Yeast Management and Pitching Calculations
Yeast health and the amount of yeast pitched are both heavily correlated with fermentation performance, consistency, and overall beer quality. Below we’ll walk through the different methods for calculating pitch amounts, some rules of thumb on yeast management, and a few tips to help make the process simpler.
Note: In this blog, we will often use million cells/mL (Mcells/mL) instead of cells/mL for ease of use. McellsmL is just cells/mL
Target Pitch Rates
Pitching the correct amount of yeast for the style and starting gravity is key. Here are some rough guidelines, though different yeast strains may require more or less, so some experimentation can help.
Ales: 1.0 million cells per mL per °P
High Gravity Ales: 1.5 million cells per mL per °Plato
Lagers: 1.5 million cells per mL per °Plato
Target Pitch Rate Calculator
Yeast Harvest Timing
There are many different methods of harvesting yeast, some of which will work better or worse for different strains and situations.
Warm Cropping
Warm cropping can offer higher viability and fermentation performance if harvested at the ideal time, though the yield may be lower. If harvesting before cold crashing, the slurry should be removed 24 to 48 hours after reaching final gravity and before cold crashing. This timing is critical; missing the harvest window will result in lower viabilities than cold harvesting. Dump the remaining slurry 1-3 hours after reaching the cold crash temperature.
Cold Cropping
Cold cropping may yield lower viability than warm cropping, but the overall yield tends to be higher, and there’s more flexibility in crop timing. The slurry should be harvested between 1 and 3 days after the beer has reached cold crash temperature. Leaving yeast at the bottom of the fermenter for longer than 3 days will result in a loss in viability and fermentation performance.
Top Cropping
Yeast harvested at this stage (the peak of fermentation) tends to be more viable and vigorous, though it requires precise timing and atypical harvesting methods. In modern unitanks, you’ll need to harvest yeast off the blow-off arm to use this method. You can check out our SOP on top cropping yeast here.
Here are some additional tips for maximum viability and yield from your yeast slurry.
24 hours into fermentation, you should bleed the trub and non-viable yeast from the bottom of the tank. This will help with the purity of your final yeast slurry.
When harvesting yeast, discard the initial portion of the cone (this will be older, less viable, and higher flocculating yeast) and capture the middle portion (this will be younger, more viable yeast). Avoid pulling yeast too quickly, or you will channel through the cone instead of pulling it down.
High-gravity fermentations stress a yeast culture—it’s not recommended to harvest yeast from beers with more than 7% alcohol by volume or 17°P original gravity.
Dry-hopping decreases yeast viability, so it’s not recommended to harvest afterward. If you need the yeast, try harvesting prior to dry-hopping. You can do this by soft crashing to 50-60°F and harvesting before dry hopping or top cropping before dry hopping.
Yeast Storage
Storing yeast slurry can affect overall viability and the performance in subsequent fermentations. In general, yeast should be stored for as little time as possible (1-3 days, but not longer than a week) at 35-40°F. Slurries insulate themselves, making the center much warmer than the outside edges, causing autolysis. Mixing the yeast slurry during storage will minimize temperature variations and help with stability, though constant mixing will introduce shear stress. Exposure to oxygen will deplete glycogen reserves, so the brink should be purged with nitrogen if possible. While you must maintain positive pressure on the storage vessel or brink, excessive head pressure adversely affects yeast and should be kept below 5psi (vent regularly if necessary).
Yeast Pitching Calculations
There are many methods for calculating the amount of yeast slurry to pitch that vary in accuracy. You’ll want to choose one that is the right balance of accuracy, time, and equipment for your brewery. Our calculations will work from the following equation, adding complexity and accuracy in each case.
Pitching by Volume (No Cell Count)
Pitching by volume is the simplest calculation, but it offers the highest level of inaccuracy due to variability in density of the yeast slurry. Yeast slurries trap CO2, causing the volume to fluctuate between harvesting and settling. It can be a viable option, but care should be taken to measure volumes at the same time in the process to limit variability as much as possible.
Since we will not be performing a cell count on the actual slurry, we’ll approximate it to be between 1.0 and 1.5 billion cells per mL at a viability of 90%. These values represent a good starting place for a healthy yeast slurry harvest. First, we need to account for the viability of the yeast slurry and our target pitch rate.
