When done correctly, re-pitching yeast is a great way to reduce costs, packaging waste and transport emissions while making your operations more self-sufficient.
However, you’ll need to manage your yeast correctly to prevent possible contamination and yeast health issues.
So, what does correct yeast management involve? The following seven-step process provides an overview.
But, before we get started, there are a few questions you’ll want to ask. Firstly, has the yeast performed as expected fermenting the current beer? Secondly, does the current beer batch taste as you’d expect? And lastly, is the current batch free of any off flavours? If the answer to any of these questions is no, it’s not advisable to re-pitch yeast from the batch.
Step 1. Determine optimal time to harvest your yeast.
Typically, you’ll want to do this on day five or six, but it will vary depending on the beer you’re brewing and the yeast strain you are using. You want to remove the yeast for re-pitching after it has fermented the beer, but before it starts dying off from lack of food. It’s also advisable to harvest yeast before adding additives or hops if you are dry hopping as these can introduce contaminants that can be passed on to a new brew via the harvested yeast.
Step 2. Dump trub and dead yeast and harvest healthy yeast.
It is recommended to harvest yeast from a conical-bottomed fermenter as these make the process easier and more efficient. After the trub has been discharged it’s important to select yeast slurry that is a light colour with a creamy texture. Harvesting from the middle of the yeast bed will give the most consistent results.
Step 3. Determine cell count and percentage viable cells.
Analysis of the yeast along with sensory analysis will let you know if the yeast slurry has a viable cell count, and will help you in the next step.
Step 4. Calculate volume/weight of yeast needed for the next brew.
Once the percentage of yeast solids per volume of the slurry has been established, you’ll be able to work out how many billion cells per mL the slurry contains. This will inform the volume of slurry that you’ll pitch into your brew.
Step 5. Setup aseptic transfer of yeast from fermenter A to fermenter B (known as going from cone to cone), or to a sterile container.
During this step, you’ll want to ensure all surfaces that come into contact with the yeast slurry are sterilised. This is a good time to record the generation of the yeast, or how many times it has been re-pitched and the brew that it has come from.
Step 6. Store yeast at 2C for up to a week.
Over time the yeast will become less viable. After two weeks in storage, it is likely viability will be down to around 75 percent or lower, which is typically too low to ensure a successful fermentation.
Step 7. Pitch the yeast.
When ready to pitch, mix yeast and transfer the appropriate volume of yeast to your fermenter.