Old Mother refers to a special batch of Kombucha that is cultivated solely to be used to inoculate future batches aka as starter liquid. The purpose is to provide low pH starter that will drop the pH of the sweet tea quickly for food safety requirements while also giving the brewer more concrete measurements for how much brew to make to satisfy production yields. It also prevents potential contamination from being put back into the system by keeping starter liquid production separate from Kombucha for sale.
Calculation to Determine Size of Old Mother
The first step is to determine how much starter liquid you need per brewing session.
- Determine the size of your batch for the brewing session.
- Calculate how much starter liquid you need – i.e. 1 cup per gallon (8oz/gal) or 10% of the batch, etc.
- Once you know how much starter you need per typical brewing session, then multiply that number by 2. That will yield the size batch and vessel that will be needed to house the “Old Mother.”
Example
I want to brew 100 gals of Kombucha with 10% starter liquid. That means I’ll need 10 gallons of starter so my Old Mother fermenter needs to have a capacity of at least 20 gallons.
The rule of thumb is to have an Old Mother that we take no more than 50% (up to 60% in a pinch) in order to maintain the proper ratio of yeast to bacteria.
How To Set Up Old Mother
- Set up a large continuous brew vessel with sweet tea, cultures & starter liquid.
- Pull off up to 25% of the vessel for the first few weeks until the starter liquid is nice and strong, then can increase to 50%. We do not recommend pulling more than that to prevent imbalance and over yeasting.
- Top off with sweet tea as soon as starter is drawn off to create a consistent supply.
- Extra cultures may be harvested to start new batches or disposed of depending on each brewers’ needs. Determine quality culture via pinch test.
- For vessels over 5 gallons, double the starter liquid (2 cups per gallon) and halve the amount of culture so there is more oxygen available during the primary fermentation. This will kickstart the fermentation while also giving enough oxygen for alcohol to be converted to acids (thus lowering the potential alcohol content).
- May need to filter the starter depending on how many yeast bodies are present and if controlling for alcohol. WARNING***Over filtering can lead to off flavors and an imbalanced brew.
- Every month or two, clean out entire vessel & filter liquid to maintain appropriate balance. Cadence will be determined by how quickly the Old Mother gets too sour or smells off.
Old Mother Maintenance
You will know its time to clean out your Old Mother when she starts to smell funky. For example, if you smell acetone, that’s a sign that the yeast have over accumulated and need to be disposed of to refresh the balance of bacteria and yeast in the Liquid SCOBY. The cadence will be determined by your own process and environment (temperature, humidity, etc).
- Decide if you are simply placing the liquid back into the vessel or if you will be refreshing with only a portion of the liquid.
- Drain the Old Mother vessel of the liquid you intend to use/keep.
- Allow the yeast bodies to remain undisturbed at the bottom of the vessel.
- Dispose of the slurry created by the yeast bodies and some liquid.
- Clean the vessel thoroughly according to your sanitation procedures.
- Remove or trim the Solid SCOBY.
- Refill the vessel with the liquid and top off with fresh sweet tea aka nutrient solution.