Example – Pitching 10-bbls of 12°P wort, targeting a pitch rate of 1.0 Mcells/mL/°P and assuming a range of 1.0 to 1.5 billion cells per mL slurry density:
Yeast Slurry Volume Calculator (no cell count)
Pitching by Weight (No Cell Count)
Pitching by weight removes the inconsistencies created by fluctuations in slurry density. In order to use weight, we must first determine the density of our slurry. Any time a measurement needs to be performed on a yeast slurry, ensuring the total volume is homogenous is critical. Gently mixing or recirculating the slurry is recommended.
The density of the slurry can be approximated by filling a 50-mL sample tube completely full and weighing it. For the highest level of accuracy, this calculation should be performed on every harvest; however, an average of 1.1 to 1.2 kg/L can be used to approximate the slurry density.
Example – Pitching 10-bbls of 12°P wort, targeting a pitch rate of 1.0 Mcells/mL/°P. Assume a range of 1.0 to 1.5 billion cells per mL slurry density. The sample weight (slurry + tuabe) is 70g, and the tube weight is 13g. NOTE: Volume must be converted from BBL to Liters to match the density measurement in this example.
Yeast Slurry Weight Calculator (no cell count)
Pitching by Cell Count
Pitching by cell count is the most accurate option since we are directly measuring the slurry's cell density. As with the previous calculation, ensuring the yeast slurry is homogenous before measuring the cell count is critical.
Since yeast slurry is orders of magnitude denser than a fermenter sample, the slurry will need to be diluted in order to perform an accurate cell count. The main consideration when performing the dilution is attention to detail in the removal of gas bubbles from the initial slurry sample.
Cell counts on yeast slurries:
Pull a sample of at least 30 mL from the homogenized brink.
Use a clean 10-mL syringe or pipette to obtain 10-mL of slurry, tapping the sides gently to remove air bubbles.
Add 10-mL of slurry to 90-mL of distilled water.
Stir diluted sample for 2-3 minutes or until fully homogenized.
Using a stir plate can help standardize the process.
Take 10mL from the first dilution, and follow steps 3-4 to dilute again.
Proceed with your normal cell counting procedure.
Multiply your cell count by a dilution of 100x and use this value in pitch calculations. Viability does not need to be adjusted.
Once a cell count and viability is obtained, these values can be substituted into either of the above calculations depending on which slurry measurement (weight or volume) is more practical for your equipment.
Example – By Volume: Pitching 10-bbls of 12°P wort, targeting 1.0 Mcells/mL/°P. The diluted cell count is 12.8Mcells/mL, and viability is 87.9%.
Dilution Calculator
Pitching By Cell Count (volume) Calculator
Example – By Weight: Pitching 10-bbls of 12°P wort, targeting 1.0 Mcells/mL/°P. The diluted cell count is 12.8Mcells/mL, and viability is 87.9%. NOTE: Volume must be converted from BBL to Liters to match the density measurement in this example.
Dilution Calculator
Pitching By Cell Count (weight) Calculator
Pitching by Percent Solids
Pitching by percent yeast solids offers an additional method for pitching by cell count that may be more repeatable, although it requires upfront time and equipment. Pitching by percent solids involves centrifuging a sample of yeast slurry, measuring the yeast solids separate from the trub layers, and correlating percent solids to a cell count. Once a full correlation curve is created for each yeast strain, the cell count can be avoided, and only a percent solids measurement needs to be taken for each slurry.
Care must be taken when performing dilutions for cell counting when creating the correlation curve to ensure yeast solids accuracy. A correlation curve should be created for each yeast strain.
Measuring percent solids of slurry:
See ASBC Yeast-5 for full details.
Pull a yeast sample from the homogenized brink.
Follow steps from “Pitching by Cell Count” to obtain an accurate cell count.
Ensure the slurry is thoroughly mixed. Measure a 50-g sample of slurry.
Place in a beaker with a magnetic stir bar and add 2mL of 30%(w/v) NaOH to slurry and mix thoroughly.
To prepare 30%(w/v) NaOH solution, dissolve 30g of NaOH in distilled water and dilute to 100 mL and store cold until use.
Transfer the slurry to 50-mL centrifuge tube and centrifuge for 15 minutes at 2400 rpm.
Remove the tube from the centrifuge.
Record the volume of the cream-colored yeast layer, the top trub layer, and the bottom trub layer to calculate the percent of yeast solids.
Calculate the percent yeast solids with the equations shown below.
When you have enough replicates (this should happen over time on different yeast slurries), plot % Yeast Solids against Cell Count in excel, and assign a line of best fit. Use this equation to predict cell count via % Yeast Solids in the future.
Pitching with Grist
Any of these calculations can be built into your yeast stages of the Grist platform. For help using custom calculations, contact Info@gristanalytics.com.
