Voss kveik optimum fermentation temperature
In my last post on kveik fermentation kinetics, I showed that kveik yeast strains vary in their response to temperature and that some of them really do prefer "hot" temperatures (>35°C) when compared to "typical" ale yeast temperature (~22°C). In that post, I tested "cool" (22°C) and "hot" (>35°C) temperatures for Voss, Hornindal, and Lutra kveik. However, this temperature range is quite limited and doesn't answer the question of optimum temperature (i.e. the ideal growth/fermentation temperature). So I decided to figure out the ideal growth/fermentation temperature for the most famous isolate - Voss kveik.
Why is this important? For two reasons: 1) brewers are interested in how temperature influences beer turnover time AND flavour, and 2) it gives us insight into the different evolutionary processes that shaped kveik yeast vs modern commercial brewers yeast. The general idea put forth by Lars Marius Garshol is that modern commercial brewers yeast may have lost the ability to tolerate high temperatures as a result of long term adaptation to commercial brewing processes.
But hasn't someone tested optimum growth/fermentation temperature for kveik? Sort of.. In the Preiss et al (2018) paper on kveik, the authors measured the optical density (i.e. cell density) of kveik and modern commercial ale yeast (WLP001) at 30°C and 40°C. Preiss and authors showed that kveik have a higher temperature tolerance (WLP001 did not grow at 40°C but most kveik did). However, they did not determine growth rates or optimum fermentation temperature. In their paper, Voss kveik reached the same cell density after 20 hours of incubation at 30°C and at 40°C. But this doesn't show the rates at the different temperatures. In fact, it's possible from their data that WLP001 and Voss kveik have the same exact optimum fermentation temperature (at a temperature much lower than 40°C).
I drew three hypothetical scenarios that could match their data (Figure 1): Voss ferments faster at 40°C than 30°C (what most of us would bet on), faster at 30°C than 40°C (seems counterintuitive but totally possible), or that the speeds are roughly similar at 30°C and 40°C (also possible, if the optimum is between). So, from their data, one cannot tell the optimum fermentation temperature or whether there is a difference in fermentation speed at 40°C vs 30°C. I should point out here that I am NOT criticizing the Preiss et al (2018) paper; this was a landmark paper on kveik and contributed hugely to our knowledge about the genetics, evolution, and genomics of this unique family of yeasts. It's certainly worth also keeping in mind that they tested the farmhouse Voss kveik, whereas I tested the Voss kveik isolate from Lallemand.
Figure 1: Theoretical possibilities for growth/fermentation of Voss kveik from Preiss et al (2018).
Another possible source to answer this question is the Lallemand info doc on Voss kveik. Under "brewing properties" they give a 2 day fermentation time at 40°C, 3-4 days at 30°C, and 5-7 days at 25°C. Not super informative. This suggests the optimum is 40°C, but it's difficult to tell. And how can we reconcile the Preiss et al (2018) paper and the Lallemand document? The Preiss et al (2018) paper suggests there isn't a difference between 30°C and 40°C, while the Lallemand document states you can shave an entire day off your fermentation time at 40°C versus 30°C. Something here is amiss.
So, I decided to test it and determine the temperature optimum for Voss kveik (from Lallemand). The base beer for the 5 batches was as follows:
60% Pilsner malt
38% Vienna malt
2% Munich malt
Mashed at 65ºC for 1 hour, 78ºC mashout for 1 min, batch sparge
Mash pH of 5.45-5.5 to counteract kveik pH drop
FG 1.007-1.010 (apparent attenuation varied from ~77% to ~83%)
60 min boil
23 IBU Hallertau magnum at 60 min
Half a teaspoon wyeast nutrients at 10 min
0.7 IBU Hallertau Mittelfrueh at flameout
Chilled to 22ºC, 30ºC, 34ºC, 37ºC, and 40ºC using immersion chiller
Aerated with a 5 micrometer stainless steel airstone (air) for 4 minutes
Pitch rate: 1 pack per 5 gallons (standard recommended pitching rate)
Fermented at the respective pitch temps: 22ºC, 30ºC, 34ºC, 37ºC, 40ºC.
I maintained temperature using a fridge fermentation chamber for my fermentation vessel coupled to an inkbird and heating pad. This allows temperature control of +/- 0.5ºC from 4ºC to 41ºC.
Once the fermentations finished (click here and go to the bottom to see how I calculate this), I transferred each beer to a corny keg and carbonated to 2.5 volumes.
I followed the gravity and instantaneous CO2 evolution rate (CER) over time to track fermentation progress at the different temperatures. As a brief reminder: CER is an instantaneous measure of yeast activity (rate per unit time) which correlates really well with sugar consumption and yeast activity in general.
Figure 2: Measured gravity and CER over time for Voss kveik fermented at 22ºC, 30ºC, 34ºC, 37ºC, 40ºC.
Faster gravity consumption = faster fermentation
Higher CER production = faster fermentation
Faster flattening of CER = faster fermentation
A) Voss kveik fermented at 22ºC and 30ºC reach terminal gravity last, with 22ºC being the slowest. The gravity decrease slopes are difficult to judge here (see Figure 3) but the CER gives a clearer picture (below).
B) I observed the highest (and earliest) CER peak at 37ºC, with activity declining back to zero faster than at any other temperature. This indicates that the yeast started fastest at 37ºC and finished first at 37ºC. On the other hand, the slowest and last CER peak was observed at 22ºC.
