Final answer:
Overmixing unshortened and chiffon cakes results in too much gluten development leading to a denser, chewier texture. It can also deflate the air bubbles, particularly in chiffon cakes, leading to a cake that does not rise properly and may even become greasy if the oil separates from the batter.
Step-by-step explanation:
When you overmix unshortened and chiffon cakes, the extra mixing can lead to the development of too much gluten. Gluten is the protein that gives bread its chewy texture; however, in cakes, you generally want to minimize gluten formation to keep the texture light and airy. Overmixing batter for these cakes results in a denser, chewier texture that can make the cake fall flat and become tough rather than having the desired soft and fluffy consistency.
In the case of a chiffon cake, it's particularly important to be careful with mixing because this type of cake relies on the air whipped into the egg whites to help it rise. Overmixing can deflate these air bubbles and lead to a cake that doesn't rise properly. Additionally, since chiffon cakes usually contain oil instead of butter (which makes them an unshortened cake type), they can become greasy if overmixed, as the oil can start to separate from the batter.