dry hops weren’t flushed suggests the impact may not be as deleterious as some think.

It might assumed that the reason for these results is that the dry hop flushing process did nothing to reduce the amount of oxygen coming into contact with the beer, meaning the lack of a perceptible difference was because both were oxidized. Valid as this may be, neither beer tasted oxidized, in fact both were well received by tasters and maintained a pleasant hoppy character the entire time they were around. An alternative explanation is that the amount of oxygen exposure that occurs when using a standard dry hopping method isn’t enough to cause harm.

The good majority of the beer I brew isn’t dry hopped, so this isn’t a variable I’m terribly worried about. In the instances I do brew IPA, I definitely make an effort keep oxygen exposure to a minimum, from fermenting in sealed vessels to packaging in CO2 purged kegs. Considering my past positive experiences along with these xBmt results, I won’t be purging my dry hop additions with CO2 in the future.

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

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