This guide teaches you the practical know-how of reusing kveik and other farmhouse yeasts. Whether you are just harvesting yeast for the next batch of beer or want to maintain a kveik culture for a lifetime, these tips will help. If you have missed the earlier parts of my practical guide for farmhouse yeast, please check also Introduction, Fermentation, and Farmhouse Yeast Descriptions.
Yeast is reused routinely in modern breweries and there are many similarities between modern and traditional yeast handling methods. But there are also remarkable differences that you should know if you are using kveik or other farmhouse yeasts.
This guide documents my long hands-on experience in brewing both traditional and modern beers. I have brewed with kveiks and Baltic farmhouse yeasts since 2015 and talked with many Norwegian farmhouse brewers. My experience comes from both original farmhouse yeast cultures and commercial versions isolated from them. I will update this guide as we learn more.
You’ll find more yeast handling tips from Lars Marius Garshol’s book Historical Brewing Techniques and his blog article How to use kveik. See also excellent articles on drying kveik by Sui Generis Brewing. For more information on farmhouse brewing traditions, please check my book Viking Age Brew.
What Is a Good Beer for Harvesting?
Yeast can be harvested from various kinds of beers but the brew you are harvesting from can make a difference in yeast purity and vitality.
Modern brewing texts advise against harvesting from strong beers but obviously this doesn’t apply to Nordic farmhouse ales. Most kveiks are harvested from ales of 6–9 % ABV. I would not harvest yeast from over 10 % ABV beer but some farmhouse brewers are doing even that.
So far I have harvested kveik mostly from traditional raw ales that are fairly similar to the originals. A few times I have successfully harvested kveik from a non-hopped sahti but I believe that hops have played a role in keeping the farmhouse yeasts clean from souring bacteria.
As far as I know, all Norwegian and Lithuanian brewers who maintain yeast cultures use hops. Some of the brewers infuse hops in hot wort but do not boil the hops. Science tells us that antibacterial properties of hops are mostly in the bitter compounds that result from the boil. Nevertheless, also non-boiled hops might inhibit souring bacteria.
I think that having some hop bitterness (10 IBU should be enough) is good insurance against bacteria. When I’m harvesting yeast from raw ales that do not involve wort boil, I use the hop tea method (hops are boiled in small amount of water).
In any case, I’d harvest yeast only from faultless fermentations.
Know Your Yeast
Throughout this guide, I advocate checking first from Farmhouse Yeast Registry how the original owner uses the yeast. Concerning harvest, you want to know this: Does the yeast contain bacteria? Does the original owner harvest from the top or bottom of the fermenter, or both. When does the brewer harvest yeast?
These details have the following implications:
- If the yeast contains bacteria, you need to use some hops to inhibit bacteria, unless you want a sour beer.
- Usually, the yeast has adapted to either top or bottom harvest and it might be easiest just to follow the original owner. More about this in the next section.
- Harvesting time may affect the vitality of the yeast. Proper timing of harvest depends along with the yeast culture on various factors such as the beer type, fermentation temperature, and the pitch rate. Nevertheless, the timing of the original owner gives some indication when you should harvest yeast.
When your goal is just to harvest yeast for your next batch, and not to maintain a yeast culture, you don’t need to do things exactly like the original owner of the yeast. But if you continually harvest and propagate differently than the original owner the evolution will probably change the yeast. That may be a good or bad thing. The yeast may adapt better to your process but it can also lose some of the characteristics that you like. It will be difficult to predict what will happen.
Top or Bottom Crop?
Most Norwegian farmhouse brewers harvest kveik always the same way, either from the top or bottom. Some brewers harvest from both top and bottom but they seem to be a minority.
Yeast that has been continually top cropped tends to form a thick yeast layer on the top. This kind of yeast is easy to harvest and gives vital yeast with little trub (protein and hop debris). For some kveiks, the yeasty top layer is very consistent and doesn’t sink easily. Some kveiks raise to top briskly but may also sink to the bottom soon after fermentation. I suspect that the brewers who harvest from both top and bottom have a this kind tricky kveik.
