Kettle Sour Blanket Experiment

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By Shana Solarte

May 14, 2024

In our overview of kettle souring, we briefly discussed the process of keeping a blanket of CO2 atop souring beer, and whether it’s necessary to prevent contamination. We decided to run this experiment and keep an eye on a split batch of kettle-soured beers to track how the two fermentations progressed and whether the presence of a CO2 blanket made a sensory impact.

We set up this experiment with a classic kettle sour grist, soured with Omega’s Lacto blend, and fermented with Voss Kveik. We chose Voss because of its relatively neutral yet lightly citrusy flavor profile, which would give us enough fermentation character to make a tasty beer, but with a relatively neutral profile that would make it easier for our sensory panelists to pick out any anomalous flavors.

Split batch kettle sour

Vital Statistics

  • Batch size ½ barrel, divided
  • Boil time 60 mins + 10 mins post-sour
  • IBU 2.5
  • SRM 3
  • OG ~8.5°P
  • FG ~2°P
  • ABV ~3.5%

Raw Ingredients


  • 2-row pale malt
  • White wheat
  • Flaked wheat
  • Cascade hops (5.5% AA)
    1oz T90

Souring & fermenting cultures

  • Lacto (OYL-605)
  • Voss Kveik (OYL-061)


Single infusion mash at 148°F (64°C) with 1.25 qts/lb water to grist ratio (2.6 L/kg). Target 5.2 mash pH. Sparge at 168°F (75°C). Boil for 60 minutes, then knockout to 95°F (35°C) and split into separate vessels, then pitch Lacto. Kettle sour until pH reaches 3.2 – 3.4 (about 18 hours). When desired pH is achieved, boil for 10 minutes and add hops. Pitch Voss and allow to ferment until ABV is approximately 3.5%.

After 18 hours of souring, the samples differed slightly in both pH and gravity—
the CO2 blanket sample had a pH of 3.42, and after a short boil & hop addition, the pH was 3.47 and the gravity was 9.27°P. After souring, the no-blanket sample had a pH of 3.5, and after a short boil & hop addition, the pH was 3.5 and the gravity was 8.78°P.


Split batch stats

With CO2 blanket8.661.943.426.7g/L77.66%3.5%
Without CO2 blanket8.171.83.486.8g/L77.64%3.3%

The table above lays out how these two beers wound up after the split souring experiment: were practically identical, with very slight differences across the board. Note that the pH and titratable acidity (TA) were very close as well, making the flavor profiles even tougher to distinguish.This made for an ideal tetrad test, so we brought the samples to our staff for a training session and discussion.

Kettle Sour results

Results from staff sensory.

About 58% of our panelists chose correctly, while the other 42% selected incorrectly. This indicates that it wasn’t immediately obvious which samples were made with the CO2 blanket and which were not, though there were some comments among those who guessed correctly that the samples made without the CO2 blanket had a slightly higher fruity aroma, and those with the blanket had a bit more sulfur. 

So, what does it mean? We hoped to take a look at whether the CO2 blanket would affect the final flavor profile of the beers, and found that for the most part, it did not. With good brewing and sanitation practices, you shouldn’t necessarily need to use CO2 during the souring process, but it also doesn’t seem to make a difference if you do. To blanket or not to blanket? That’s entirely up to your preference and process. 

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