Six months ago I embarked on an experiment to assess how well drying kveik (Norwegian landrace yeasts) works as a method to preserve and reuse these yeasts. Today, six months later, this experimental series comes to its end.
Kveik are a relatively “new” set of yeast strains to commercial and home brewers, but are a very old family of landrace yeasts that have been used in Norwegian farmhouse brewing for generations. These yeasts have been written about extensively by Lars Garshol, and I would direct you to his blog for all of the information on these yeasts’ that you could ever want.
One thing that intreged me about kveik is the traditional method of storing them dried on wooden logs or rings. Most brewers yeast will not survive drying with a reasonable (for brewers) survival rate without significant technological intervention. That these yeasts could apparently be stored for long periods of time simply dried on wood was unexpected. So I set forth to take a slightly more modern (but very budget-conscious) approach to measure the viability of dried kveik over time.
This is going to be a deep-dive into the results of this experiments, the limitations to the approach I used, and a detailed description of what it all means. If you simply want the 6-month result, skip to the middle of the post. If you just want the take-home message, skip to the end.
The Drying Kveik Series
- Drying the yeast and viability immediately after drying.
- Viability at 1 month.
- Viability at 2 months.
- Viability at 3 months.
- Viability at 4 months.
- Viability at 5 months.
- Bonus Entry: Viability at 965 days.
- This entry: viability at 6 months and conclusion to the series.
Background & Approach
This experiment started back on February 24th 2019, with the completion of my Jörmungandr Black IPA. This was a massively over-hopped IPA which I fermented with the Voss kveik strain. I took the yeast/hop slurry from this beer and dried it using a $40 food dehydrator. The resulting dried yeast cake was placed into a zipper-sealed bag and stored in the freezer. Viability was then assessed on a monthly basis, using Trypan Blue staining. My plan was to assess viability monthly for six months, which should give enough measurements to get an accurate measurement of the usable lifespan of the dried kveik.
For those wishing to repeat this experiment, here are step-by-step instructions on how I dried, stored and rehydrated/tested the kveik
- Brew a beer with kveik. As soon as the beer is ready, transfer it off of the yeast cake.
- Suspend the yeast cake. Ideally this should be done by shaking the fermenter. If the yeast cake is too thick to suspend, add a small amount of boiled & cooled water.
- Optional: pour the yeast cake into a sterilized jar to make pouring it onto the dehydrator easier.
- Pour the slurry onto parchment paper-lined trays of a dehydrator.
- Dehydrate on the lowest setting until completely dried.
When dried the yeast should have a texture similar to leather, but will crumble when bent. If you don’t have a dehydrator, you should be able to use a warm (not hot) oven and parchment paper-lined baking sheets.
Storing Dried Kveik:
- Wearing rubber gloves, pre-cleaned with sanitiser and dried on a lint-free towl, break up the dried kveik into smaller pieces.
- Place these pieces into a zipper-sealed bag.
- Seal the bag, removing as much air as possible. I used a drinking straw to suck out the air prior to sealing.
- Store in a home deep-freezer or refrigerator.
Rehydrating Dried Kveik:
Note that this step is only for viability testing and is unnecessary if you are brewing with the kveik or adding it to a starter.
- Add a small amount of distilled water to a small, clean container.
- Add a pinch of the dried kveik (~1/10th the volume of the water) to the tube and shake to mix.
- Shake the tube after 5 minutes.
- Repeat step 3 an additional two times.
Trypan Blue Staining:
Trypan blue can be purchased as a dry powder, or as a 2X solution. While more expensive, I recommend the 2X solution (0.4% w/v) for ease-of-use. If you purchase the powdered form, dilute 0.4g into 100 ml of distilled water to make the staining solution. You will also need an eye dropper/pipette, glass slide, cover slip, and microscope for this step. For more information on microscopy, see my video and post.
- Using the eye dropper, place 2 drops of trypan blue staining solution (or 50 ul, if using a micropipettor) onto the slide.
- Using the same eye dropper, place 2 drops of the yeast suspension into the drop of trypan blue on the slide. You want to mix an equal volume of yeast and trypan blue.
- Place a cover glass on top of the yeast/dye mixture and transfer the slide to your scope.
- Focus on the sample, and view the sample using a 400X to 600X magnification using widefield illumination.
- Count the blue (dead) versus white (live cells) as quickly as possible. For accurate measurements, count at least 200 cells.
Drying Kveik – Viability at 6 Months
Viability continues to drop at the rate I observed before – 6% to 8% per month. Viability at 6 months was 51%, nearly dead-on the value I predicted at the 3-month time point.