The downside of top cropping is that in the long run it can lead to yeast that rises out of suspension prematurely. Many of the top cropped yeasts need to be stirred back to beer towards the end of the fermentation. On average, bottom-harvested yeast attenuates further and makes dryer beer.
Although top crop cropping is the favorite method for farmhouse brewers, several farmhouse brewers harvest also from the bottom, Sigmund Gjernes for example. Usually, the beer is racked or scooped off the yeast before harvesting. Therefore, these brewers harvest a bit later because the yeast needs more time to sink to the bottom. Nevertheless, they want to harvest promptly. Doing this kind of early bottom crop requires some practice. You need to monitor fermentation and inspect the beer. The beer should be relatively bright before racking, otherwise, you’ll have little yeast on the bottom.
In theory, bottom cropped yeast is less vital because trub (hop and protein debris) is mixed with the yeast. I haven’t noticed any loss of vitality with bottom harvested yeast but the amount of trub depends a lot on your beer and the process.
I have occasionally bottom cropped kveik even when that particular kveik is normally top cropped. It has worked but doing this for continually would probably change the yeast character.
Time to Harvest
Most farmhouse brewers harvest soon, typically 20–84 hours after pitching. When the yeast contains bacteria the early harvest is crucial because later harvest may favor bacteria. Early harvest probably favors yeast health too.
Once I harvested Hornindal kveik after four days. The beer had been already fermented but a thick yeast cake was still on the top and I needed yeast. I top cropped this kveik and pitched it two days later into fresh wort. The fermentation was sluggish and I believe that the late harvest had caused the loss of vitality. This is only one vague data point but from now on I stick to earlier harvests.
I firmly believe that this is the best practice for top cropping: harvest after the midpoint of fermentation but before the yeast has finished. When cropping from the bottom, you need to wait for the yeast to drop down, but don’t wait for a week.
How Much Yeast Do I Get?
The top crop should yield at least one gram per liter of beer. Once you learn the techniques and get to know the yeast you can gain a lot more. A good bottom crop can yield six grams of yeast per liter. These are my experiences from raw farmhouse ales. A lot depends on the beer as well.
Can you harvest so much from the top that the beer won’t ferment well? No, in my experience. Plenty of yeast will remain in the beer.
Yeast Storage Options
Farmhouse brewers store their yeast either dried or as a slurry.
Drying works only with yeasts that have been accustomed to drying. All kveiks can be dried but Baltic yeasts may not always survive the drying process. Both top and bottom harvested kveiks can be dried. An outstanding advantage of drying is longevity: with a proper cold storage dried yeast can survive for years or even decades. Against all odds, kveik dried in the open air can be very vital and clean from bacteria.
Storing yeast as a slurry is easy. Just put top or bottom harvested yeast into a jar and store the jar in a cold place. Both farmhouse brewers and commercial brewers have been doing this for a long time. More than a hundred years ago some farmhouses stored their house yeast in a glass bottle that was kept in a well.
The slurry can survive a year in cold storage but to ensure good yeast health it is better to use the slurry within six months. I have found this rule useful in my home brewery and I heard it also from Sigmund Gjernes.
I like to use both methods. The slurry is an easier option when I need yeast within the next few months. With slurry maintaining good brewing hygiene is easier. So far I have used dried yeast only for my traditional farmhouse ales. With beers that are bottled and stored longer, I cannot help thinking all the dust and dog hair that settles on yeast while it dries.
Yet, I couldn’t maintain several farmhouse yeasts as a slurry. I simply don’t brew often enough with each yeast. Also, many farmhouse brewers use both methods. When the slurry is the main method, dried yeast can be a backup.
Tips for Harvesting
When harvesting from the bottom, you can just scoop the yeast after racking or swirl the yeast into the remains of beer. You can also pour a small amount clean water to fermenter to make swirling easier. If you plan to dry the yeast and you have thin slurry, pour the slurry first into a jar so that you can decant off the most liquid.
