Place blackberries, raspberries and half the water into large pan; bring to boil, reduce heat; simmer gently for 3 minutes, or until pulpy; strain through some cheesecloth; return strained mixture to pan; add remaining water.
Add the sugar and lemon juice and stir over low heat until the sugar dissolves; remove from heat and allow to cool.
Pour mixture into freezer trays and freeze until sorbet is a slushy consistency.
Using electric mixer, beat egg whites until stiff peaks form; using a metal spoon, fold egg whites into berry mixture; freeze, stirring occasionally to ensure egg whites are evenly distributed.
May be served in scoops or scraped; garnish with finely shredded fresh mint leaves and lemon zest.