Ingredients

• 4 teaspoons finely grated washed unpeeled organic ginger, divided (grate 1 teaspoon at a time over 4 days)
• 4 teaspoons sugar, divided
• 3 pounds red seedless grapes
• Special equipment: A clean 1-liter plastic soda bottle with cap; cheesecloth

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Instructions

1. Stir 1 teaspoon ginger, 1 teaspoon sugar, and 4 cups water in a 32-ounce canning jar to combine. Cover jar with cheesecloth; secure with a canning jar band (without lid). Let sit 1 day out of direct sunlight at room temperature.
2. Stir in 1 teaspoon ginger and 1 teaspoon sugar and let sit 1 day. Repeat twice more. Mixture (ginger bug) should have bubbles throughout and release more when agitated.
3. After 3 days, purée grapes in a blender until smooth. Strain, pressing on solids (you should have about 4 cups juice).
4. Combine juice and 1/2 cup ginger bug in a large nonreactive bowl. Cover with cheesecloth; secure with a large rubber band. Let sit out of direct sunlight at room temperature, skimming white mold from surface, until mixture is slightly foamy and releases bubbles when stirred, 3-4 days. Strain into bottle, cap, and let sit at room temperature until bottle feels pressurized, about 1 day longer. Chill.
5. DO AHEAD: Soda can be made 2 weeks ahead. Keep chilled.