Brewing Mountain Sour #1-4

The Mountain Sour brand will be a catch-all for a variety of sour beer recipes, including barrel-aged mixed-fermentation sour beer, Catharina sour beer, and spontaneously fermented sour beer.

The Mountain Sour brand will be a catch-all for a variety of sour beer recipes, including barrel-aged mixed-fermentation sour beer, Catharina sour beer, and spontaneously fermented sour beer. The former is the focus of today’s blog post.


Mountain Sour was inspired by The Rare Barrel’s Golden Sour but altered for use with 100% Mecca Grade Estate Malts. After listening to Dave McLean of Admiral Maltings discuss intentionally under-modifying malt for nuance in mixed-fermentation beers, I jumped on the opportunity to use Mecca Grade’s Gateway Malt in Mountain Sour. Gateway is an under-modified “wind-malt” inspired by a long-forgotten style of air-dried Belgian malt. I paired Gateway with Wickiup (Red Wheat Malt) which touts a protein content of 16%+ to give my “house” microbes plenty to snack on during the long secondary fermentation. I also utilized Vanora (Vienna-style Malt) for its rich aroma and deep golden hue. The final grist was 76% Gateway, 18% Wickiup and 6% Vanora.


Last summer I purchased 5 lb of choice debittered hop pellets from Hops Direct specifically for the Mountain Sour brand. Aged hops contribute low bitterness while retaining some antimicrobial properties and important flavor/aroma precursors. Despite the low alpha acid content of aged hops, some brewers have reported high-than-expected IBU levels in their beers, slowing the souring process. Therefore, I leaned heavily on the experience of fellow Milk The Funk member Caleb Buck who examined aged hopping rate in spontaneous beer on the Archaic Pursuit blog. From his 2017 coolship experiment, Buck recommends an aged-hopping rate of 0.15 oz/gal to target ~35 IBUs in his spontaneously fermented beer which is exactly the rate I used for Mountain Sour. I chose to further reduce the IBUs by only adding hops to the whirlpool for 30 minutes at 185°F.

Yeast and Bacteria

Mountain Sour was fermented with our house yeast before being transferred into the Squarrel Square Barrel and inoculated with our house microbes.


In the spirit of preserving as much terrior as possible, I used 100% unfiltered Kuna, Idaho water. The only water treatment I used was adding lactic acid to the mash to target a mash pH of 5.2.

Mountain Sour Recipes

Mountain Sour #1:

Mountain Sour #2:

Mountain Sour #3:

Mountain Sour #4:

The recipes were nearly identical but there were some changed from batch-to-batch.

Mountain Sour #1 was mashed at 158°F for 20 minutes to intentionally leave unconverted starches for the house microbes. Unfortunately, I significantly undershot my target original gravity SG (1.033 instead of 1.048). Therefore, Mountain Sour #2 underwent a total mash duration of 90 minutes at 158°F which got me a lot closer to my target SG (1.047 instead of 1.048). Anticipating final gravity SG of 3.0% and 4.9%, respectively, I increased the grist weight for Mountain Sour #3 and #4 by 20% to target 6.0% ABV. Average ABV across all four iterations would result in 5.0% Unfortunately, Mountain Sour #3 undershot my target SG (1.054 instead of 1.058) so I decided to introduce stirring of the mash every 15-20 minutes for Mountain Sour #4. Counter-intuitively, this lead to an even lower SG (1.048), which leads me to believe my hydrometer may no longer be calibrated. Regardless, all four batches were transferred into the Squarrel Square Barrel on May 4, 2020 with 100 ml of my house microbes. The final gravity SG for the blended batches was 1.014.

I intend to use the 10 gallon Squarrel Square Barrel as a single-stage solera for perpetual sour beer blending. Every ~6 months, I will draw ~2.5 gallons of sour beer. If necessary, I will blend the sour beer with some non-sour beer to dial in the appropriate acidity level and round out the overall character of the beer. Next I will transfer the beer onto 49 oz of Vintner’s Harvest Fruit Purée for ~4 weeks. Lastly, I will replace the ~2.5 gallons of drawn sour beer with an equal volume of fermented beer to keep the Squarrel full.

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