Author: Matt Del Fiacco

Aeration of chilled wort with pure oxygen is commonly practiced by brewers as a means of optimizing the fermentation, as yeast use the oxygen for the synthesis of sterols and unsaturated fatty acids, which in turn facilitates cell memberane health and reproduction. Often contended as an important step for all beers, many brewers view wort aeration as critical to avoiding problematic fermentations and off-flavor development in the production of higher OG styles.

In their book Yeast: The Practical Guide To Beer Fermentation, White and Zainasheff note that for beers with a strength of 1.083 OG or higher, a second dose of oxygen after 12 to 18 hours may result not only in faster fermentation and higher attenuation, but reduced risk of diacetyl and acetaldehyde. Purportedly, the yeast cells undergo at least one cell division during the time between oxygen doses.

Considering the fact higher OG worts put more osmotic pressure on yeast and have lower oxygen solubility, the aforementioned concepts made enough sense for me to dogmatically embrace the practice of aerating such styles with pure oxygen at both yeast pitch and a day or so into fermentation. Curious if the second dose of oxygen was having any impact, I decided to put it to the test.

| PURPOSE |

To evaluate the differences between a high OG ale dosed only at yeast pitch and one dosed at both yeast pitch and a day into fermentation.

| METHODS |

I opted to go with an Imperial Stout for this xBmt, relying on a reiterated mash method to achieve the high OG using my smaller brewing system.

Altitude Sickness

Recipe Details Batch Size Boil Time IBU SRM Est. OG Est. FG ABV 5.2 gal 60 min 71.6 IBUs 60.5 SRM 1.123 1.050 10.0 % Actuals 1.123 1.017 14.5 % Fermentables Name Amount % Pale Malt, 2 row (Gambrinus) 13.5 lbs 59.02 Oats, Flaked 2.375 lbs 10.38 Chocolate Malt (Simpsons) 1.562 lbs 6.83 Munich II (Weyermann) 1.562 lbs 6.83 Rye, Flaked 1.187 lbs 5.19 Roasted Barley (Muntons) 1 lbs 4.37 Crystal, Medium (Simpsons) 8 oz 2.19 Sugar, Table (Sucrose) 1.187 lbs 5.19 Hops Name Amount Time Use Form Alpha % Magnum 60 g 60 min First Wort Pellet 12 East Kent Goldings (EKG) 67 g 30 min Boil Pellet 5 Yeast Name Lab Attenuation Temperature Flagship (A07) Imperial Yeast 75% 60°F - 72°F Notes Water Profile: Ca 50 | Mg 5 | Na 8 | SO4 36 | Cl 80 Download Download this recipe's BeerXML file

I made two starters of Imperial Yeast A07 Flagship a day ahead of time.

After collecting two sets of water on brew day, I turned on the elements to heat them up.

I then weighed out and milled the grain for each batch.

With the water adequately heated, I began the reiterated mash process by incorporating half of the grains for each batch into either kettle then checked to ensure both were at the same temperature.

After 60 minutes, I removed the grains from each batch then began heating the first “iteration” of wort to mash temperature. After cleaning the grain baskets, I added the second half of the grains to each batch of wort. When the second 60 minute mash rest was complete, I removed the grains and combined the worts in a single kettle to ensure homogeneity.

Next, I transferred half of the homogenized wort back to its own kettle and turn the elements on to heat them up, at which point I weighed out the kettle hop additions.

The worts were boiled for 60 minutes with hops added as stated in the recipe.

With each boil complete, I ran them through my plate chiller directly to fermentation kegs then took refractometer readings showing both hit the same 1.123 OG.

At this point, I hit both worts with 60 seconds of pure oxygen before pitching a yeast starter into each.

The fermentation kegs were placed next to each other in my chamaber controlled to 66°F/19°C.

With both beers actively fermenting 18 hours later, I gently removed the lid on one keg and proceeded to dose it with another 60 seconds of pure oxygen.

Following 2 more weeks of fermentation, neither beer was showing signs of activity, so I took hydrometer measurements showing a difference in FG.

I then pressure transferred the beers to sanitized serving kegs.

The filled kegs were placed in my cool keezer where they were burst carbonated and left to condition for several weeks before I served them to tasters.

| RESULTS |

A total of 22 people of varying levels of experience participated in this xBmt. Each participant was served 1 sample of the beer dosed once with oxygen and 2 samples of the beer dosed twice with oxygen in different colored opaque cups then asked to identify the unique sample. At this sample size, 12 tasters (p<0.05) would have had to identify the unique sample in order to reach statistical significance, though only 9 (p=0.29) did, indicating participants in this xBmt were unable to reliably distinguish a beer dosed with oxygen just at yeast pitch from one dosed with oxygen at yeast pitch and a day into fermentation.

My Impressions: Out of the 7 semi-blind triangle tests I attempted, I chose the unique sample just twice. To my palate, the beers were identical in every perceptible way. Both were clean with prominent bitter chocolate and roasted coffee character, and despite the high ABV, I didn’t detect any alcohol warmth from either.

| DISCUSSION |

Yeast require oxygen to grow effectively and ferment healthily, hence the the importance placed on wort aeration. Given positive relationship between osmotic pressure and specific gravity, many brewers opt to double dose their higher OG beers with oxygen, first at yeast pitch then again once fermentation has kicked off, the goal being to reduce stress on the yeast. The fact tasters in this xBmt were unable to tell apart a 1.123 OG Imperial Stout dosed just once at yeast pitch from one dosed at yeast pitch and the following day suggests both methods had a similar perceptible impact.

While the beers were indistinguishable on a sensory level, the one double dosed with oxygen finished 0.004 SG points, indicating slightly higher attenuation. It’s possible this relatively small difference in FG was caused by something else, but considering the the effort made to keep things between these beers as similar as possible, it seems marginally more likely the additional dose of oxygen played a role.

Based on both my personal experience and the blind taster results, I’ve no plans to start regularly double dosing my beers with oxygen, regardless of OG. In addition to being quiet messy, the difference in attenuation wasn’t enough to convince me the extra work is worth it. I would be interested to further explore the impact double dosing with oxygen has in less favorable conditions, for example, when pitch rates are low. Until then, I’ll stick to my normal routine of hitting wort with oxygen at yeast pitch.

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

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