Combine the sugar, water and lemon juice in a saucepan, and bring to a boil over medium heat. Reduce heat and simmer until all the sugar is dissolved and the mixture is clear. Skim off any foam that forms. Cool the syrup in the refrigerator. When the syrup is cold, combine it with the tangerine juice, zest and black pepper. Freeze the sorbet in an ice cream maker according to the manufacturer's directions. Pour the tangerine mixture into a shallow metal dish and place in the freezer. Every hour, stir to cause ice crystals to form, until the mixture resembles shaved ice.) Serve the sorbet in chilled martini glasses with a sprig of mint.