A fat is a solid triester of glycerol. It is formed when a molecule of glycerol, an alcohol with three hydroxyl groups, reacts with three molecules of fatty acids. A fatty acid is a long-chain aliphatic carboxylic acid. The more correct name for a fat is a triglyceride.
The three fatty acid fragments in a fat may be all the same (a simple triglyceride) or they may be different from each other (a mixed triglyceride). The fat known as glyceryl tripalmitate, for example, is formed when a molecule of glycerol reacts with three molecules of palmitic acid. Glyceryl palmitate distearate, on the other hand, is produced in the reaction between one molecule of glycerol, one molecule of palmitic acid and two molecules of stearic acid.
Fats and oils are closely related to each other in that both are triesters of glycerol. The two families differ from each other, however, in that fats are solid and oils are liquid. The difference in physical state between the two families reflects differences in the fatty acids of which they are made. Fats contain a larger fraction of saturated fatty acid fragments and have, therefore, higher melting points. Oils contain a larger fraction of unsaturated fatty acid fragments and have, as a result, lower melting points.
As an example, beef tallow contains about 56% saturated fatty acid fragments and about 44% unsaturated fatty acid fragments. In comparison, corn oil contains about 13% saturated fatty acid fragments and 87% unsaturated fatty acid fragments.
Both fats and oils belong to the family of biochemicals known as the lipids. The common characteristics that all lipids share with each other is that they tend to be insoluble in water, but soluble in organic solvents such as ether, alcohol, benzene, and carbon tetrachloride.
Fats are an important constituent of animal bodies where they have four main functions. First, they are a source of energy for metabolism. Although carbohydrates are often regarded as the primary source of energy in an organism, fats actually provide more than twice as much energy per calories as do carbohydrates.
Fats also provide insulation for the body, protecting against excessive heat losses to the environment. Third, fats act as a protective cushion around bones and organs. Finally, fats store certain vitamins, such as vitamins A, D, E, and K, which are not soluble in water but are soluble in fats and oils.
Animal bodies are able to synthesize the fats they need from the foods that make up their diets. Among humans, 25-50% of the typical diet may consist of fats and oils. In general, a healthful diet is thought to be one that contains a smaller, rather than larger, proportion of fats.
The main use of fats commercially is in the production of soaps and other cleaning products. When a fat is boiled in water in the presence of a base such as sodium hydroxide, the fat breaks down into glycerol and fatty acids. The sodium salt of fatty acids formed in this process is the product known as soap. The process of making soap from a fatty material is known as saponification.
See also Lipid.