Mix the sour cream with the powdered sugar, vanilla pulp, lemon juice and zest. Mix the cream well with the starch in a saucepan. Bring to the boil while stirring, transfer to another container and allow to cool. Mix the cream with the sour cream and freeze to ice cream in an ice cream maker or sorbetière for 30-40 minutes. Then keep the ice cream in the freezer until serving.