Preheat the oven to 190°C. Combine the buttermilk and egg yolk in a small bowl; put aside.
Combine the flour, sugar, baking powder, bicarbonate of soda and a pinch of salt in a large bowl. With a pastry blender or two knives, cut in the butter until the mixture resembles coarse crumbs. Add the blueberries and lemon zest, stirring until well mixed.
Make a well in the centre of the dry ingredients, add the buttermilk mixture and, with a fork, combine until a soft dough is formed. (If the mixture is too dry, add up to 2 tablespoons more buttermilk.)
Transfer the dough to a lightly floured surface and knead 4 or 5 times until well mixed. Transfer to an ungreased baking tray and shape into an 18 cm round. Lightly beat the egg white and brush over the dough. Cut the round into 6 wedges. Bake for 25 minutes or until a skewer inserted in the centre comes out clean. Cool on a wire rack.
Did you know?
Blueberries have natural antibacterial properties that can inhibit the growth of bacteria such as E. coli, thus guarding against stomach upsets and other digestive problems.