Inspired by Sandor Katz's The Art of Fermentation, I wanted to make some soda flavors I've never had before. According to Katz, a soda can be made by adding an active starter culture (in this case, whey derived from yogurt) to any sweetened liquid. I thought I'd riff on ginger ale and make sodas from fresh turmeric and galangal, ginger's rhizomatic relatives.
Step 1: Gather Whey
Whey can be derived from any cultured milk product, including kefir, buttermilk, raw milk and yogurt. Kefir, buttermilk and raw milk will naturally separate into curds and whey after sitting unrefrigerated for a few days. But if you collect whey by straining yogurt, not only is the whey available immediately, but the yogurt becomes labneh: a tangier, thick and spreadable yogurt.
Line a bowl with a few layers of cheesecloth and empty some yogurt into it. A 32 oz. tub of Nancy's yogurt yields around 2 cups of whey. Close up your package of yogurt and suspend it so the whey can freely drip out. If your bowl is deep enough, you can lay a wooden spoon across it and tie the cheesecloth sack to the spoon.
It won't take long to get enough whey for the sodas, but I set mine up overnight for the sake of the labneh.
Step 2: Brew Decoction
Using a microplane, grate turmeric into one pot and galangal into the other. For each soda, I steeped 100g of rhizome in 6 cups of water, but you have a lot of leeway with these numbers. Bring each pot to a boil, then cover and simmer for about 15 minutes. Taste it at this point: if the decoction comes out too strong, you can water it down; if it's a bit weak, make a smaller, concentrated batch to add to the original batch.
Step 3: Strain Decoction
When you're happy with the decoction's concentration, strain it into a jar or bowl. I used a mesh fabric bag I bought at a brew store, but a few layers of cheesecloth or a metal strainer would do the job. While the liquid is still hot, add sugar, tasting as you go until it's the right sweetness for you. I ended up adding about 1 cup of sugar to 6 cups of liquid.
Heat kills the living cultures in the yogurt, so let the decoctions cool off to body temperature, either on the counter or in the fridge.
Step 4: Add Whey to Decoctions
Once your decoctions are cool, add the whey at a ratio of roughly 1/4 cup whey for every 4 cups liquid.
The decoctions have been brought to life! Stir them up and cover with some cheesecloth.
Allow the cultures to get nice and active by letting them sit out for a day at room temperature.
Step 5: Transfer to Sealable Bottles
Once they appear active, transfer to sealable containers and close them off. Crack open a lid after a couple days to gauge how much carbonation has built up.
Once they're bubbly enough for you, refrigerate and drink up!