Atriglyceride (fromtri- andglyceride; alsoTG,triacylglycerol,TAG, ortriacylglyceride) is anester derived fromglycerol and threefatty acids.[1]Triglycerides are the main constituents ofbody fat in humans and other vertebrates as well asvegetable fat.[2]They are also present in the blood to enable the bidirectional transference ofadipose fat and blood glucose from the liver and are a major component ofhuman skin oils.[3]
Many types of triglycerides exist. One specific classification focuses onsaturated and unsaturated types.Saturated fats haveno C=C groups;unsaturated fats feature one or more C=C groups. Unsaturated fats tend to have a lowermelting point than saturated analogues; as a result, they are oftenliquid at room temperature.
Example of a natural mixed triglyceride with residues of three different fatty acids. The first fatty acid residue is saturated (blue highlighted), the second fatty acid residue contains onedouble bond within the carbon chain (green highlighted). The third fatty acid residue (a polyunsaturated fatty acid residue, highlighted inred) contains threedouble bonds within the carbon chain. All carbon-carbon double bonds shown arecis isomers.
The three fatty acidssubstituents can be the same, but they are usually different. The positions of the three fatty acids are specified usingstereospecific numbering as sn-1, sn-2, and sn-3. The compositions of many fats and oils have been determined.[4] Many triglycerides are known because manyfatty acids are known. The chain lengths of the fatty acid groups vary in naturally occurring triglycerides, Those containing 16, 18, or 20carbon atoms are defined aslong-chain triglycerides, whilemedium-chain triglycerides contain shorter fatty acids. Animalssynthesize even-numbered fatty acids, but bacteria possess the ability to synthesise odd- and branched-chain fatty acids. As a result,ruminant animal fat contains odd-numbered fatty acids, such as 15, due to the action ofbacteria in therumen. Many fatty acids are unsaturated; some are polyunsaturated (e.g., those derived fromlinoleic acid).[5]
Most natural fats contain a complex mixture of individual triglycerides. Because of their heterogeneity, they melt over a broad range of temperatures. Cocoa butter is unusual in that it is composed of only a few triglycerides, derived frompalmitic,oleic, andstearic acids in the 1-, 2-, and 3-positions of glycerol, respectively.[5]
The simplest triglycerides are those where the three fatty acids are identical. Their names indicate the fatty acid:stearin derived from stearic acid,triolein derived fromoleic acid,palmitin derived frompalmitic acid, etc. These compounds can be obtained in three crystalline forms (polymorphs): α, β, and β′, the three forms differing in their melting points.[5][6]
A triglyceride containing different fatty acids is known as amixed triglyceride.[7] These are more common in nature.
If all three fatty acids on the glycerol differ, then the mixed triglyceride ischiral.
Triglycerides are colorless, although degraded samples can appear yellowish. Stearin, a simple, saturated, symmetrical triglyceride, is a solid near room temperature, but most examples are oils. Their densities range from around 0.89 with very long-chain fatty acids, through about 0.93 to 0.98 with medium-chain, and above 1.0 for very-short-chain acids.[8]
In nature, the formation of triglycerides is not random; rather, specific fatty acids are selectively condensed with the hydroxyl functional groups of glycerol. Animal fats typically have unsaturated fatty acid residues on carbon atoms 1 and 3. Extreme examples of non-random fats arecocoa butter (mentioned above) andlard, which contains about 20% triglyceride withpalmitic acid on carbon 2 andoleic acid on carbons 1 and 3.[5] An early step in the biosynthesis is the formation of theglycerol-1-phosphate:[5]
The three oxygen atoms in this phosphate ester are differentiated, setting the stage for regiospecific formation of triglycerides, as thediol reacts selectively with coenzyme-A derivatives of the fatty acids, RC(O)S–CoA:
Fats are often named after their source, e.g.,olive oil,cod liver oil,shea butter,tail fat. Some have traditional names of their own, e.g., butter, lard,ghee, andmargarine. The composition of these natural fats are somewhat variable. Theoleic acid component inolive oil can vary from 64% to 86%.
Triglycerides are then commonly named as esters of those acids, as in glyceryl 1,2-dioleate 3-palmitate, the name for a brood pheromone of the honey bee.[9] Where the fatty acid residues in a triglyceride are all the same, names likeolein (for glyceryl trioleate) andpalmitin (for glyceryl tripalmitate) are common.[citation needed]
an 18 carbon chain ("octadec-") with the carbon of the carboxyl ("-oic acid") given the number 1
all carbon-carbon bonds are single except for the double bond then joins carbon 9 ("9-en") to carbon 10
the chain connects to each of the carbons of the double bond on the same side (hence,cis, or "(9Z)"—the "Z" being an abbreviation for the German wordzusammen, meaning together).
IUPAC nomenclature can also handle branched chains and derivatives where hydrogen atoms are replaced by other chemical groups. Triglycerides take formal IUPAC names according to the rule governing naming of esters. For example, the formal name propane-1,2,3-tryl 1,2-bis((9Z)-octadec-9-enoate) 3-(hexadecanoate) applies to the pheromone informally named as glyceryl 1,2-dioleate-3-palmitate,[9] and also known by other common names including 1,2-dioleoyl-3-palmitoylglycerol, glycerol dioleate palmitate, and 3-palmito-1,2-diolein.
