Depending on their molecular structure, some fats are solid at room temperature, while others are liquid at the same temperature. Liquid fats are known as oil. Solid fats soften and eventually melt into a liquid state when exposed to heat.
In addition to being a vital nutrient, fat preforms a number of culinary functions. It provides a rich flavor and silky mouthfeel or texture that most people find enjoyable and satisfying. Fat also carries and blends the flavors of other foods, and makes available to us flavor compounds and nutrients that are soluble only in fat.
Fat provides an appealing visual element when a food appears to be moist, creamy, fluffy, or shiny, among other things. During the baking process, fat performs a multitude of chemical function, such as tenderizing, leavening, aiding in moisture retention, and creating a flaky or crumbly texture. In cooking, fat transfers heat to foods and prevents them from sticking. It also holds the heat in food, emulsifies or thickens sauces, and creates a crisp temperature when used for frying.
One important aspect of fat is its ability to be heated to relatively high temperatures without boiling or otherwise breaking down. This is what allows fried foods to cook and brown quickly. If heated to high enough temperatures, however, fat will begin to break down and an acrid flavor develops, effectively ruining anything cooked in it the temperature at which this occurs, known as smoke point, is different for each type of fat.
Generally vegetable oils begin to smoke around 450 F/232 C, while animal fats begin to smoke around 375 F/191 C. Any additional materials in the fat (emulsifiers, preservatives proteins, carbohydrates) lower the smoke point. Because some breakdown occurs at moderate temperatures and food particles tend to get left in the fat, repeated use of fat also lowers the smoke point.