Preheat the oven to 175C (350 F). In a clean bowl, begin whipping the egg whites. Add the lemon juice when the whites become frothy. Whisk the egg white on medium speed to stiff peaks. In a separate bowl, beat the egg yolk until pale yellow. Add the salt, baking power and butter to the egg yolks and whisk till it doubles in volume. Fold the almond flour into the egg yolks in three additions. Once the almond flour is completely incorporated into the egg yolk mixture, fold in the egg whites. First add a third of the whites in, folding gently to lighten the mix. Then fold in the remaining egg white in two batches. Grease your loaf tin and pour in your batter. Bake the bread for 30 minutes or until a toothpick inserted in it comes out clean. Allow the bread to cool, then slice and serve.