Last weekend I brewed a black IPA, using the Voss Kveik strain (post on that beer coming soon). Five days after brewday I found myself with 23 L of glorious IPA and a litre of yeast slurry. And so I embarked on an attempt to try something I’ve always wanted to do since learning about the magical powers of kveik – drying kveik for long-term storage. I kinda tried this once, drying small amount of slurry onto mini yeast logs that I handed out as part of an advent exchange. But I’ve never tried drying kveik in a more formalised and scientific manner.
Drying kveik is, on the surface, a very simple process. Traditionally, Norwegian farmhouse brewers would dip a yeast log or yeast ring, into the completed beer, and then hang that ring someplace to dry. More modern attempts have used everything from home ovens to food dehydrators, and most of them have reported success.
As luck would have it, SWMBO had borrowed a dehydrator from her dad for making some rabbit jerky. Before she had a chance to use it, I “borrowed” it, cut two rings of parchment paper to fit onto the trays, and poured out a half litre of slurry onto each rack. I then added am empty tray – it needs at least 3 to work – and then turned the unit on. This is not a temperature-controlled unit, but it generated a strong flow of air at about 40C (105F).
Five hours later most of the kveik had darkened and taken on an odd texture. It was leathery to the touch and somewhat flexible, but would shatter like glass if bent too far. I placed this kveik into a bag, but there were some thicker sections which still had some moisture. These sections were left on the dehydrator an additional half-hour, at which point they took on the same leathery texture. Once bagged, I tossed the Kveik into my home freezer for long-term storage.
The next step was to determine how viable the kveik was after drying. To determine this, 48 hours after trying I added a pinch of kveik to a 1 mL tube of 1.042 wort. 15 minutes later I gave the tube a shake, let the debris settle , and then sampled from the mid-point in the tube. This sample was stained with Trypan Blue. This is a viability dye that is excluded by living cells but stains dead cells a deep and vibrant blue.
Much to my surprise, nearly 100% of the cells in the sample were alive. This is well above the 5% that survive drying in my mailer system. The number of dead cells was too low to calculate viability – it was at least 99%! The yeast was active, fermenting enough to push the cap off the tube half hour after rehydration.
While my viability was exceptional, I am not confident that it is representative. Nearly a 3rd of the dried material I added would sink to the bottom a few seconds after mixing. It is very well possible that this materials contains a lot of non-viable yeast. Likewise, by sampling the middle of the tube, I may have biased my measurements towards yeast that had successfully rehydrated and were generating enough CO2 to begin to rise in the wort.
None-the-less, this was a very successful attempt at drying yeast. There is easily enough viable yeast in my zipper-sealed bag for a half-dozen batches of beer.
I am planning on repeating this test monthly for at least the next six months. I’ll post these results as they become available.