Author: Matt Del Fiacco

One of most commonly cited brewing faults responsible for unpalatable beer is inappropriate fermentation temperature, and for the most part, the concern is on making sure fermenting beer doesn’t get too warm. This is especially true for lagers, which are believed to require temperatures around 50°F/10°C or lower in order to avoid undesirable off-flavors. Indeed, various past xBmts have called this idea into question, nearly all of which compared lager beers fermented at traditional temperatures to ones fermented at ale temperatures, a delta of about 20°F/11°C.

Reactions to the aforementioned results ranged from interested disbelief to self-serving proof of Brülosophy’s evil and dishonest plot to topple the brewing establishment. It makes sense. Brewers have been fermenting lagers cool for centuries, and results questioning such convention ought to be scrutinized. Curiously, Kveik yeast, which is purported to ferment cleanly at temperatures far warmer than typically accepted, seems to have received general acceptance among brewers. One such example that was recently released by Bootleg Biology is OSLO, a strain reportedly sourced from a raw beer made by a couple Norwegian brewers, which has the following description:

OSLO can comfortably produce beautifully clean, lager-like beers at temperatures as high as 98°F/37°C without noticeable off flavors.

At nearly 50°F/27°C warmer than commonly accepted lager fermentation temperatures, the idea that OSLO could be used to produce beers with the clean, crisp character expected in most lager beers is perplexing. However, users of this unique strain have been reporting positive results, many of whom are using it to ferment otherwise standard lager styles at strikingly warm temperatures. As a lager lover who has loosened my grip a bit on fermentation temperature dogma, I was interested see how clean OSLO would be fermented warm and decided to compare it to a version fermented cooler.

| PURPOSE |

To evaluate the differences between a lager style beer fermented with Bootleg Biology OSLO Kveik yeast at either 64°F/18°C or 98°F/37°C.

| METHODS |

Given OSLO’s purported ability to ferment a clean lager at warm temperatures, I went with a simple Helles Exportbier for this xBmt in hopes of accentuating any impact of the variable.

Sauna

Recipe Details Batch Size Boil Time IBU SRM Est. OG Est. FG ABV 5 gal 90 min 20.6 IBUs 3.9 SRM 1.053 1.006 6.1 % Actuals 1.053 1.007 6.1 % Fermentables Name Amount % Barke Pilsner (Weyermann) 9.625 lbs 98.09 Melanoidin (Weyermann) 3 oz 1.91 Hops Name Amount Time Use Form Alpha % Hallertauer Mittelfrueh 33 g 60 min Boil Pellet 4.7 Yeast Name Lab Attenuation Temperature Oslo (BBOSL-OSLO) Bootleg Biology 81% 75°F - 98°F Notes Water Profile: Ca 54 | Mg 11 | Na 25 | SO4 81 | Cl 65 Download Download this recipe's BeerXML file

I started my brew day by collecting two similar volumes of water then turning on the elements to heat them up.

Next, I weighed out and milled two identical sets of grain.

Once strike temperature was reached, I stirred in the grains then checked to ensure each batch was at the same mash temperature.

Twenty minutes into each mash rest, I took samples for pH measurement and found they were identical.

When each 60 minute mash was complete, I removed the grains and brought the worts to a rolling boil.

When the 60 minute boils were finished, I transferred the worts through a plate chiller to identical fermentation vessels, one of which was placed in a chamber controlled to 64°F/17°C while the other went into a 98°F/37°C chamber.

Hydrometer measurements showed the worts hit the same target OG.

It took 6 hours for the worts to stabilize at their respective temperatures, at which point I pitched a pack of OSLO into each.

While both beers quickly showed signs of active fermentation, the warm ferment batch predictably beat the cool ferment batch to the punch by a few hours. After a week, activity was absent so I took hydrometer measurements showing the beer fermented cool finished 0.004 SG higher than the warm ferment beer.

At this point, the beers were racking into sanitized kegs.

The filled kegs were placed in my keezer where they were burst carbonated and allowed to condition for a week before being served 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 cool ferment beer and 2 samples of the warm ferment beer 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, while a total of 13 (p=0.012) did, indicating participants in this xBmt were able to reliably distinguish a Helles Exportbier fermented with Bootleg Biology OSLO yeast at 64°F/18°C from the same beer fermented at 98°F/37°C.

The 13 participants who made the accurate selection on the triangle test were instructed to complete a brief preference survey comparing only the beers that were different. A total of 7 tasters reported preferring the cool ferment beer, 4 liked the warm ferment beer more, 1 had no preference despite noticing a difference, and 1 reported perceiving no difference.

My Impressions: I attempted 5 semi-blind triangle tests and identified the odd-beer-out every time with relative ease. While I didn’t perceive either of these beers as being necessarily lager-like, I was startled by how clean both turned out to be. To me, the warm ferment beer had a more floral aroma with perhaps a touch more ester character, though far less than I’d expect for a beer fermented at 98°F/37°C. The cool ferment beer, on the other hand, had a more noticeable malt flavor with hints of the same fruity character, just a bit more subdued.



| DISCUSSION |

The temperature at which beer is fermented has been deemed one of the most important aspects to creating a quality product, leading many brewers to integrate methods of controlling the temperature of their fermenting beer. Whereas it’s typically recommended to ferment ales around 68°F/20°C, lagers are traditionally fermented much cooler to avoid stylistically inappropriate esters. However, Bootleg Biology’s latest Kveik strain, OSLO, is purported to produce clean lager-like beers when fermented as outrageously high as 98°F/37°C. The fact tasters in this xBmt were able to reliably tell apart pale lagers fermented with OSLO at either 98°F/37°C or 64°F/18°C suggests that fermentation temperature did have a perceptible impact.

What makes this result so interesting is that OSLO is marketed as a strain that produces lager-like cleanness even when fermented very warm, leading one to wonder what the nature of “clean” is in regards to this yeast. In follow-up conversations with tasters, some noted both as having a slight fruitiness that was a bit stronger in the warm ferment version, which likely contributed to their perceptible difference. Furthermore, while both beers were generally well received by tasters, many of whom were caught off-guard once learning how warm they were fermented, none felt either accurately represented a more traditionally fermented Helles Exportbier.

I thoroughly enjoyed both of these beers, despite perceiving neither as being very lager-like. Having used other types of Kveik yeast in the past, I will say that OSLO did seem to produce a generally cleaner beer in comparison, which I think makes it ideal for brewers who might not have the means to control fermentation temperatures. While I was pleased with the way both of these xBmt beers turned out and will use OSLO again in the future, I’m compelled to stick with traditional lager strains when making lager styles, as even when fermented at ale temperatures, they still produce the character I expect.

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

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