Mar 14, 2024
Reusing a yeast pitch is one of the best ways to improve consistency in your beers while also helping your brewery save on material costs. However, it isn’t as simple as grabbing a spoonful of fresh yeast cells from inside the fermentor. With proper sanitation and procedures in place, yeast can be repitched for multiple generations and can sometimes experience improved performance during subsequent generations.
Harvesting yeast
The method you use to harvest yeast cells depends on the fermentation vessel. Some tanks have ports that make it easy to pull yeast from the bottom, while others require a bit more finesse to get a successful crop. Here’s a quick overview of some common tank shapes and how to go about harvesting the next generation of your yeast pitch.
CCVs are easily identifiable by their cylindrical top and cone-shaped bottom that provides a convenient place for yeast and trub to settle.
Cylindroconical vessels
Many modern breweries use cylindroconical vessels (CCVs) to complete fermentation. These tanks are designed with a cone-shaped bottom where yeast can settle in the later stages of fermentation and can be easily separated from the beer. This is typically where brewers will harvest yeast for future propagation and reuse.
At the very bottom of the cone, there is a layer of trub, proteins, hop matter, and the now-dead yeast which flocculated first and is no longer usable. This layer is usually dark beige/brown in color. When harvesting, avoid collecting this bit of precipitated mass and wait for a lighter beige-colored slurry to come through. This is where the freshest, healthiest yeast is, and this is the part of the cone that will give you the best results. As the slurry begins to thin out, stop harvesting, as what’s left will be less dense and will contain more of the less flocculent cells.
Harvesting from the cone is as simple as opening the valve and letting the slurry flow out and into a sanitized yeast brink. It’s important, however, to pay attention to the flow rate: too fast and you may wind up creating a channel through the slurry and beer will be all that comes through the pipes. Take it slow and if you start to find that some beer is sneaking out with the yeast, close the valve and resume collection once the cone has re-settled. You’ll also want a small amount (1 – 3 psi) of pressure on the tank to avoid creating a vacuum when the yeast slurry begins to flow out of the valve.
Another method of harvesting yeast is to fully rack the beer off the yeast slurry before collecting from the cone. This will allow you to dispose of the trub and collect the rest of the slurry without worrying about wasting any beer.
Flat-bottom fermentors are often — but not exclusively — seen in lager-focused breweries and look like long, horizontal cylinders. Some of the world’s largest beer producers have large, horizontal flat-bottom fermentors that measure 17 feet in diameter and more than 75 feet in length.
Flat-bottom fermentors
Harvesting from flat-bottom fermentors, like horizontal lagering tanks, is more difficult than harvesting yeast from CCVs, requiring beer to be carefully racked out of the fermentor with the yeast cake left intact. While this slurry will have distinct layers like the yeast in the cone of a CCV, they will be thinner and less distinct, which may increase the possibility that harvested yeast is less viable. This method is perfectly viable, but requires diligent record keeping and viability testing at each generation.
Kräusening
Kräusening is a term derived from the German word kräusen for using beer from an active fermentation (kräusen) to initiate fermentation in a fresh batch of wort. It can be an effective way to repitch yeast without having to manually crop it. If one tank is actively fermenting (about 48 hours after the initial yeast pitch), you can take about 20% of the volume of that tank and pitch the fermenting wort into fresh, uninoculated wort to kick off fermentation. While this will reduce the volume of the finished beer in the first tank, it is an effective way to maintain consistency from batch to batch.
Kräusening is also used to prime beer before packaging. The fermenting beer brings fresh sugars, nutrients, and yeast to the finished beer, which has had much of its sugar and nutrient stock consumed by the initial pitch of yeast. By adding kräusen, a limited fermentation can occur in the package to carbonate and condition the final product.
Open fermentors are basically large vats with an open top, which reduces pressure within the vessel as carbon dioxide is able to escape freely. Brewers using open fermentors take great care to ensure the beer is transferred to a closed system before fermentation activity slows and contamination can occur.
