Triglycerides
About 95 percent of all lipids are triglycerides, which consist of three (“tri”) fatty acids attached to a glycerol molecule. Two fatty acids linked to the glycerol molecule, form a diglyceride, while one fatty acid linked to glycerol is a monoglyceride. The fatty acids on the glycerol can be identical (simple triglyceride) or different (mixed triglyceride).
Fatty Acid Structure
Fatty acids differ in two major ways: 1) their length, which is determined by the number of carbon atoms, and 2) their degree of “saturation,” which is determined by the number of double bonds. The number of carbons can range from 2 to 22, with the number usually being even. A fatty acid is said to be saturated if there are no double bonds—every carbon on the chain is bonded to two hydrogens and therefore fully loaded. If one hydrogen from each adjacent carbon is missing, the carbons form double bonds with one another and form a point of unsaturation. A fatty acid with one double bond present is called a monounsaturated fatty acid. If there are two or more double bonds in the carbon chain of a fatty acid, the fatty acid is called polyunsaturated.
The degree of unsaturation of the fatty acids in a fat affects the temperature at which the molecule melts. Generally, the more unsaturated a fat, the more liquid it remains at room temperature. In contrast, the more saturated a fat, the firmer its consistency. Generally, oils are liquid at room temperature, while fats are solid.
Fatty Acid in Food
Most foods contain all three types of fatty acids—saturated, monounsaturated, and polyunsaturated—but one type usually predominates. Generally speaking, most vegetable and fish oils are high in polyunsaturates, while olive and canola oils are rich in monounsaturates. The animal fats, as well as coconut and palm oils, are more saturated. Overall, foods of animal origin usually contain approximately a 50:50 P/S ratio of polyunsaturated and saturated fats, while for plant foods the ratio is usually 85/15. The higher the P/S ratio, the more polyunsaturated fats the food contains.
Fatty Acid Nomenclature
Each fatty acid is identified by a common name, a systematic name, chemical configuration, or numerical ratio. Usually a fatty acid is referred to by its common name, while the systematic name is used when a more formal or correct chemical nomenclature is required. Approximately 40 fatty acids are found in nature. Some of the more common fatty acids are butyric acid, found in butter, and the two essential fatty acids, linoleic and linolenic. The long number of carbons is abbreviated in a type of chemical shorthand that conveys the length and saturation of fatty acids in a numerical ratio. For example, palmitic acid is a saturated fatty acid that is represented by 16:0, meaning that it is sixteen carbons long with zero double bonds.
KEY TERMS
P/5 ratio
The ratio of polyunsaturated fats to saturated fats. The higher the P/S ratio, the more polyunsaturated fats the food contains.
Essential nutrients
Nutrients that the body cannot synthesize at all or in necessary amounts to meet the body’s needs.
CHEMIST’S CORNER
Few fatty acids occur free in foods, but rather are incorporated into triglycerides. Each fatty acid consists of an acid group (-COOH) on one end and a methyl group (-CH3) on the other end. The fatty acids are attached to the glycerol molecule by a condensation reaction. The hydrogen atom (H) from the glycerol and a hydroxyl (-OH) group from a fatty acid form a molecule of water. When a fatty acid reacts like this with an alcohol such as glycerol, it results in an ester. Since “acyl” defines the fatty acid part of an ester, what is called “triglyceride” should actually be named “triacylglycerol”.