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Shortened cakes, made with oil as the added fat, adding moistness and density, include the carrot, zucchini, apple and pumpkin, and spice cakes. Cake mix cakes, are commonly made with the same liquid fat, and can be prepared either layered, in a loaf pan, or as a cupcake. The fat and sugar are not "creamed", but rather the cakes are mixed using the One or Two Bowl or Muffin Mix Method. Oil-shortened cakes instead rely on chemical leaveners such as baking powder and/or baking soda or physical leavening from whipping air into the batter, especially egg whites, if beaten alone with sugar and folded into the batter at the end of mixing.
The liquid oil acts more rapidly than softened butter to coat the wheat flour's gluten forming protein molecules, rendering them tenderer by preventing the formation of gluten which makes the cakes tough. The key is to not add too much oil, otherwise the cake becomes too heavy and compact, or worse, greasy.
Liquid oils always stay liquid and never set into a solid form, even when refrigerated; always retaining their tenderness and moistness, also a perfect match for perishable frostings, such as cream cheese based ones. However, some people add a small amount of olive oil, which contains natural emulsifiers, to cakes shortened with solid fats, such as stick butter or margarine or shortening, to make them more tender and moist.