After getting everything organized (equipment, staff, etc.), yeast propagation can began. As mentioned previously, timing is one of the most difficult parts of yeast propagation. This guide is written in regards to yeast propagation for white juice, but the primary difference when dealing with red juice is that yeast starter creation uses saignee.
If possible, I like to have a stock of juice designated solely to creating yeast cultures. This can pose several different issues depending on how the winery operates. Larger wineries will be more successful including this in their yeast propagation program, which will in turn help streamline the production process significantly
- More tank space. On top of the tanks for yeast starts, this is another tank that needs to be dedicated to yeast propagation.
- Juice crossover. Creating yeast starters from a single stock of juice will mean blending small amounts of juice from one batch into other batches.
- Holding juice stable for several days. This is always easier said than done, particularly when only small levels of SO2 can be used to ensure the juice is inoculation-ready when needed.
- Ability to quickly heat juice. Holding juice stable usually means keeping it really cold, but the juice needs to be at yeast-happy temperatures during procedures.
Since most wineries won't be able to commit to the above, I will discuss propagation on a batch-to-batch level. Feel free to contact me to discuss propagation for larger wineries.
My experience with yeast propagation shows an average inoculum volume of 1-2%. This means the volume of inoculum is 1-2% of the total volume of juice to be inoculated. In smaller scale propagation procedures, this will likely not be achievable for several reasons, Aiming for a 5-10% inoculum volume may be advisable.
Once the juice to be inoculated is clarified, it will arrive in its fermenting tank. A small portion of this juice should be transferred to a yeast propagation tank, ideally via heat exchanger to arrive around 25° C (77° F).
The yeast starter juice should be prepared for fermentation in a similar fashion as any juice to be inoculated.
- Keep SO2 levels to a minimum. Free SO2 will ideally be below 5 ppm, but less than 10 ppm is acceptable.
- Add extra nitrogen (NH4) and fermentation nutrients.
- to the yeast starter just to make it an extra happy environment.
- Ensure the oxygen connection to the tank is working and set sparging to the necessary level. If you don't do it before inoculation, you will definitely regret it.
- Inoculate following typical procedures at the regular rate, typically 200-250 ppm. I highly recommend using a yeast rehydration nutrient during preparation.
Once prepared and inoculated, the culture can then be left for several hours. Depending on time constraints at the winery, 6-12 hours is a good period of time before the first round of analysis. Determining the analysis schedule moving forward can be based on the progress during this first time frame.
Cell count is the most essential measurement, but I also like to have brix, pH, and VA checked to ensure everything is going well. Sensory evaluation is also a must.
This is when experience with yeast propagation becomes increasingly important. Knowing how to balance all the variables involved to reach the end goal of inoculating the juice to be inoculated at the correct temperature and inoculating rate is a lot easier said than done. The balancing act becomes exponentially more difficult once you begin talking about multiple inoculations from a single culture on top of the timing of everything.
Here are a couple situational examples assuming that the culture is not sufficient for inoculation:
- Cell count for yeast starter is >300 mcell/mL. I would feed the culture more juice, decrease the temperature, and adjust oxygen flow rate upwards.
- Cell count for yeast starter is <300 mcell/mL, but >150 mcell/mL. I would check brix levels, if somewhat low than I would feed the culture more juice. I would maintain the temperature and adjust oxygen flow rate.
Determining when the culture is ready to be used for inoculation depends on several variables.
- Desired inoculation rate.
- Volume of the juice to be inoculated and the yeast starter volume.
- Yeast starter cell count.
- Temperature of juice to be inoculated and yeast starter temperature. These should be within 5° C of each other.
Inoculation is rather simple. The yeast starter needs to be transferred into the juice to be inoculated tank and mixed through. Just keep in mind that yeast populations are constantly changing within the yeast starter, so all processes are time sensitive. After inoculation, the fermentation can be tracked as per usual.