Fats, Oils, Fatty Acids, Triglycerides

Triglycerides are the main constituents of vegetable oils and animal fats. Triglycerides have lower densities than water (they float on water), and at normal room temperatures may be solid or liquid. When solid, they are called "fats" or "butters" and when liquid they are called "oils". A triglyceride, also called triacylglycerol (TAG), is a chemical compound formed from one molecule of glycerol and three fatty acids.

Oleic Acid

Glycerol or Glycerin

Glycerol is a trihydric alcohol (containing three -OH hydroxyl groups) that can combine with up to three fatty acids to form monoglycerides, diglycerides, and triglycerides. Fatty acids may combine with any of the three hydroxyl groups to create a wide diversity of compounds. Monoglycerides, diglycerides, and triglycerides are classified as esters which are compounds created by the reaction between acids and alcohols that release water (H₂O) as a by-product.

C18:1 C18:1 C16:0

C18:0 C18:0 C18:0

The triglyceride structural formula on the left is typical of olive oil. It consists of two radicals of oleic acid and one of palmitic acid attached to glycerol (the vertical carbon chain). The small squares represent the fatty acid components of the glyceride molecules. The picture on the right shows the three-dimensional molecular structure of tristearin, a triglyceride with three stearic acid radicals. Oxygen atoms are shown in red, carbon atoms as dark gray, and hydrogen atoms as blue. Tristearin is found as a minor component in many natural fats.

Soap is made traditionally by heating an alkali like sodium hydroxide (NaOH) with animal fat. The chemical reaction (hydrolysis) produces glycerol and soap, which consists of the sodium salts of the fatty acids, e.g., sodium stearate (CH₃(CH₂)₁₆C(O)O^- Na⁺).

C18:1   -  C16:0

1,3-diglyceride

-    -  C16:0

1-monoglyceride

A diglyceride, or diacylglycerol (DAG), has two fatty acid radicals and exists in the 1,2 form and the 1,3 form depending on how the fatty acids are attached to the glycerol molecule. A monoglyceride, or monoacylglycerol (MAG), has only one fatty acid radical per molecule of glycerol. The fatty acid may be attached to carbon 1 or 2 of the glycerol molecule.

All esters of glycerol and fatty acids are metabolized in the same way. Monoglycerides, diglycerides, and triglycerides all have 9 Calories per gram, but some nutrition labels hide the calories of mono- and diglycerides under the contention that "fat" consists only of triglycerides.

Structure of Olestra. R represents fatty acids.

Artificial fats and fat substitutes have become more common as manufacturers target people who through misinformation have acquired aversions to fats or who would like to diet without reducing food intake. Olestra is an artificial fat created from sucrose (a carbohydrate) and up to eight fatty acids. In the olestra chemical structure, sucrose takes the place of glycerol. The olestra molecule is too large to be metabolized and passes through the body unchanged, but because it acts as a lipid, it can cause depletion of fat-soluble vitamins. Polyglycerol fatty acid esters (glyceran fatty acid esters) are mixtures of the esters of fatty acids with polyglycerol. These compounds have the general structure R-(OCH₂-CH(OR)-CH₂O)_n-R, where R represents fatty acids and the average value of n is about 3. Polyglycerol esters of fatty acids are almost completely metabolized like fats, so they are not calorie-free. The polymerized glycerol moiety is not digested and is excreted primarily in the urine. The main purpose of these compounds is to create products that are technically "fat free" and whose calories and fatty acid compositions are not reported on the Nutrition Facts of food labels.

Fatty acid composition of some common edible fats and oils.

Fatty acid compositions depend on the sources of the oils. Canola oil is made from selectively bred rapeseed plants that contain less than 2% erucic acid. Some crops have produced canola oil with 76% oleic acid. The table lists the linoleic type of safflower oil; oleic types of safflower oil have approximately 78% monounsaturated, 15% polyunsaturated, and 7% saturated fatty acids. Not shown in this table: Coconut oil, also called copra oil, has 8% caprylic acid (C8:0). Cod liver oil has 7% palmitoleic acid (C16:1), 17% C20 unsaturated fatty acids (10% EPA), and 11% C22 unsaturated fatty acids (6% DHA). Peanut oil has approximately 5% of C22:0 and C24:0 fatty acids. Palm olein is the liquid fraction obtained by the fractionation of palm oil after crystallization at controlled temperatures. Cow butterfat has 4% butyric (C4:0) and 2% caproic (C6:0) acids. Goat butterfat has 4% butyric (C4:0), 3% caproic (C6:0), and 3% caprylic (C8:0) acids. Beef tallow, cow butterfat, human butterfat, and lard all have about 3% palmitoleic acid (C16:1). Human depot fat, usually found in the abdomen of men and around the thighs and hips of women, has a composition similar to lard.

What are the triglyceride profiles of these fats and oils? The percentages in the table above reflect the overall proportions of the fatty acid radicals in the triglycerides. If we had 33 representative triglyceride molecules containing 99 fatty acid radicals, the number of each fatty acid radical in these 33 molecules would be proportional to its percentage in the table. For example, 33 representative molecules of lard triglycerides would contain, on average, 26 radicals of palmitic acid (C16:0), 14 radicals of stearic acid (C18:0), 44 radicals of oleic acid (C18:1), and 10 radicals of linoleic acid (C18:2). These fatty acid radicals would be distributed randomly among the 33 triglyceride molecules. The typical lard triglyceride molecule would have one or two radicals of oleic acid and one radical of palmitic acid. Frequently, there would be triglycerides with one radical each of oleic, palmitic, and stearic acids. Only rarely would one encounter triglycerides with only palmitic and stearic acid radicals.

This profile was constructed using a random distribution of the appropriate percentages of the fatty acids in 33 representative triglyceride molecules. Red is used for saturated, green for monounsaturated, and blue for polyunsaturated fatty acids. Although the composition of the individual triglyceride molecules may vary, the relative proportion of fatty acids remains constant. The profiles for canola oil or olive oil would be mostly green and blue with very little red, whereas the profile for coconut oil would be mostly red.