Given this level of viability, I would expect that for most beers, kveik dried and stored in this fashion can be directly pitched into low-to-medium gravity beers (<1.060 OG) without a starter up to 6 months after drying. With a starter, this yeast should be usable in the brewery for several years.
But more on that at the end of the post.
Before I do my deep-dive into the results of this experiment, I want to bring to your attention several limitations to my approach. I do not consider my approach to be optimal, and more stable storage may very well be possible. Should you try this approach yourself, consider these limitations, and if possible, try to mitigate them:
Older Yeast Cake: This experiment was performed using yeast from a completed ferment and which had settled into a cake. As a consequence, this yeast would not have been as viable or had as much energy stores as fresher yeast. This could very well affect viability over time, as the carbohydrate used by yeast to store energy (trehalose) is also the substance used by yeast to resist the stress of dehydration. Therefore, yeast with better energy stores may maintain viability longer than yeast harvested from a yeast cake. Thus, yeast from a starter or yeast collected by top-cropping may be better.
High Hop Content: The yeast used in this experiment was from a heavily dry-hopped IPA. I estimate as much as 50% of the yeast cake (by volume) was hop matter. While hops are not normally toxic to yeast, the concentration of hop compounds during dehydration may have added further stress to the yeast. Using yeast from a lower-hopped beer may be beneficial.
Bad Dehydrator: The dehydrator I used was a very cheap unit with no temperature control. While I didn’t do a good job measuring temperature during the kveik drying, I did measure it during a rabbit jerkey run, and found that it held between 50C and 65C – likely higher than what would be optimal for yeast drying. In addition, drying was uneven, leaving the outer edges moist when the middle portion was dry. A higher quality dehydrator with temperature control and more even drying may work better. I would suggest a drying temperature of 35-40C.
Trypan Blue: While trypan blue is a better viability dye than other easily-accessible dyes (e.g. methylene blue), it is not perfect. In particular, trypan blue will stain dead cells and cells undergoing cell division. Thus, my viability measurements are likely under-estimates. Unfortunately, more accurate dyes typically require fluorescence microscopy, which is beyond the budget and availability of most commercial and home brewers. Of course, this has no effect on your ability to use dried kveik in your brewery, but it is something to keep in mind if you are ever doing viability counts for any purpose.
Bringing it all Togeather
How Quickly Does Viability Decrease?
Plotted over time (graph to the right), the viability of the dried kveik decreased at a very consistent rate of 8.2%/month. Much to my surprise this decay best fit a linear decay, rather than the expected 1-phase decay.
1-phase decay (e.g. half-life decay) is the norm when talking about microbiological survival, and it may simply be the case that I did not run this experiment long enough to get an accurate fit to this expected profile. Alternatively, decay may be linear – although this result is not consistent with my “surprise” drying experiment. Regardless, a fit to a 1-phase decay equation gives a half-life of 6.4 months; not far off of the linear decay prediction of 6.1 months.
How Much Yeast Is In A Teaspoon of Dried Kveik?
I did direct counts of yeast in my liquid (pre-dried) slurry, and of the yeast in the dried slurry. My yeast slurry started off with a slightly below average yeast density of 1.8 billion/ml (2.5-3.5 billion/ml is more typical), and the dried kveik lost ~50% of its volume, meaning the dried material had an estimated ~3.6 billion cells per ml (e.g. 18 billion per teaspoon).
Consistent with this, I measured a density of the dried kveik of 0.62 g/ml, and when rehydrated, produced ~2.16 billion cells/g of dried kveik (total cells, e.g. dead + viable). Dried kveik with lower hop content would likely be 50%-100% denser, but I am going to use my values moving forward for the sake of consistency. Keep in mind that you could easily exceed the numbers I am going to discuss below.
How Long Is Dried Kveik Good For Direct Pitching?
Standard practice is to pitch 4-6 teaspoons (e.g. 2.5 g – 3.75 g) of dried kveik into 5 gallons (19 L) of average gravity wort. Assuming the kveik is relatively fresh (75% viability), this works out to a pitch rate of 20 billion to 35 billion/batch. For a 1.050 gravity wort (12.5 plato) this works out to 0.085 to 0.15 million/ml/plato, or about 1/10th normal ale-pitch rates. I’ve pitched kveik as low as 0.02 million/ml/plato with success (1/50th normal ale rates), so lets assume that is the “floor” in terms of minimum pitch rate.
