Combine the sugar and lemon juice powder in your blender. Pulse 8 to 10 times to combine well. Lemon juice powder does not easily dissolve in liquids, so it must be blended with the sugar first., Add the cold lemon juice, milk and cornstarch to the blender and blend for 30 seconds to combine well. If using the bowl method, whisk together until very well combined., Pour the cold mixture into your ice cream machine and freeze according to the manufacturer's directions.