Mix 1 cup (250ml) of the juice with the sugar, or sugar and corn syrup, and heat – stirring occasionally – until the sugar is completely dissolved.
Remove from heat and pour the mixture back into the tangerine juice. Add the zest and the orange liqueur (if using). Chill the mixture thoroughly, at least 8 hours, or overnight.
Freeze the tangerine sorbet mixture in an ice cream maker according to the manufacturer’s instructions.