A number of cake recipes incorporate applesauce as an ingredient, and it's also commonly added to boxed cake mixes. Applesauce's moisture, sweetness, acidity and fresh fruit flavor all have beneficial effects in some recipes. It can be added as a supplement to a cake prepared normally, or used as a substitute for some or all of the fat or liquids.
Arguably the most obvious reason for adding applesauce to cake mix is its flavor. Applesauce adds a pleasant tang and depth of flavor to white or yellow cakes. In butterscotch or caramel-flavored cakes, it suggests the flavor of caramel apples, a pleasantly nostalgic childhood memory. Applesauce in spice cakes or gingerbread complements their warm spices, making the finished cake even more aromatic. In more strongly flavored cakes, especially rich chocolate cakes, the delicate apple flavor is overwhelmed by the taste of the cake's other flavoring ingredients.
Applesauce added to a cake's batter also has the effect of giving it a moister, richer crumb. This is why bakers often add it to chocolate cakes, even though it can't be tasted. Usually you'll reduce your other liquids to compensate, so if your mix calls for 1 1/3 cups of liquid, use 2/3 cup of the regular liquid and 2/3 cup of applesauce. If you don't reduce your liquids, the cake will be dense and tough when it's baked.
Applesauce can be added to your cake mix as a direct replacement for oil or melted butter. This reduces the total amount of fat and calories in your recipe, improving its healthfulness. Cakes made with only applesauce will be slightly denser and chewier, so some bakers prefer to substitute only half of the oil. Applesauce can also be used as a substitute for eggs or milk in a cake mix, if you're baking for vegetarians or allergy sufferers. Using applesauce in place of milk makes little difference in the end result. Applesauce in place of the eggs makes the cake chewier, but the cake's texture can be improved by adding an extra tablespoon of oil.
Applesauce's relatively light flavor, low cost and easy availability make it the most versatile addition for cake mixes, but other fruit purees can be used in similar ways. Pumpkin puree works well in chocolate or spice cakes, and can substitute for all the other ingredients. Tomato puree can be also be used in chocolate cakes, where it adds moisture and tang. Pear puree can be used interchangeably with applesauce, if you have it. Finely shredded zucchini can be added to cake mixes for extra moisture, though it should be peeled for light-colored cakes. Vegetable purees -- such as baby food -- can be added to any cake for moisture and additional nutrition.