If the yeast sediment on the bottom contains lots of trub, you may want to rinse the yeast with water. This classic method is well documented in the homebrewing text, see for example this guide by American Homebrewers Association. I haven’t seen that rinsing makes a big difference but that may well be because normally I have little trub in the fermenter. However, the amount of trub varies a lot depending on the beer style and the process.
Modern top cropping guidelines usually advise discarding the first dirty skim. This is good advice if lots of trub arise to the top along with yeast. I haven’t seen a need for this “dirt skim” but again your mileage may vary.
Procedure for Storing Yeast
Storing yeast slurry is simple. Just put harvested yeast into a clean disinfected jar and store the jar in the fridge or other cool place. Colder the better, as long as the yeast doesn’t freeze. Wait a few hours before fully tightening the jar lids, because the yeast might be still releasing some carbon dioxide.
The procedure for drying farmhouse yeast is:
- Spread top or bottom harvested yeast onto a baking paper. 1/2–1 cm thick layer works well.
- Put the sheet into a dry and warm place at 25–40°C (77–104°F). Beware temperatures above 45°C (113°F).
- The yeast has been completely dried when it looks like cracked earth and breaks easily into flakes. Depending on your drying method, this may take anything from five hours to a few days.
- Crack the yeast it into smaller flakes and put them into ziplock bags or vacuum-sealed bags. Store cold. The freezer is the best long term storage but the fridge or cold cellar will also work.
You have several options for the warm place for drying. I dry kveik on the top of a wood-fired oven that doesn’t get too hot. Food dehydrators can work very well but some models can heat up above 50°C (122°F) which is probably too much for yeast. Microbiologist Richard Preiss informed me that yeast begins to die above 48°C (118°F). A simple solution is to put the yeast on a baking parchment into a kitchen oven with a thermostat set to 30–40°C (86–104°F). Check the temperature yourself because oven thermostats can be very imprecise.
Norwegian farmhouse brewers generally dry kveik under the open air without any cover. I have been doing this as well and I can attest that it works. No doubt some bacteria and wild yeast floating in the air will settle on the kveik but the hardy kveik remains in charge. Incredible!
Here’s a neat trick that some farmhouse brewers are using: spread the yeast on a baking paper and let it partly dry. Then spread another layer of slurry on top of that. In this way, you’ll get thicker flakes that don’t pulverize so easily.
Although slurry and dried yeast will usually survive for the timespans mentioned above, the yeast will gradually lose vitality. This can be taken into account by adding more yeast and making a starter early enough (see Fermentation part of this guide). Sui Generis Brewing has reported fascinating viability tests of dried kveiks.
Keeping the Farmhouse Yeast Going
A modern yeast handling rule says that yeast can be pitched at most ten times before it needs to be recultured. This is probably a good rule with a single strain brewer’s yeast but it doesn’t apply to Nordic or Baltic farmhouse yeast.
Before modern laboratory methods, brewers just kept repitching endlessly and borrowed yeast from a friend if something went wrong. With farmhouse yeast, you can follow the same traditional route. This method will work with both original mixed cultures and commercial strains isolated from them.
When repitching, you need to monitor the yeast vitality and purity. Lactic and acetic acid bacteria are the typical enemies of farmhouse yeast culture. Hops will inhibit most lactic acid bacteria but they aren’t a cure-all solution. Some lactic acid bacteria and most acetic acid bacterias are tolerant to hop bitterness. Acetic acid bacteria need oxygen and for this reason, it isn’t the most typical farmhouse brewery contaminant. Nevertheless, occasionally also acetic acid bacteria can be a nuisance.
It is best to store yeast from several batches. If one batch goes sour the yeast from earlier batches might still be good. Sometimes you might have to dump the yeast and start over with fresh yeast.
This ends my four-part guide to farmhouse yeast. I hope that you found the guide useful. I’ll update this guide whenever we learn more, and there’s certainly a lot to learn.