Combine 1 cup granulated sugar, 120 ml water and 1 tablespoon light corn syrup (if using) in a small saucepan over medium heat. Stir until the sugar dissolves and the mixture comes to a gentle simmer. Once clear and fully dissolved, remove from heat and let cool for 10 minutes.

Fruit & acid

Add brightness

While the syrup is still warm but not hot, whisk in 30 ml fresh lemon juice and 1.5 teaspoons citric acid until fully dissolved. Taste and adjust acidity: add up to 0.5 tsp more citric acid for a sharper sour profile, if desired.

Color & flavor

Flavor and color the syrup

Stir in 15 ml fruit concentrate or extract (raspberry for rood, blueberry for blauw) and 0.25 tsp vanilla extract (optional). Add 6 drops of the appropriate food coloring (red for rood, blue for blauw) and mix until uniform. Allow the syrup to cool completely to room temperature, then refrigerate for at least 30 minutes to chill and let flavors meld.

Carbonation & finishing

Assemble the drink (batch)

Fill a large pitcher with 2 cups ice. Pour the chilled flavored syrup into the pitcher (use the entire syrup above — yields roughly 200–250 ml flavored syrup). Slowly add 750 ml chilled sparkling water or club soda to preserve bubbles. Stir gently once to combine; avoid vigorous stirring to keep carbonation.

Serve

Pour into tall glasses over fresh ice if desired. Garnish each glass with fresh raspberries (for rood) or blueberries (for blauw). Serve immediately.

Notes on scaling & storage

Syrup stores in a sealed jar in the refrigerator for up to 10 days. For single servings, use about 30–40 ml syrup per 250 ml sparkling water depending on desired sweetness and tartness.