Place popcorn in two greased 15x10x1-in. baking pans. Keep warm in a 225° oven.
Meanwhile, in a heavy saucepan, combine sugar, corn syrup and water; bring to a boil over medium heat. Stir occasionally until mixture reaches 290° on a candy thermometer (soft-crack stage). Remove from the heat; quickly stir in extract and baking soda. Pour over warm popcorn. Sprinkle with lemon zest; stir until well coated. Store in an airtight container.
Editor's Note: 1/2 cup of unpopped kernels equals about 4 cups popped popcorn.