Pureed bananas, as well as other fruit and vegetable purees, are rich in moisture, so adding them to a yellow cake mix can create a moister, more tender cake. Banana puree can even stand in for some of the oil, for a cake that's healthful and delicious. Substitute no more than half the oil with banana puree.

Select only very ripe bananas to use in a yellow cake. The peels should be dark brown, but the bananas should not be oozing liquid or have an alcohol odor, which indicates that they're spoiled. Peel the bananas and smash them with a fork in a bowl until they're evenly mashed. You can also toss them in a blender for a more uniform consistency. Add the bananas to the cake mix when you add the oil and water. Mix with a hand mixer for 1 to 2 minutes and bake in an oven set at 350 degrees Fahrenheit for 25 minutes, or until the cake springs back lightly when you touch it.

Not only do pureed bananas add moisture to a cake, but they also add flavor. Use two to four bananas to transform your yellow cake into a banana cake; serve with cream cheese frosting for a classic dessert. To add moisture without the concentrated banana flavor, use no more than one small banana. Even then, you may detect a slight banana flavor. Add the bananas in addition to the oil, water and eggs called for in your method. If you wish to use the bananas instead of some of the oil, use 1/2-cup banana puree for every 1-cup oil.

Bananas aren't the only product that can add moisture to a yellow cake mix. For an even better flavor and texture, add a few tablespoons of yogurt, sour cream or instant vanilla pudding mix in addition to the pureed banana, oil, water and eggs. If you want moisture without the banana flavor, try applesauce instead. Applesauce has a mild flavor, which is barely discernible in a cake. It can stand in for some of the oil at a ratio of 1-part applesauce to 2-parts oil.