The malic and oxalic acids contained in rhubarb not only provide the typical fine-sour taste of the vegetables, but are also effective against bacteria in the stomach and intestines that cause illness and stimulate digestion.
If you have problems with kidney or gallstones or suffer from rheumatism or gout, you should rather avoid rhubarb because of its high oxalic acid content. Alternatively, you can use finely chopped strawberries, raspberries or apple cubes for the muffins.
Preheat oven to 180°C / 350°F. Line wells of a muffin tin with paper liners.
Rinse rhubarb, trim and cut into small pieces. Rinse orange in hot water, pat dry and finely grate zest. Squeeze out juice. Mix egg with orange zest, juice, vegetable oil, sugar, vanilla, salt and buttermilk. Mix flour with baking powder in a mixing bowl. Pour in egg mixture quickly and stir just until all ingredients are moist. Fold in rhubarb and pour batter into paper liners. Bake on middle rack until golden brown, about 25 minutes.
Carefully remove muffins from tin and cool on a wire rack. Serve dusted with powdered sugar.