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Muffins may seem like a healthy breakfast treat, but many are high in fat from added vegetable oil or butter. Reduce the calories and fat content by replacing the oil with mashed bananas, which provide moisture and volume. Most hearty muffins recipes, such as bran, carrot and spice, do fine with the replacement, but those with a more delicate taste and texture, such as lemon poppyseed or corn, will not turn out as well with a banana replacement.

Measure for Measure

Mashed bananas replace oil measure for measure in your muffin recipe. Use overripe bananas, which are quite soft and have a high moisture content. Peel the bananas, place them in a large bowl and mash thoroughly with a potato masher or fork. Make the mixture as smooth as possible, unless you want chunks of banana noticeable in the cooked muffins.

Measuring Banana Puree

Measure accurately when adding bananas in lieu of oil. Place the mashed banana into a measuring cup and gently tap the bottom on the counter or press down on the puree with a spoon to remove any air bubbles. Refill and repeat until the cup is full.

Recipe Effects

Expect the resulting muffin to turn out denser and heartier than one made with oil. You can lighten the recipe a bit by using pastry flour instead of whole-wheat or all-purpose. Separating the eggs and whipping the egg whites before folding them in at the end of the batter-making process can also add more lightness to the muffins. The muffins may brown more quickly and need less time to bake, too. Check on them in the oven about 10 minutes before the recipe says they're due to be ready.

Substituting just half of the oil with bananas will have less of an impact on the final outcome of the muffin recipe but will still save you some fat and calories. So, if a recipe calls for a cup of oil, substitute with 1/2 cup of banana puree and 1/2 cup of oil.

Flavor Matches

Mashed banana works well in chocolate or chocolate chip muffins. Hearty muffins with lots of spice, dried fruit or nuts also benefit from the banana flavor. You can use bananas in blueberry or other fruit muffins, but the banana will compete with the flavor of the other fruit.