In a saucepan, combine one cup of the juice and sugar, and heat, stirring occasionally, until the sugar is completely dissolved.

Remove from heat and pour the mixture back into the remaining tangerine juice. Add zest and the orange liqueur, if using, and mix. Pour the mixture into a freezer storage container and freeze overnight, until frozen solid.

Because citrus is mostly water, the sorbet will freeze quite hard after being stored in the freezer overnight. Remove sorbet from the freezer and allow to defrost slightly, about 15 to 20 minutes, until it pulls away from sides of container. In a food processor, briefly pulse the mixture until slushy, with ice crystals still visible. Working quickly, pour the mixture back into the container and return to freezer to keep it from melting completely. Allow mixture to freeze for at least six hours, until it returns to a solid soft freeze.

Remove ices from the freezer five minutes or so before serving, so it’s at the right temperature for scooping and serving.