Open-top fermentors
While open fermentors are less common in modern brewing, there are still many breweries around the world who use these vessels and take advantage of the ability to top crop. Not all strains of yeast are ideal for top cropping, but many ale strains have the potential to rise to the top of the fermentor during the first 24 – 36 hours of fermentation, or when about 50% of the original gravity has been consumed. The initial “head” to appear on the fermenting beer typically contains trub and oxidized hop polyphenols and should be skimmed off and discarded, but the second bit that rises will contain fresh, healthy yeast cells that can be skimmed into the yeast brink. Take care when skimming not to go too deep, however, as the bottom of the “head” will have more trub and undesirable material that should be skimmed and discarded.
Lagers can also undergo fermentation in open-top fermentors, but the fresh yeast created during fermentation tends to flocculate to the bottom, rather than rise to the top. This yeast can be collected from a valve at the bottom of the fermentor, just as it’s done in CCVs.
Storing yeast
A yeast brink is a vessel designed to store your fresh yeast slurry between pitches. These vessels can be dedicated sanitary brinks designed for this purpose, a repurposed fermentation vessel, or kegs that have been converted to be used as a brink. Some brewers use stainless steel containers with lids or even plastic buckets, but we wouldn’t recommend using plastic as it can be incredibly difficult to ensure cleanliness. No matter what kind you use, sanitation is paramount: the ideal yeast brink is easy to include in your CIP routine, and even better if you can sanitize it with steam.
Your yeast brink should have the ability to both hold and relieve pressure. Remember that the yeast you collect has been creating CO2 during fermentation, which can build up in storage. Vent CO2 pressure daily during storage, or outfit your brink with an airlock or blow-off tube. Yeast stored warm, with exposure to oxygen, and/or under extreme pressure (more than 2 – 3 psi) can lose viability. Keep your brink in cold storage (0 – 4°C/32 – 39°F) and use your stored yeast quickly! We recommend a few days, no more than a week, in the brink — fresher is always better.
Another great feature to have in your yeast brink is some kind of method for measuring the fill volume and/or total weight of the slurry in the container. This will help you when determining how much yeast to pull for your next pitch. It can also be nice to have sanitary ports where you can pull samples for viability and contamination testing, too.
Repitching yeast
Once the yeast is harvested and stored cold in the brink, you should plan to use it as soon as you can. Remember that yeast is an active, living organism, and much like a fresh pitch of liquid yeast, it will start to lose viability as time goes on and you may see increased lag times. When stored correctly — cold and away from light and oxygen — yeast can last anywhere from a few days to a few weeks in the brink.
Repitching methods
The actual method you use to repitch yeast will depend on how the yeast is stored. If using a converted keg, you can push the yeast into the cone with CO2 before or after filling the tank with fresh wort, or even inject it directly into the wort line if you have the proper connection. You can also pour yeast into a sanitized fermenter through a manway, if your storage container is pourable.
Repitching by volume or weight protocol
Now that you’ve got yeast in the brink, it’s time to figure out how much you need for your next batch. Start by pulling a sample and doing a quick sensory check — compared to a fresh pitch, how does the yeast smell? Bad yeast will smell sulfury, meaty, or just plain bad. If you’re getting the sense that it doesn’t smell like it’s alive, you probably won’t want to use it.
Estimating your pitch
You can estimate your cell count by seeing how much of your slurry is yeast solids. Pull and chill a sample and allow it to settle, then see roughly how much of it is solid matter. Assuming about 40% of your harvest will be solids, you can estimate approximately 1kg yeast slurry per barrel for “normal strength” ales (4 – 6% ABV), and about 2kg per barrel for lagers. This estimation will vary depending on the yeast strain.
If conducting cell counts
If your slurry sample passes the smell test, degas it and then dilute it at a 1:500 ratio. Mix well, then do a stain test to check viability followed by a cell count. If you are seeing 70 – 200 cells present in a 4x4 grid, you’re in good shape. Use your viability and cell count data to calculate the volume or weight you’ll need to get a full pitch.
Cone-to-cone repitching
Repitching cone to cone is an option if your timing is right and you have a clean tank ready to go. With a healthy fermentation, you should have enough yeast to pitch two tanks of equivalent volume. Be sure to dump the trub, then take a yeast sample from your initial fermentation vessel (FV) and check cell counts and viability before you start this process — no need to waste an entire batch because you gave it unhealthy yeast! If your yeast in the cone of FV1 is good to go, outfit each tank with a sight glass so you can visually track progress and get a rough estimate of transfer times.