There are two approaches we can use to determine how long the dried kveik can be used as a direct pitch: 1) use a set volume of yeast and determine how old yeast can be before the number of viable yeast in that volume drops below our “floor”, or 2) determine the volume of yeast you need to add to get a desired pitch rate. I’ve plotted out both these approaches in the graphs below.
On the left are two models of viable yeast numbers (linear versus half-life) assuming that 10 tsp (50 ml) of dried kveik is pitched, based on the decay rates discussed above. The dotted blue line indicates the “standard” 1/10th (0.1 million/ml/plato) ale pitch rate assuming a 1.050 wort, and the dotted red line indicates the minimum (0.02 million/ml/plato) “floor” assuming a 1.050 wort. On the middle and right are graphs of the volume of dried kveik required to get the 1/10th pitch rates (middle) or “floor” pitch rates (right) for a 1.050 OG beer comparing the linear to half-life decays:
As you can see the predictions of the models vary greatly, with the linear models giving a much shorter useful lifespan. Assuming you pitch 10 ml of slurry, the linear model predicts that you need to use the slurry within 7 months to get a 1/10th ale pitch rate, or in 11 months to exceed our minimum “floor” pitch rate. The half-life model gives longer values of 8 months and 24 month, respectively. The linear model also predicts no viable cells after 13 months.
Of course, the decreasing cell counts can be overcome by pitching more dried slurry, but this quickly becomes limiting and with the potential for autolysis off-flavours from the increasing portion of dead yeast. I’m not sure at what volume these flavours would become apparent, but personally I’d avoid anything over 100 ml into 5 gallons of wort. For the linear models these numbers are incomplete, due to the predicted total death of the yeast sample at 13 months.
How Long Is This Yeast Viable For Starters?
In theory you only need one cell to generate a viable starter, although from a more practical perspective, you likely want at least a million. Again, we can always increase the amount of slurry we add to our starter to increase the number of viable cells, meaning *in theory* the total cells in our initial batch of dried slurry determines what we have to work with. More practically, you can only add about 50 ml (10 tsp) to a lab-sized starter (250 ml) starter.
I’m going to ignore the linear model in this section, as we know from my unexpected kveik drying experiment that dried kveik maintains viable cells for over 900 days. That is far longer than the 13 month maximum lifespan predicted by the linear model. Assuming a 50 ml innoculum of dried kveik into a 250 ml starter, you should be able to generate a functional starter with dried kveik:
- 58 months (4 years and 9 months), assuming you need 1 million viable cells in the starter.
- 228 months (19 years), assuming you need 1 viable cell in the starter.
But what if we use the entire volume of dried kveik? This isn’t likely possible in the real world, but its a fun question to think about. In this experiment I ended up with 1200 ml of liquid yeast slurry, or about 2160 billion (2.16 trillion) cells! Using these numbers, you should be able to generate a functional starter using all of the dried kveik from this slurry:
- 113 months (9 years and 5 months), assuming you need 1 million viable cells in the starter.
- 262 months (21 years and 10 months), assuming you need 1 viable cell in the starter.
Edit/Update: It was brought to my attention in the Milk the Funk Facebook group that others have reported 9-18 billion cells per ml of dried kveik - e.g. 4 to 8 times what I found in this study. While I am suspicious of that higher number, the lower number is certainty within the bounds allowed for given the typical range of yeast concentration in wet slurry (2-4 billion/ml) and the degree of shrinkage I observed during drying (50% decrease in volume). I suspect the difference may be due to a large portion of the yeast in my samples getting trapped in the hop material, which quickly settles out prior to trypan blue staining, and therefore potentially reducing my cell counts. What does this mean in terms of what I wrote above? The answer is surprisingly little. Because the decay in viability is exponential, a 4-8 fold increase in yeast density would add only one to three months to the "effective" life span for direct pitching, three to six months in regards to making a starter, and 1-2 years in terms of maximum recoverable lifespan.
So that’s a lot of math and big numbers…but what does it really mean, and what can we actually conclude?
- Beer of modest starting gravity (1.060 or lower) can be fermented by directly pitching reasonable volumes of dried kveik (25-50 ml) that is under 6 months of age.
- Dried kveik can be directly pitched at as much as 2 years in age, though-be-it at pitch rates which may be risky (in terms of other organisms establishing themselves in the wort) and at higher risk of off-flavours.
- If using a starter, drying kveik is a reasonable method to store the yeast over significant periods of time (up to 5 years with a good chance of recovery using a modest sized pitch (50 ml of dried slurry) and a 250-500 ml (1 to 2 cup) starter.