![]() These compounds can be obtained in three crystalline forms ( polymorphs): α, β, and β′, the three forms differing in their melting points. Their names indicate the fatty acid: stearin derived from stearic acid, palmitin derived from palmitic acid, etc. The simplest triglycerides are those where the three fatty acids are identical. Cocoa butter is unusual in that it is composed of only a few triglycerides, derived from palmitic, oleic, and stearic acids in the 1-, 2-, and 3-positions of glycerol, respectively. Because of this, they melt over a broad range of temperatures. Most natural fats contain a complex mixture of individual triglycerides. Many fatty acids are unsaturated some are polyunsaturated (e.g., those derived from linoleic acid). As a result, ruminant animal fat contains odd-numbered fatty acids, such as 15, due to the action of bacteria in the rumen. Bacteria, however, possess the ability to synthesise odd- and branched-chain fatty acids. Natural fatty acids found in plants and animals are typically composed of only even numbers of carbon atoms, reflecting the pathway for their biosynthesis from the two-carbon building-block acetyl CoA. The chain lengths of the fatty acids in naturally occurring triglycerides vary, but most contain 16, 18, or 20 carbon atoms. The three fatty acids (RCO 2H, R′CO 2H, R″CO 2H in the above equation) are usually different, as many kinds of triglycerides are known. HOCH 2CH(OH)CH 2OH + RCO 2H + R′CO 2H + R″CO 2H → RCO 2CH 2CH(O 2CR′)CH 2CO 2R″ + 3H 2O In triglycerides, the hydroxyl groups of the glycerol join the carboxyl groups of the fatty acid to form ester bonds: The glycerol molecule has three hydroxyl (HO–) groups and each fatty acid has a carboxyl group (–COOH). Alcohols and organic acids join to form esters. Organic acids have a carboxyl (–COOH) group. Triglycerides are tri- esters consisting of a glycerol bound to three fatty acid molecules. All carbon-carbon double bonds shown are cis isomers. The third fatty acid (a polyunsaturated fatty acid, highlighted in red) contains three double bonds within the carbon chain. One fatty acid is saturated ( blue highlighted), another contains one double bond within the carbon chain ( green highlighted). Example of a natural triglyceride with three different fatty acids.
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