Preheat the oven at 180 degree Celsius for 10 minutes and line the muffin tray with liners.Sieve the Sathumaavu mix, whole wheat flour, cinnamon powder, baking powder, baking soda and salt together two times.In a large mixing bowl, mash the bananas using a fork. Add ½ cup sugar to it and beat well until the sugar completely dissolves.To this mix, add the refined oil and blend well.Add the sifted dry ingredients to this banana mixture and fold delicately, making sure there are no lumps in the mixture. Do not over mix; just make sure that the dry ingredients are moistened.Add the vanilla extract, lemon juice and chopped walnuts to the batter. Mix well.Fill in the muffin liners up to three fourths of their volume. Tap the muffin tray gently 2-3 times to remove the trapped air bubbles.Bake the muffins in a preheated oven at 180 degree Celsius for 25 minutes or until the tester tooth pick comes out clean. As the temperature may vary from oven to oven keep a close watch on the muffins once 20 minutes of baking time are over. If the tooth pick is sticky , bake for another 2-3 minutes.Once done , carefully remove the muffins and allow them to cool on a wire rack.