A simplified diagram of how to repitch yeast from cone to cone. This method takes some practice, but can be an efficient way to keep your yeast pitch moving without extended storage times. Note the color change in the hoses — blue represents when hose sections should be open and wort, yeast, sanitizer or water are able to flow through, while red represents what sections of the hose should be blocked off at each step.
Before you begin to move yeast from FV1 to FV2, run a CIP loop through all transfer hoses, starting at the whirlpool, and then direct wort into the tank. Slowly transfer wort through a sight glass at FV1 and use a stopwatch to get a rough estimate of how long it takes the wort to start moving through the sight glass at FV2. This will give you a general idea of your wort flow rate so you can estimate how long your overall transfer will take. Keep in mind that wort moves at a different rate from yeast in the cone — you may want to move your yeast from FV1 past FV2 and collect a sample into a bucket to estimate how quickly the more-viscous yeast slurry is moving, then close off the loop and begin transferring yeast into FV2.
After you’ve transferred some wort to FV2, pause the transfer by closing the valve from your wort supply. Then, slowly open the valve on FV1 to allow yeast to flow through. Remember that transferring yeast too quickly can allow beer to punch through the slurry. Keep an eye on both sight glasses to track yeast transfer, and using your rough flow rate calculation, close the valve on FV1 once you’ve transferred enough yeast slurry. This method isn’t as precise as harvesting, counting, and repitching the exact amount of yeast, but your pitch accuracy using this method can be +/- 20% of the desired volume and you can still get good results.
Managing several strains
If you aren’t storing yeast in a brink and you know you’ll be transferring yeast from cone to cone or harvesting and reusing the same day, your fermentor is going to be occupied for about three weeks. If you want to be pitching from that yeast every week, you’ll need three tanks to manage it effectively. You’ll be flipping and cleaning a tank, getting fresh wort in, and pitching again. It has to be on that three tank rotation.
If using strains with a really quick fermentation timeline, you can minimize the number of fermentors that you need. Kveiks for example can tie up fewer fermentors because you’ll be pitching more frequently. On the other hand, if you’re making lagers you’ll need more tanks because you’ll have more occupancy time.
Monitoring performance
The main question we get about repitching is “how many generations can I get out of one pitch?” The short answer is: it depends. If you are only brewing minimally hopped 4% ABV beers, and doing so frequently enough that the newly cropped yeast isn’t sitting around very long, you could get 10+ generations. On the other hand, if you are brewing 8% DIPAs or strong stouts, you’ll be lucky to get 2 – 3 generations. To truly understand how far you can take your yeast, you will need to monitor its performance from batch to batch and make sure it’s always in good shape for a healthy fermentation.
As you monitor your yeast, keep an eye on details like:
- Yeast source. This one may seem obvious, but keep track of where you sourced your initial pitch, the cell count you received, and what strain it was.
- Generation number. The initial pitch of yeast would be considered the first generation, while the second generation refers to the yeast culture that is cropped from the first fermentation. Once the second generation is used to ferment a beer, the yeast culture cropped from that fermentation would be the third generation, and so on.
- Diacetyl clearance times. How long does it take for diacetyl to reduce to your house specifications?
- Harvest timing. At what point in fermentation are you harvesting yeast?
- Sensory. Take notes on how the yeast smells each time you harvest it. Eventually you’ll start to recognize when your strain has changed to the point where you won’t want to use it again by just taking a few sniffs.
- Dry hop. If you are dry hopping your beer, there are some variables you need to consider when harvesting yeast — see our guide for harvesting yeast in dry-hopped beers.
As your beer ferments, take daily cellaring records. Variations can indicate yeast stress and aging. This is a great practice for every batch and will help you better understand trends in the cellar and from generation to generation. By taking small samples throughout fermentation and testing, you can check for contamination, viability, and overall cell counts to see how things are progressing, and whether you need to adjust your processes.
Related Collections
