1 Boil champagne, sugar, corn syrup, zest: Put champagne, sugar, corn syrup, and zest into a saucepan. Bring to a vigorous boil so that the sugar completely dissolves, remove from heat.
2 Strain: Strain into a stainless steel bowl (will help cool down faster), add the grapefruit juice and lemon juice.
3 Chill: Chill completely. To do this, either place bowl in a larger bowl half-filled with ice water, and stir until champagne solution is completely cold, refreshing the ice in the outer bowl if necessary.
Or you can cover with plastic wrap and chill in your refrigerator overnight.
I put the bowl in the ice compartment of our freezer for a couple hours, because it fits, chills quickly, and is far enough away from the other food in the freezer.
4 Process in ice cream maker: Process the mixture in your ice cream maker according to the ice cream maker directions.