Triacylglycerol simple

When most lipids circulate in the body, they do so in the form of lipoprotein complexes. Simple, unesterified fatty acids are merely bound to serum albumin and other proteins in blood plasma, but phospholipids, triacylglycerols, cholesterol, and cholesterol esters are all transported in the form of lipoproteins. At various sites in the body, lipoproteins interact with specific receptors and enzymes that transfer or modify their lipid cargoes. It is now customary to classify lipoproteins according to their densities (Table 25.1). The densities are... 

FIGURE 12.4 (A) Diagrammatic representation of the separation of major simple lipid classes on silica gel TLC — solvent system hexane diethylether formic acid (80 20 2) (CE = cholesteryl esters, WE = wax esters, HC = hydrocarbon, EEA = free fatty acids, TG = triacylglycerol, CHO = cholesterol, DG = diacylglycerol, PL = phospholipids and other complex lipids). (B) Diagrammatic representation of the separation of major phospholipids on silica gel TLC — solvent sytem chloroform methanol water (70 30 3) (PA = phosphatidic acid, PE = phosphatidylethanolamine, PS = phosphatidylserine, PC = phosphatidylcholine, SPM = sphingomyelin, LPC = Lysophosphatidylcholine). 

Structure and Triacylglycerols (also called fats or triglycerides) consist of three fatty acid function chains esterified to a glycerol backbone. Simple triacylglycerols have three... 

Fig. 1. Structure of (a) a simple triacylglycerol (1,2,3-tripalmitoyl-glycerol) and (b) a mixed triacylglycerol (1 -palmitoyl-2,3-dioleoyl-glycerol).

It should be noted that simple capillary GC, while convenient, just separates triacylglycerols into groups of similar molecular weight, and does not provide information on individual triacylglycerols - carbon number 38, for example, will consist of several different triacylglycerols (e.g., 4 0, 16 0, 18 0 4 0, 16 0, 18 1 6 0, 14 0, 18 1, etc.). 

Simple triacylglycerols are composed of three identical fatty acid side chains, whereas mixed triacylglycerols have two or three different fatty acids. Table 29.2 lists the most common fatty acids used to form triacylglycerols. 

Simple biochemical rules are made more complex by other factors, and in fish oil triacylglycerols, the 20 1 and 22 1 fatty acids of a high melting point confuse the issue (82). The traditional enzymatic approach to fatty acid distribution will soon be replaced by nondestructive instrumental methods, particularly nuclear magnetic resonance (NMR). It can distinguish the proportion of DHA between the 1,3- and 2-positions (74) and otherwise provide the details shown in Table 7 for a few oils, which shows verihcation of the method through an international exchange (83). A... 

Overall, DSC is an excellent method to measure the wax appearance and crystallization temperatures of vegetable oils. Due to the complexity of the vegetable oil composition with respect to their FA distribution, the situation is not as simple as pure triacylglycerol molecules. Moreover, there is significant influence of the nature, relative abundance, and orientation of C=C bonds on the wax appearance temperatures. Further, presence of other saturated short-chain-length FAs in vegetable oil structure is found to affect the crystallization process. Statistical analysis of NMR-derived vegetable oil structure support the influence of several predictor variables associated with FA unsaturation on the crystalhzation process. 

Mobile phases of hexane or light petroleum ether as main components and acetone or diethyl ether as polar modifiers are used for the separation of simple lipids. Acetic or formic acid is often added to keep the free fatty acids in the fully protonated form. The retention of simple lipids increases in the order waxes, sterol esters, methyl esters, triacylglycerols, free fatty acid, sterols, dia-cylglyerols, and monoacylglycerols (see Fig. la). If no acid is present in the mobile phase, the free fatty acids migrate between diacylglyerols and monoacylglycerols. The... 

Simple (or neutral) lipids release two types of products, e.g., fatty acids and glycerol. The most abundant simple lipids are the triacylglycerols. 

A) Reference mixture of simple lipid classes, silica gel TLC, mobile phase hexane-ethyl ether-acetic acid, 80 20 2 (v/v/v) SE, sterol esters ME, fatty acid methyl esters TG, triacylglycerols EEA, free fatty acids S, sterols 1,3-DG, 1,3-diacylgly-cerols 1,2-DG, 1,2-diacylglycerols MG, monoacylglycerols C, complex lipids detection by spraying with 5% ethanolic phosphomolibdic acid and heating for several minutes at 180°C. 

Fat is mainly composed of triacylglycerol (TAG) molecules. These TAG are polymorphic, i.e., they can crystallize under several crystalline forms (2). To get a stable food at the end requires the fat to be crystallized in the stable polymorphic form to avoid possible further transformation. Cocoa butter does not form in the stable Form V by simple cooling. It needs a well-controlled thermal path, performed under shearing conditions, called tempering. This specific temperature program is necessary to first nucleate enough crystals and then only keep the Form V crystals and melt the other metastable ones. Shear is also required during this process to get sufficiently rapid and intense nucleation of the stable forms. 

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