Reduce, Reuse, Repitch (Part 1)

Almost a year ago, we started a project geared toward better understanding how our core yeast strains behave when they are reused in the brewery. The common industry term for this is 'repitching'. The goal was to look at the health of our four core strains over 12 generations, so that we could track changes and spot some potential areas of improvement for the yeast management practices that we recommend. We've shared this data at a few conferences, but also think it might be helpful to a broader audience of brewers! 

Knowing your best practices when repitching is important and makes financial sense to brewers. There really wasn’t a lot of research out there on repitching yeast other than lager strains in macro settings, so it was important for us to try to develop this knowledge in house. For us yeast nerds, we want to ensure that craft brewers using our product get as much value out of it as they can, so we set out to investigate exactly what kind of TLC each strain needs to give them the best quality and flavour results, and last for many generations.

The strains we tested were Vermont, Cali, Foggy London, and Old World Saison (OWSB) blend. The recipe was brewed weekly and kept consistent (20L pilot batches), and yeast was added at the recommended rate for each strain (10 Mcells/mL for the ale strains, and 7 M cells/mL for OWSB). Once each of these was finished fermenting, we collected the slurry not only to repitch, but also to test viability (and a few other parameters) weekly for 8 weeks. We also ran a sensory panel on the beers produced by each ferment.

Experimental design of the repitching project

Because of the way we set up the experiment, the first 3 generations of these had worts oxygenated using compressed air. Using a dissolved oxygen (DO) meter, we measured the wort to have about 6 - 6.5 ppm of O2 after aeration. After that, generations 4-12 were oxygenated with pure O2, to about 15 ppm. From this change we could determine one of our most important findings: which strains benefit the most from the addition of oxygen. Here are some of our results below.

Percent viability of yeast at time of harvest (end of fermentation) over 12 generations
 

The first thing we noticed was that viability of yeast cells at the end of fermentation was much higher with more O2 in the wort (generation 4 – onwards). This wasn’t true so much for Foggy (it went on to show itself as a sturdy, resilient strain in a few other metrics too). Cali, Vermont, and OWSB showed strong improvements in viability when they were fermented in heavily oxygenated wort.  

The next graph shows the impact of oxygenation on shelf life of harvested yeast. Darker lines are earlier generations, greener lines are later generations.


 

Viability of harvested yeast slurry over time in cold storage. 

From this we can see that Cali and Vermont in particular had a much improved shelf life with a higher oxygenation before fermentation. In a real world scenario, this would be like a brewer storing a slurry of Vermont in the cold room, then coming back 3 weeks later to repitch into a new batch, and finding out that the yeast has only 50% viability, and probably shouldn’t be reused. We don’t want that. 

As you can see from the Cali and Vermont graphs, the slurries that were cropped from oxygenated wort ferments (later gens, lighter green colour) fared much better and maintained a healthy viability for much longer. This is definitely something to consider when using Cali or Vermont yeast, and repitching for many generations.

As far as the actual fermentation goes, for Vermont, oxygenation also sped up attenuation, especially in the first 24h. The other strains were less sensitive to oxygen in that regard. Cali and Foggy were fairly consistent throughout all 12 generations, while OWSB fermentation was primarily dependent on pitch temperature, not O2 levels. 

 Fermentation curves for the repitching experiment. 

An important takeaway from this is that different strains have different oxygen requirements. To make a consistent, good quality product with many generations of yeast, know your strain’s particular needs and quirks. We’re happy to answer questions about our products if you have any.

Next up in Part 2 of our repitching project, I will talk about our sensory panel results, and trends in flavour profile changes when yeast is repitched.