Yeasts are tremendous bubblers—the carbon dioxide they exude is responsible for inflating breads and pastries, and they also add sparkle to many drinks. Beer, champagne, and hard ciders get their carbonation (and their alcohol) from yeasts, and so do some soft drinks, which ferment for shorter periods and therefore aren't so alcoholic. This sweet, fizzy Concord grape soda is set to bubbling with a purchased culture of champagne yeast, which gives it a pronounced effervescence. To attain ample carbonation, you have to pressurize the soda in sealed vessels, preferably plastic bottles with screw tops that won't explode if too much CO2 builds up.
- 2 3⁄4 cups pure Concord grape juice
- 2 3⁄4 cups filtered water
- 1⁄4 cup sugar (or 1/2 cup agave nectar)
- 1⁄8 tsp. champagne yeast
Bring grape juice, filtered water, sugar (or agave nectar) to a simmer in a 4-qt. saucepan over medium-low heat, and cook, stirring occasionally, until slightly reduced, about 30 minutes. Stir in yeast, and drape a large kitchen towel over saucepan; let sit for 24 hours.
Using a funnel, pour soda into a sterilized 1-liter plastic soda bottle, filling to within 1" of the top or lower. Close bottle tightly with top and let sit at room temperature (ideally 70°—75°) for 24 hours (the bottle should become rock-hard with the pressure that builds up within). Refrigerate soda for 2 days before opening and serving. Drink soda within 1 week of opening, and store in the refrigerator.