Figure 3: Maximum gravity points per minute drop, maximum CER, and time to finish fermentation for Voss kveik fermented at 22ºC, 30ºC, 34ºC, 37ºC, 40ºC.
Higher max gravity points per minute (MGPPM) consumption = faster fermentation
Higher max CER production = faster fermentation
Lower time to finish fermentation = faster fermentation
C) Max sugar consumption per minute (MGPPM) and max CER is highest at 37ºC. Correspondingly, time to finish is shortest (i.e. "fastest" fermentation) at 37ºC.
The reason there are two points for 37ºC for max CER and time to finish fermentation is because I did two fermentations at this temperature (but only measured gravity decrease in parallel once).
D) Activity (MGPPM and max CER) starts to decline at 40ºC; this means that Voss kveik is already stressed and starting to show inhibited growth at 40ºC. Activity at 34ºC is roughly equivalent to 40ºC.
E) Fermentation is slowest at 22ºC (taking ~70 hours to finish); but you can shave 20 hours off your fermentation time if you go to 30ºC.
Conclusions on physiology:
Optimal fermentation temperature for Voss kveik is ~37-38ºC.
At 40ºC, we start to see growth inhibition and stress.
But this doesn't mean we shouldn't ferment at 40ºC (see below).
Figure 4: Impact of fermentation temperature on sensory properties of Voss kveik
I chose a very simple malt bill to let the yeast flavour shine. I wanted as much yeast flavour as possible to get a real sense of how the expressiveness changed with temperature.
Voss at 22ºC: very light fruity and malty aroma, perceptible "kveiky" acidity/twang, with a noticeable "alcoholic" taste coming through. The "kveiky" acidity was the dominant tasting note even though I adjusted the mash pH to 5.5. I was a bit surprised how neutral Voss could be, as I've seen several members of the "Brewing with Kveik" group using it for pseudo-pilsner beers, but it wasn't nearly neutral enough to make this beer enjoyable. Low body and low mouthfeel, with the beer tasting somewhat stripped due to the relatively high attenuation but "low" crispness.
Voss at 30ºC: light-to-medium fruity aroma and flavour. The "kveiky" acidity is still there but slightly more balanced by the heavier fruity notes. The malt comes through better here and the beer tasted strangely more neutral, likely because it was a bit more balanced by the esters. I enjoyed this one more than that at 22ºC.
Voss at 34ºC: Very similar to 30ºC but with a slightly heavier citrusy aroma and flavour. The beer is less balanced and notes of red apple and "kveiky" acidity come through.
Voss at 37ºC: Medium citrusy flavour and aroma with the malt notes almost completely hidden by the esters. The alcoholic flavour and the "kveiky" acidity are both less perceptible than at any lower temperature - perhaps because the citrusy esters are starting to overwhelm the other flavours. The resulting beer is no longer "neutral" at all. I enjoyed this one better than at any lower temperature.
Voss at 40ºC: Heavy notes of citrus and bitter orange peel. The "kveiky" acidity is almost completely un-perceptible but notes of red apple come through slightly. The mouthfeel and body is slightly fuller than at lower temperatures. This beer tasted like the original citrus 5 Alive (a commercial fruit juice)! I was amazed at how much more citrusy this beer came out compared to 37ºC; the citrus notes are really turned up in an unexpected way. My favourite beer out of the series, by far! I should note here that the "acid" and "alcoholic" flavours may not be gone at 40ºC - rather it's likely that the other esters dominated over/overwhelmed those notes and I could no longer pick them out.
Here I will note that the increased expressiveness of Voss kveik at 40ºC is similar to that of hefeweizen and saison yeasts at elevated temperatures. For example, if a brewer wants more banana notes (isoamyl acetate) in the finished beer, then the brewer would generally 1) underpitch, 2) increase wort density (>1.055), and 3) ferment warmer than usual. These three approaches all work together to stress the yeast a bit and result in an increase of the desired flavours. Wyeast 3068 (Weihenstephan Weizen) produces almost no banana character if overpitched! So should we ferment Voss kveik at 40ºC?? Yes, if you desire the corresponding flavours.
I didn't add any clarity process aids post-fermentation to see how the clarity of the beer might change at different fermentation temperatures. I carbonated to 2.5 volumes in corny kegs and tapped the beer ~7 days after kegging. There was a noticeable decrease in clarity of the beer with increasing temperature, but none of the beers came out very clear. (I recommend looking at the bottom of the glass each time in the next figure to assess clarity)
Figure 5: Clarity of beer fermented with Voss at 22ºC, 30ºC, 34ºC, 37ºC, and 40ºC.
Conclusions on tasting and clarity:
Fermentation has a massive impact on flavour of the yeast (with 40ºC being the most expressive by far, even compared to 37ºC).
Fermented at 40ºC was my favourite beer to drink, by far.
Sedimentation (i.e. clarity) was lowest at the highest temperatures (34ºC to 40ºC).
The citrus and bitter-orange flavours that predominate in Vossøl are really only produced by Voss kveik at high temperatures (>38ºC). Correspondingly, when making classic Vossøl, Sigmund Gjernes would pitch Voss kveik at 39ºC and observe that the endogenous activity caused the temperature to exceed 40ºC. So not only is the yeast optimized and adapted to work at this temperature, but the flavours that the brewers desired from the yeast is only produced at these high temperatures.
This series of experiments has changed my perspective of yeast and kveik a lot. I was often trying to shoehorn kveik into a role that other yeasts could do better. Using kveik strains where they really shine (i.e. high expressiveness at high temperatures) seems to produce flavours that I like more.