The best way to freeze citrus is to freeze the juice in ice-cube trays until solid, then transfer the cubes to a freezer bag. Freeze it in 1- to 2-Tbsp. portions―it should keep indefinitely. Thaw at room temperature or in the fridge (or use lemon cubes in iced tea). You can also freeze the zest: Zest the fruit onto a sheet of plastic wrap, wrap tightly and place in a freezer bag.