In a small mixing bowl, dissolve the yeast in the warm water. Let stand until creamy, about 10 minutes.
In a large bowl, combine the yeast mixture, a quarter of the flour, 3 whole eggs, 4 egg yolks, sugar, milk and orange zest; mix well. Add the remaining flour, 125g at a time, until the dough comes together. Turn the dough onto a lightly floured surface and knead until smooth and supple, about 5 minutes.
Flatten the dough out with your hands and smear 1/3 of the butter on the dough. Fold up the dough and knead gently to incorporate the butter into the dough. Repeat this twice until all of the butter is incorporated. Use the same method to incorporate the raisins.
Lightly oil a large mixing bowl. Place the dough in the bowl and turn the dough to coat with oil. Cover with a damp cloth and let stand in a warm place until doubled in volume; about 1 hour.
Preheat oven to 180 C / Gas 4.
Turn the dough onto a lightly floured surface and divide into two equal size pieces. Form each piece into a loaf and place them in two lightly greased 900g loaf tins. Cover with a damp cloth and let rise until doubled, about 40 minutes.
Brush the tops of the loaves with the reserved egg whites and bake in the preheated oven until the top of the loaves are golden and the bottom of the loaves sound hollow when tapped, about 30 to 40 minutes.