Blanch the mint leaves for 2 minutes and refresh. Drain. Blend all the ingredients together
Using a vegetable peeler, peel the grapefruit zest; blanch in boiling water and drain. Slice into a julienne. Bring the syrup to the boil and gently candy the grapefruit zest.
Blend the flesh. Heat, and then add the pectin with some of the sugar. Bring to the boil and add the remaining sugar. Add the citric acid solution and leave to cool.
Blend the pulp, sugar and xanthan gum. Combine with the mascarpone. Strain through a china cap sieve and transfer to a siphon fitted with two gas cartridges.