Author: Jason Cipriani

Making sour beer can be intimidating. For some, the added cost of acquiring separate equipment to avoid contamination of future clean beers is enough to keep them from trying it out, though perhaps the biggest deterrent has to do with the time required for the bacteria do its souring magic. Traditionally brewed, a sour beer can take anywhere from 1 to 4 years before it’s ready to consume, much longer than many are interested in waiting.

An approach that has grown massively in popularity over the last few years due to its ability to significantly reduce grain-to-glass time is kettle souring, which involves souring wort prior to boiling and fermenting it. Unlike traditional souring methods, the lactic acid bacteria (LAB) is generally pitched on its own and allowed to blaze its way through the sweet wort before proceeding, reducing the pH to desirable levels in as little as 18 hours.

Kettle souring also adds an element of control not found when relying on methods like sour mashing, as the wort can be pasteurized prior to pitching the LAB to ensure its free from other spoilage microbes.

While there exist many critics of this arguably less conventional method, most who claim it produces beers with lacking complexity, there’s also some disagreement among its advocates regarding proper technique. One very common recommendation, often spun as dogmatic mandate, is that the wort be soured in an environment free of oxygen, as not doing so could lead to the production of isovaleric acid, a compound that reeks of stinky cheese and dirty gym socks.

However, there’s been some talk lately about this not being the case, that the LAB used to sour beer will perform similarly even without such special treatment. As someone who had adopted the anti-oxygen position, I was skeptical of souring in an open vessel and decided to test it out for myself!

| PURPOSE |

To evaluate the differences between a beer soured in an oxygen free environment and one soured in an open vessel.

| METHODS |

I thought a nice Sour Blonde would allow any differences caused by the variable to shine and went with a grist inspired by a clean Blonde Ale recipe of mine.

Coulter

Recipe Details Batch Size Boil Time IBU SRM Est. OG Est. FG ABV 5.5 gal 15 min 4.2 IBUs 4.5 SRM 1.057 1.013 5.7 % Actuals 1.057 1.009 6.3 % Fermentables Name Amount % 2-Row - US 10 lbs 90.91 Caramel/Crystal 10 - US 8 oz 4.55 Honey Malt - CA 8 oz 4.55 Hops Name Amount Time Use Form Alpha % Willamette 14 g 15 min Boil Pellet 5.1 Yeast Name Lab Attenuation Temperature Dieter (G03) Imperial Yeast 75% 60°F - 69°F Download Download this recipe's BeerXML file

I started my brew day collected the required RO water for both batches then adjusting each to the same profile. While one batch of strike water was heating, I weighed out identical sets of grain and had my assistant help mill it.

Staggering the start of each batch by 30 minutes, I began heating the strike water on the second just as I mashed in on the first; they were treated identically otherwise. Thermometer readings showed both hit the same mash temperature.

While mashing, I heated the sparge water for both batches with my electric heat stick.

The first batch was just coming to a boil when the second batch was sparged.

To reduce the risk of spoilage by unwanted contaminants, the worts were boiled for 15 minutes without hops for the sake of pasteurization before being chilled during transfer to souring vessels.

Hydrometer measurements showed both worts had achieved the same OG.

An initial pH measurement showed a delta of 0.06 pH between the worts, not too much, but odd since they were treated identically up to this point.

My souring source of choice for these beers was GoodBelly, which is teeming with Lactobacillus plantarum, a microbe known for its ability to fairly quickly reduce wort pH.

After dosing each wort with the same amount of GoodBelly, I sealed one keg, purged the headspace with CO2, then left the gas attached at low pressure to ensure no oxygen ingress. The opening of the other keg was loosely covered with a piece of sanitized foil to prevent debris from falling in.

At this point, I stole some leftover wort and built equal sized starters of Imperial Yeast G03 Dieter.

I measured the pH after 24 hours and found both worts had dropped about the same relative to their starting pH.

I generally aim for about 3.4 pH when kettle souring, which usually takes about a day, but this ended up being one of the slowest kettle sours I’ve ever made– it took 85 hours for the worts to reach this target pH.

L. plantarum being heterofermentative, a drop in SG is expected as a result of souring and was observed in this instance, though it seemed the wort soured in a sealed keg dropped a bit more than the wort soured in an open keg.

I proceeded with boiling each wort for 15 minutes to rid it of any LAB.

Each wort was hit with a small amount of hops.

At the completion of each boil, I chilled the wort during transfer to fermentation vessels, pitched the yeast starters I’d made on brew day, and let them ferment in a temperature controlled chamber. After a week, I took hydrometer measurements confirming FG had been reached.

Each beer was then pressure transferred to identical CO2 purged Torpedo kegs.

The filled kegs were placed in my keezer where they were chilled and burst carbonated. After a few days of conditioning, they were ready to serve to participants.

| RESULTS |

A total of 18 people of varying levels of experience participated in this xBmt. Each participant was served 1 sample of the beer soured in a closed vessel and 2 samples of the beer soured in an open vessel in different colored opaque cups then asked to identify the unique sample. At this sample size, 10 tasters (p<0.05) would have had to identify the unique sample in order to reach statistical significance, though a mere 3 (p=0.97) made the accurate selection, indicating participants in this xBmt were unable to reliably distinguish beers soured in either closed or open conditions.

My Impressions: Despite the stench from the boil and the bias it undoubtedly produced, I was unable to tell these beers apart in my own semi-blind triangle test attempts, randomly guessing the odd-beer-out just 2 out of 5 times. I enjoyed these beers for their simplicity, though in the future I’ll likely add some fruit post-fermentation to up the complexity a bit.

| DISCUSSION |

Lactobacillus, the most common microbe used for kettle souring, is a facultative anaerobe, which basically means it functions in both the absence or presence of oxygen. Despite this fact, and seemingly based on a few anecdotal reports, many brewers of kettle sour beer strive to limit oxygen exposure during the souring phase for fear of producing off-flavors.

In researching the topic, I discovered little in the way of objective data supporting this notion, mostly just homebrew horror stories and a partially relevant study referenced on the Milk The Funk wiki. Coincidentally, episode 004 of Milk The Funk: The Podcast dropped while I was collecting data for this xBmt, in which the idea that Lactobacillus produces off-flavors in the presence of oxygen was deemed “a myth.”

During the process of making the beers for this xBmt, there were points I was absolutely convinced they were going to be different. During the post-souring boils, the open vessel wort emitted a noticeable aroma of dirty socks that left my wife none too pleased. The odor was so pungent it actually made me a bit queasy, and I began to feel guilty about the beer I would eventually serve to blind participants.

However, in sampling that very beer when it was done fermenting, I was shocked to find the nasty characteristics had completely diminished, its flavor and aroma were the same as the beer soured in a closed vessel to my palate. And the fact blind participants in this xBmt were unable to tell them apart with any consistency lends even more credence to the claim that oxygen exposure during the kettle souring process won’t necessarily lead to off-flavors.

One caveat to this xBmt is that it focused specifically on souring with a known LAB, L. plantarum, and thus the results cannot be extrapolated to include wort soured with other microbes. As such, I’ll likely continue souring under a blanket of CO2 when making kettle sours pitched with a known LAB in the future, if for no other reason than to prevent the smell during the boil, though in those times I detect a stench, I’ll no longer presume the batch is dead.

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

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