Mix the gelatine, lemon juice and water in a mixing bowl. Set aside to bloom.
Mix the sugar, corn syrup and water in a heavy-bottomed pot; bring to a boil over medium-high heat. Stir gently just until the sugar is dissolved. If any sugar remains above the level of the liquid, wash down the sides of the pot with cold water using a pastry brush. This is important so the mixture doesn't crystalize. Once the mixture boils, boil hard for one minute. Remove from heat and with the mixer running on high (using the whisk attachment, slowly pour the hot sugar into the gelatine down the side of the bowl, trying not to touch the whisk. Once all the sugar is in, add the zest and turn the mixer on high to whisk for about 6-8 minutes. The mixture will be very fluffy and sticky.
Line a loaf pan with plastic wrap. Spray the wrap with cooking spray. Transfer the mixture to the pan and, using lightly greased hands, press the marshmallow into the pan so that it is evenly spread. Allow to cool and set at room temperature for about 2 hours.
Lightly dust your counter with icing sugar, take the marshmallow out of the pan, remove the plastic and cut into the desired size. Dust with icing sugar if it gets too sticky to handle.
Yield: 6-8 servings