Warm the raspberries, sugar and water together in a saucepan over medium heat, stirring until the sugar fully dissolves. Then, turn off the heat and allow the soda base to cool to room temperature.
Strain the soda base into a pitcher, discarding the raspberries. Stir in the strained ginger bug.
Pour the raspberry soda base into flip-top bottles, allowing 1 to 2 inches of headspace. seal the bottles, and allow them to ferment at room temperature 2 to 3 days, keeping in mind they'll ferment faster at warm temperatures and more slowly in a cool kitchen. Transfer the bottles to the fridge to allow the bubbles to set, about 3 days. Open carefully and serve over ice.
Notes
How long will the soda keep? Homemade, fermented sodas should keep about 3 months in the fridge, and potentially longer.Substituting the sugar. You can substitute honey, maple syrup, coconut sugar or any caloric sweetener for the whole, unrefined cane sugar.Substituting ginger bug. Ginger bug works best as a starter culture for raspberry soda, but you can also use fresh, active whey from kefir or yogurt, kombucha, jun tea or any other fermented drink.