Preheat oven to 375ºF.
Line a muffin pan with muffin liners.
In a large bowl, stir together the flour, sugar, baking powder, baking soda, salt and coriander.
In another bowl, whisk together the egg, oil, orange juice, and orange zest, if using.
Stir the wet ingredients into the dry ingredients until just combined. Fold in the rhubarb.
Divide the batter between the muffin liners.
Bake for 20-25 minutes or until a toothpick comes out clean, rotating the pan halfway through baking.