Slightly dissolve the yeast in warm milk and a little sugar and allow to intensify. Dissolve baking soda in sour milk. Sift 2.21 lb (1 kg) flour in a large pan and add the dissolved yeast in it, along with yogurt, sugar, oil and eggs. Add lemon rind and make a soft and smooth dough, adding more flour when kneading.
Transfer the dough to a large lightly greased bowl, so there is room for it to rise well and cover it with aluminum foil. Put the bowl in the refrigerator for 1 night. On the next day, with greased hands, knead it again and let it rise, but while still warm.
Bake the cakes in greased molds or pans until ready to moderate heated oven. Before they go in the oven, brush them with beaten egg and sprinkle with sugar.