Beat the egg white until stiff peaks form. Set aside.
In a separate bowl, beat the egg yolks with the powdered sugar until pale, thick and creamy.
Add the salt, milk, sparkling water and baking powder. Gradually add the flour and mix until the batter is smooth.
Gently fold the beaten egg white into the batter, taking care not to knock out too much air.
Peel and core the apples, then cut them crosswise into ½-inch-thick rings. Sprinkle the rings with a little lemon juice to prevent them from browning, then lightly coat them in flour. Shake off any excess.
Heat the oil in a deep frying pan over medium-high heat. Dip each apple ring into the batter, allowing the excess to drip off, then carefully place it in the hot oil.
Fry the apple rings in batches until golden brown on both sides. Do not overcrowd the pan.
Transfer the fritters to a plate lined with paper towels to drain.
Combine the powdered sugar with ground cinnamon and sprinkle generously over the warm apple fritters. Serve immediately.