Preheat oven to 375 degrees. Spray muffin pan with non-stick spray. Measure 1 tbsp lemon juice into a measuring cup and pour in skim milk for a total of one cup. Do not stir. Allow to sit for 5 minutes. Alternatively, you can simply use 1 cup of buttermilk if you have this ingredient on hand.Mix together the first six ingredients in a large bowl: oat bran, whole wheat flour, flaxseed, wheat bran, baking powder and salt.
In a blender or food processor, blend oranges, brown sugar substitute, milk, oil, eggs, and baking soda until well combined.
Create a well in the middle of the dry ingredients. Pour the orange mixture into the dry ingredients and stir until well mixed.
Add walnuts and raisins (if desired) and mix.
Fill muffin compartments approximately 3/4 full. Bake 20 minutes for regular muffins. Note: mini-muffins will take less time.