Apr 23, 2024
While you may already be tracking parameters like gravity and pH throughout fermentation, adding cell counts to your data-collection checklist can provide valuable insights into the health and performance of your yeast. The advice we give most often to brewers is to take detailed notes, and with good reason — notes are your key to understanding the past and planning for the future.
This introduction is designed to explain some of the theory behind why we count cells and what to do with that information, rather than getting into the specifics of cell counts and understanding viability (for now).
Why cell counting is important
So, you’ve invested in the equipment as well as time and effort into performing cell counts during fermentation, but what’s the payoff? What do you do when you’ve got a number, and what do those numbers actually tell you?
When you obtain a cell count, you’re essentially gaining a glimpse into the health and predictability of your yeast population. There’s a lot you can do with that information:
Assess yeast health: At the start of fermentation (“high kräusen” or the first 24 – 48 hours), high cell counts indicate a robust and healthy yeast population, primed to ferment wort efficiently. Conversely, a low cell count may suggest stressed or sluggish yeast, which could lead to incomplete fermentation or off flavors in your beer. What is considered to be “high” or “low” cell counts depends on your yeast and process. High kräusen yeast counts can range from 60 – 120 million cells/mL and are influenced by pitch rate, wort nutrients, oxygenation, and temperature. The key here is to monitor and develop a target cell count that you can test early in fermentation that can be used to optimize fermentations and potentially get ahead of a problematic fermentation.
Monitor fermentation progress: Cell counts provide a snapshot of fermentation activity at various stages. By comparing cell counts over time, you can track the rate of yeast proliferation and gauge the progression of fermentation.
Determine harvest timing: Late-stage cell counts help determine the optimal timing for yeast harvesting. By assessing viability and cell density, you can identify the peak window for harvesting yeast for future batches.These counts can be less reliable from the sample port as the yeast begins to flocculate and settle. At this point you can shift to counting the yeast in the cone and determining the timing for harvesting and repitching.
Calculate pitching rates: With thorough cell count data, you can calculate pitching rates for subsequent batches. Whether aiming for consistent flavor profiles or experimenting with different fermentation characteristics, understanding your pitch rate is key.
When to count cells
Timing is crucial when it comes to cell counts. Consider taking measurements during key phases of fermentation, like during the growth phase, approaching terminal gravity, and even during storage.
Counting early
Early cell counts offer a window into yeast health and growth during the critical initial stages of fermentation. By knowing your pitch rate, you can monitor how vigorously your yeast are multiplying and adapting to their environment in the first 24 to 48 hours and get a better understanding of growth rates.
During the growth phase: Begin counting shortly after you observe signs of yeast activity. How quickly are your cells multiplying? A good way to gauge yeast activity is to determine the percent of budding to total yeast cells (budding index). Early fermentation can have a budding index of 1, where a stressed or fermenting population might be <0.2.
If you see that all of your visible cells are budding, you have a budding index of 1: this indicates very active growth, when all the cells are multiplying. The example on the right indicates a budding index of 0.2, where only 20% of the cells are budding.
Throughout peak kräusen: Take measurements at 24 and 48 hours post-pitch. Does this growth correlate with changes in gravity? If your typical peak cell counts are 80 million cells per ml, 60 million per ml might indicate a slower/sluggish fermentation or potential for a longer period for diacetyl clearance. It might also indicate that the tank temperature is improperly set, or your oxygen supply is low and in these cases allow you to make changes to prevent further issues.
Mid-fermentation is when yeast activity is in flux, with healthy cells flocculating, and both dead and active cells still in suspension. Taking cell counts at this stage may result in inaccurate data.
Mid-fermentation counting
Cell counts during the bulk of fermentation (about 48 hours after pitch to about 24 hours prior to terminal gravity) can be challenging and unreliable due to a couple of factors:
Tank stratification: The inside of your tank is churning as yeast does its thing, and in most cases this leads to adequate mixing and homogenous counts in active fermentation. However it is possible for your tank to stratify into separate zones of fermentation, leading to uneven distribution of yeast cells throughout the vessel.
Flocculation: Cell counting is most effective when your sample is homogeneous and representative of the overall batch. As the yeast begin to flocculate, cells will begin to settle and lead to areas of concentrated yeast or no yeast. Cells may collect at the level of your sampling port, skewing your counts from day to day, or even hour to hour. Another issue related to flocculation is viability — you may see a drop in viability when collecting from beer because dead cells don’t flocculate. So in the beer you may see a dramatic drop in viability but in the cone the majority of cells are viable.
Counting later
Late-stage cell counts provide insights into yeast viability and help determine the optimal timing for yeast harvesting and repitching. Since yeast strains behave differently, understanding the specific harvesting and repitching windows for each strain is essential for maintaining consistency from batch to batch.
Approaching terminal gravity: About 24 hours before you anticipate reaching terminal gravity, ask yourself a few questions:
- Can you see the yeast settling in the cone?
- Is the density high enough for harvest?
- When fermentation is complete, how many cells can you expect to harvest?
- Can you see the yeast concentration increasing in the cone?
- Is today too early to harvest?
Day-to-day cell counts from the cone can double and tracking this will allow for more successful harvesting and repitching.
The chart above tracks gravity readings throughout fermentation, with the shaded areas representing your ideal windows for taking cell counts — the beginning and end of fermentation.
Counting during storage
If you are planning on keeping your yeast in storage for a number of days, tracking cell viabilities can be a great indication of how long is too long and ways to improve your yeast storage practices. It can be challenging to track cell viability in stored yeast, but pairing the cell viability with the resulting yeast performance can be a good place to start setting limits to what you can repitch.
Takeaways
Data isn’t useful unless it has purpose, and understanding when and why to count cells will help you refine your processes and ensure each batch meets its full potential. By incorporating cell counts into your data collection, you’ll gain valuable insights into yeast behavior and fermentation activity. Plus, the more you do it, the more valuable the data becomes — tracking throughout generations will help you understand what would be considered a high or low cell count, plus how many generations you can expect from a single pitch. Leverage this data to make informed decisions and optimize your cellaring procedures to take the guesswork out of fermentation.
