First, make a sponge: Dissolve the yeast in 1/2 cup lukewarm milk. Add 2 tsp. Sugar and 2 tbsp. Flour and beat together well. Set aside, covered, and allow the sponge to rise until doubled in bulk. Sift the rest of the flour and the salt together. To scalded and slightly cooled milk, add margarine or butter and sugar and stir until melted. Cool to lukewarm. Beat the three eggs well, and add the lukewarm milk mixture to them. Put the yeast sponge into a large bowl. Add the milk mixture to it and mix. Add flour and raisins and beat together well with a wooden spoon. Turn out onto a floured board and knead for about 8 minutes. Put back into bowl, cover, and allow to rise in a warm, draft free place until doubled in bulk. Punch dough down: knead lightly again for a few minutes. Divide into number of babkas desired, placing enough dough in each greased pan to fill about halfway. Cover and allow to rise again until doubled in bulk. Preheat oven to 350 F. Brush top of babkas with slightly beaten egg white. Bake 45 minutes to an hour, until golden brown on top and a straw or wooden skewer inserted in center comes out clean. Cool and serve with butter or margarine.