Fat Substitutes (Fruit Purees)
Fruit purees, especially applesauce, are often used as fat substitutes. The pectin from the fruit forms a film around the tiny air bubbles in the batter, similar to what occurs when you cream solid shortenings with sugar, but not as effectively.
My favorite fruit puree for baking is unsweetened applesauce. Not only is it readily available but it is inexpensive and versatile because it doesn't impart any strong flavor to the final result. Applesauce contains more pectin than other fruit purees, which helps to retain the moistness of baked goods. Even if a recipe is flavored with another fruit puree, I always add a little applesauce as well.
You'll see recipes here that use pumpkin, banana, and prune purees, among others.
Sugars provide sweetness and flavor. Crystalline sugars, such as granulated white sugar and brown sugar, are integral to the creaming process that incorporates air into batters. Sugar also inhibits gluten formation, which means that sugar helps make baked goods tender. Honey and corn syrup are liquid sweeteners, and while they do provide sweetness, they do not cream well, just as liquid vegetable oils can't substitute for solid shortenings.
Most desserts are leavened with baking soda or baking powder, or by the air beaten into egg whites. (Yeast, not used in this book also makes baked goods rise.)
Baking soda (bicarbonate of soda) is alkaline. When it comes in contact with an acidic ingredient like applesauce, buttermilk, or lemon juice and is moistened, the alkali/acid combination creates carbon dioxide. This carbon dioxide expands the air bubbles previously formed by creaming, and makes the baked good rise. In some recipes, depending on the quantity of acidic ingredients included, a combination of baking soda and baking powder is used for better flavor and texture.
Baking powder does not need an acidic ingredient to release its leavening power. Double-acting baking powder begins releasing carbon dioxide as soon as it is moistened, and again when heated in the oven. Some baking powders include sodium aluminum sulfate, but there are aluminum-free baking powders that work just as well, and I prefer them. Look for a brand like Rumford's at natural food stores or many supermarkets.
Not every recipe includes a thickener, although flour certainly has thickening attributes. But many fruit fillings include cornstarch to thicken the juices. I occasionally use tapioca as a thickener, as well.
Flavorings enhance a baked good's aroma and taste. The butter in traditional recipes contributes to and carries flavors throughout the batter. Even more important, butter has flavor of its own that, when it interacts with sugar, is responsible for the caramelized baked taste we associate with baked goods. In reduced-fat baking, the flavorings must be increased to compensate for the reduction in butter.