Dissolve yeast in warm water. Combine flour and salt; make a well in the center of the mixture. Blend together egg, mashed potatoes, potato water, honey and oil. Combine baking soda and yogurt or buttermik; add to egg mixture. Pour mixture into well of flour and salt; blend thoroughly. Turn onto lightly floured surface and knead 10 minutes. Cover and let rise 1.5 to 2 hours, until a 1 cm hole in dough made with a wet fingertip remains. If the hole fills in slightly, give the dough more time to rise. Deflate gently. Let rest 10 minutes. Shape into loaves and let rise until almost doubles, about 45 minutes. Bake at 180ºC or approximately 1 hour.