A notation specific for fatty acids with unbranched chain, that is as precise as the IUPAC one but easier to parse, is a code of the form "{N}:{D}cis-{CCC}trans-{TTT}", where {N} is the number of carbons (including the carboxyl one), {D} is the number of double bonds, {CCC} is a list of the positions of thecis double bonds, and {TTT} is a list of the positions of thetrans bonds. Either or bothcis andtrans lists and their labels are omitted if there are no multiple bonds with that geometry. For example, the codes for stearic, oleic, elaidic, and vaccenic acids are "18:0", "18:1cis-9", "18:1trans-9", and "18:1trans-11", respectively.Catalpic acid, (9E,11E,13Z)-octadeca-9,11,13-trienoic acid according to IUPAC nomenclature, has the code "18:3cis-13trans-9,11".[citation needed]
For human nutrition, an important classification of fats is based on the number and position ofdouble bonds in the constituent fatty acids.Saturated fat has a predominance ofsaturated fatty acids, without any double bonds, whileunsaturated fat has predominantlyunsaturated acids with double bonds. (The names refer to the fact that each double bond means two fewer hydrogen atoms in the chemical formula. Thus, a saturated fatty acid, having no double bonds, has the maximum number of hydrogen atoms for a given number of carbon atoms – that is, it is "saturated" with hydrogen atoms.)[11][12]
Unsaturated fatty acids are further classified intomonounsaturated (MUFAs), with a single double bond, andpolyunsaturated (PUFAs), with two or more.[11][12] Natural fats usually contain several different saturated and unsaturated acids, even on the same molecule. For example, in most vegetable oils, the saturatedpalmitic (C16:0) andstearic (C18:0)acid residues are usually attached to positions 1 and 3 (sn1 and sn3) of the glycerol hub, whereas the middle position (sn2) is usually occupied by an unsaturated one, such asoleic (C18:1, ω–9) orlinoleic (C18:2, ω–6).[13])
Saturated fats generally have a higher melting point than unsaturated ones with the same molecular weight, and thus are more likely to be solid at room temperature. For example, the animal fatstallow andlard are high in saturated fatty acid content and are solids. Olive and linseed oils on the other hand are unsaturated and liquid. Unsaturated fats are prone tooxidation by air, which causes them to become rancid and inedible.[citation needed]
The double bonds in unsaturated fats can be converted into single bonds by reaction with hydrogen effected by a catalyst. This process, calledhydrogenation, is used to turn vegetable oils into solid or semisolidvegetable fats likemargarine, which can substitute for tallow and butter and (unlike unsaturated fats) resistrancidification. Under some conditions, hydrogenation can creates some unwantedtrans acids fromcis acids.[14]
In cellularmetabolism, unsaturated fat molecules yield slightly less energy (i.e., fewercalories) than an equivalent amount of saturated fat. The heats of combustion of saturated, mono-, di-, and tri-unsaturated 18-carbon fatty acid esters have been measured as 2859, 2828, 2794, and 2750 kcal/mol, respectively; or, on a weight basis, 10.75, 10.71, 10.66, and 10.58 kcal/g – a decrease of about 0.6% for each additional double bond.[15]
The greater the degree of unsaturation in a fatty acid (i.e., the more double bonds in the fatty acid) the more vulnerable it is tolipid peroxidation (rancidity).Antioxidants can protect unsaturated fat from lipid peroxidation.
While it is thenutritional aspects of polyunsaturated fatty acids that are generally of greatest interest, these materials also have non-food applications.
Linseed oil and related oils are important components of useful products used inoil paints and related coatings. Linseed oil is rich in di- and tri-unsaturated fatty acid components, which tend to harden in the presence of oxygen. This heat-producing hardening process is peculiar to these so-calleddrying oils. It is caused by apolymerization process that begins with oxygen molecules attacking the carbon backbone. Aside from llinseed oil, other oils exhibit drying properties and are used in more specialized applications. These includetung,poppyseed,perilla, andwalnut oil. All "polymerize" on exposure tooxygen to form solid films, useful inpaints andvarnishes.[16]
Triglycerides can also be split into methyl esters of the constituent fatty acids viatransesterification:
Staining for fatty acids, triglycerides, lipoproteins, and other lipids is done through the use oflysochromes (fat-soluble dyes). These dyes can allow the qualification of a certain fat of interest by staining the material a specific color. Some examples:Sudan IV,Oil Red O, andSudan Black B.[citation needed]
^"Densities of Triglycerides"(PDF).BioNumbers—The Database of Useful Biological Numbers. Harvard. Retrieved25 February 2025. table from Gouw, T. H., and J. C. Vlugter. "Physical properties of triglycerides. I. Density and refractive index." Fette, Seifen, Anstrichmittel 68, no. 7 (1966): 544–549.
^abN. Koeniger and H. J. Veith (1983): "Glyceryl-1,2-dioleate-3-palmitate, a brood pheromone of the honey bee (Apis mellifera L.)".Experientia, volume 39, pages 1051–1052doi:10.1007/BF01989801
^Krisnangkura, Kanit (1991). "Estimation of heat of combustion of triglycerides and fatty acid methyl esters".Journal of the American Oil Chemists' Society.68:56–58.doi:10.1007/BF02660311.S2CID84433984.
