Imagine, if you will, for a second; a pie crust made with no fat. What do you see? Not much, right? It’s effectively just a pile of flour, sugar, salt, and a little bit of water. Not very pie-y. Now, were you to add butter, suddenly it actually works. If you bake flour, salt, sugar and water, you’ll get a burnt pile of powder. If you bake all that with butter, you get a baked flaky, delicious pie crust.
Butter is a solid composed of the protein and fat components of milk or cream. There are various types of butter, such as salted or unsalted, or butter from other animals. The most common type of butter that you generally buy in grocery stores is made from a cow’s milk, and may contain some salt depending on the type you buy. Butter is a fat due to its composition as a protein and fat rich solid, and as such is wildly useful in baking, much like other fats.
In case you were wondering, a fat is a macronutrient important for bodily function. Fat in this context is different from fat present in one’s body. Fat molecules are mostly just carbon and hydrogen linked together in long chains. These chains can either stack together tightly or loosely. The tighter they stack, the thicker the liquid and the higher the melting point. Oils and fats are similar in their molecular composition, and therefore in baking can occupy similar roles. Now, let's talk about the different types of fats.
Butter: As previously mentioned, butter is a dairy product made of the fat and proteins of milk or cream. Most fats, like butter and animal fats, are solid at room temperature. Butter is an emulsion of fat and water at room temperature. When baked, the water evaporates, leaving pockets of air in the crust. Take a croissant, for example. The process of making a croissant involves creating many layers of butter and flour in the pastry, that when baked, results in the flaky layers croissants are known for. Without the water in the butter, this wouldn’t be possible. There are vegan variants of butter, although they are made using vegetable fats instead of milk fats.
Oil: As mentioned before, oil is similar to fat in its molecular composition. The fat molecules in oils pack more loosely, which is why they tend to be liquid at room temperature. Oils used for baking tend to be neutrally flavored, such as canola or vegetable oil. Baked goods made with oil have the property of being more tender than those made with a solid fat such as butter or shortening.
Shortening: Shortening is oil that has been partially hydrogenated (hydrogen added to it) to make it solid at room temperature--think Crisco. Shortening commonly refers to both margarine (a butter substitute traditionally made of beef tallow) and vegetable-based solid fats. Due to shortening being made of vegetable fats, it tends to lack any particular flavor when used for baking, imparting no taste. Shortening is useful for making especially flaky pastries or baked goods like biscuits, although this effect can also be produced through the use of butter. Typically, it is used with butter to create a flaky, yet tender, baked good.
Fun Fact: Shortening was originally synonymous with lard, a fatty substance created from rendering the fatty tissue of pig. Margarine was invented as a cheaper alternative to lard by a French chemist named Hippolyte Mège-Mouriès. The main reason he invented margarine was due to a challenge issued by Emperor Napoleon Bonaparte to make a cheaper alternative to butter/lard using beef tallow. Ironically, most margarine made nowadays is made exclusively using vegetable fats, and at one point, this version of margarine was more expensive than lard.
As you can tell, fat is very important in baking. Without it, many of your favorite pastries and baked goods wouldn't even exist!
Cousminer, J. Jeffrey, editor. Culinology: The Intersection of Culinary Art and Food Science. Wiley, 2017.
Field, Simon. Culinary Reactions: The Everyday Chemistry of Cooking. Chicago Review Press, 2012.