Glycerides, analysis

Olive oil is a mixture of fatty acid glycerides. Analysis of olive oil shows a high proportion of unsaturated fatty acids, and a typical analysis shows that the composition of the fatty acids is as follows ... 

An important feature of glyceride analysis may be the distinction between mono-, di- and triglycerides in a mixture or in a commercial product. Clearly there will be major molecular mass differences ... 

The partition of different lipids between two immiscible solvents (countercurrent distribution) is useful for crude fractionation of lipid classes with greatly differing polarities. Repeated extractions in a carefully chosen solvent pair increase the effectiveness of the separation but in practice mixtures of lipids are still found in each fraction. A petroleum ether-ethanol-water system can be used to remove polar contaminants (into the alcoholic phase) when interest lies in the subsequent analysis of neutral glycerides, which may be recovered from the ether phase. Carbon... 

Siekmann, B. and Westesen, K., Thermo analysis of the recrystallization of melt-homogenized glyceride nanoparticles. Colloids Surf. B., 3, 159-75, 1994. 

Breckenridge, W. C. 1978. Stereospecific analysis of triacylglycerols. In Handbook of Lipid Research, Vol. I Fatty Acids and Glycerides. A. Kuksis (Editor). Plenum Press, New York pp. 197-232. 

Gas-Liquid Chromatography. Gas-liquid chromatography (GLC) affords both a qualitative and, if adequate internal standards are used, a quantitative analysis of the products of lipolysis. It is necessary, however, first to isolate the acids by a suitable method and then to inject them as free acids or as esters. The partial glycerides can be isolated by thin-layer chromatography and can also be determined by GLC of suitable derivatives. The acid(s) remaining in the partial glycerides can be identified readily by GLC following transesterification. Jensen and co-workers have utilized these techniques in their studies of lipase specificity (Jensen et al 1964). 

The phosphoric acid esters of diacyl glycerides, phospholipids, are important constituents of cellular membranes. Lecithins (phosphatidyl cholines) from egg white or soybeans are often added to foods as emulsifying agents or to modify flow characteristics and viscosity. Phospholipids have very low vapor pressures and decompose at elevated temperatures. The strategy for analysis involves preliminary isolation of the class, for example by TLC, followed by enzymatic hydrolysis, derivatization of the hydrolysis products, and then GC of the volatile derivatives. A number of phospholipases are known which are highly specific for particular positions on phospholipids. Phospholipase A2, usually isolated from snake venom, selectively hydrolyzes the 2-acyl ester linkage. The positions of attack for phospholipases A, C, and D are summarized on Figure 9.7 (24). Appropriate use of phospholipases followed by GC can thus be used to determine the composition of phospholipids. 

Many of the tests described involve physical properties such as refractive index, viscosity or melting point of the fat, of the fatty acids or of the lead salts of the fatty acids. However, there were also many chemical tests such as Reichert, Polenske, iodine, saponification and acetyl values. These all gave information as to the composition of the fat, some information as to fatty acid composition, others as to other non-glyceride components of the fat. Thus the iodine value is a measure of unsaturated fatty acids in the fat, now obtainable in more detail from a fatty acid profile. Similarly the Reichert value is a measure of volatile fatty acids soluble in water. For most purposes this means butyric acid, and so the modem equivalent is the determination of butyric acid in the oil. The modem method for milk-fat analysis is thus carrying out the analysis in a similar way to the Reichert determination, but uses a technique that is less dependent on the exact conditions of the analysis and is thus less likely to be subject to operator error. The Reichert value could be useful, in theory, even if milk fat was not present. Lewkowitsch notes that some other oils do give high values. Porpoise jaw oil has a value almost twice that of milk fat, while some other oils also have significant values. It is unlikely that one would have come across much porpoise jaw oil even in 1904, and even less likely today. 

Peritectics are observed in mixed saturated-unsaturated systems. Figure 17.12(c) is a theoretical peritectic phase diagram. Several authors have reported mixtures of SOS and SOO to exhibit peritectic behavior (Rossell 1976). The peritectic mixture contained 24%SOS melted at 27.4°C, determined using differential thermal analysis. Table 17.4 list common phase behaviors of binary mixtures for different triacyl-glycerides adapted from Rossell (1967). 

A typical analysis of refined cottonseed oil indicates the composition of the acids present as glycerides to be as follows linoleic acid 39.3% oleic acid 33.1% palmitic acid 19.1% stearic acid 1.9% arachidic acid 0.6%, andmyristic acid 0.3%. Also present are small quantities of phospholipid, phytosterols, and pigments. The toxic polyphenolic pigment gossypol is present in raw cottonseed and in the oil cake remaining after expression of oil it is not found in the refined oil. 

A typical analysis of refined sesame oil indicates the composition of the acids, present as glycerides, to he arachidic acid 0.8% linoleic acid 40.4% oleic acid 45.4% palmitic acid 9.1% and stearic acid 4.3%. Sesamin, a complex cyclic ether, and sesamolin, a glycoside, are also present in small amounts. 

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