Pour the freshly squeezed lemon juice through a fine mesh strainer to remove larger pieces of pulp and seed.
Add the cane sugar to a heatproof half-gallon jar (or divide it between two quart-sized jars).
Pour in 1 cup of hot water, stirring it together with the sugar until it dissolves.
Add in about 4 cups of the cool water, stirring to combine.
Stir in the lemon juice and whey.
Pour in more cool water until the jar is filled to within 1” of the top.
Add a plastic lid or pickle pipe (if your home is warm) and secure it.
Place the jar in a warm place and ferment for 2-3 days.
Once the lemonade has fermented you can add the jar directly to the fridge to chill or decant it into swing-top bottles for single servings.
Serve chilled and with ice (if desired). Keeps for up to 2 weeks in the fridge.
Notes
Alternative starter cultures: If you don’t have any whey leftover from straining yogurt or kefir, you could use finished water kefir, kombucha, OR ½ cup of ginger bug if you don’t mind a lemon and ginger flavor.