Combine 1 cup granulated sugar and 1 cup water in a small saucepan over medium heat. Add the orange zest and lemon zest. Stir until the sugar dissolves and bring just to a gentle simmer. Do not boil vigorously. Simmer 2–3 minutes to extract the zest oils, then remove from heat.

Allow the syrup to cool for 5 minutes, then stir in 1 cup strained fresh orange juice and 2 tablespoons strained fresh lemon juice. Whisk in 1 teaspoon citric acid, 1 teaspoon vanilla extract, and 1/4 teaspoon orange oil or extract (if using) until fully incorporated.

Taste the syrup and adjust acidity or sweetness: add an additional 1/4–1/2 teaspoon citric acid if you want more bite, or up to 2 tablespoons more sugar dissolved while still warm if you prefer sweeter. Let the syrup cool to room temperature. For a very clear beverage, strain the cooled syrup through a fine-mesh sieve or cheesecloth to remove zest and any pulp.

Transfer the cooled, strained syrup to a clean bottle or jar. The concentrated syrup can be stored refrigerated for up to 1 week.

Carbonation & Finishing

Assemble the Fanta-style soda

Fill four 12-oz (about 350 ml) glasses or a large pitcher with ice if serving immediately. For each 8 oz (1 cup) serving of finished soda, add 2 tablespoons (30 ml) of the orange syrup to the glass or pitcher per cup of finished beverage. As a guideline, combine 1 cup chilled carbonated water with 2 tablespoons syrup for a bright, balanced flavor; adjust syrup to taste (1–3 tablespoons per cup).

Slowly pour chilled carbonated water over the back of a spoon into the glass to preserve fizz, stirring gently just once to combine. If making in a pitcher, add chilled carbonated water and gently stir once to mix.

Garnish each glass with an orange slice and an optional mint sprig. Serve immediately while cold and fizzy.

Notes on scale: The recipe makes roughly 1 1/4 cups of syrup (concentrate). Using 2 tablespoons syrup per 1 cup carbonated water yields approximately 4 cups finished soda (serves 4). Adjust syrup-to-water ratio to get sweeter/tarter results; homemade syrup keeps refrigerated up to 1 week.