Lipid, analysis fatty acid composition

Accurate determination of lipids in foods is required for nutritional labeling, certification, or for evaluation of standard of identity and uniformity, as well as examination of their effects on functional and nutritional properties of foods. Following lipid extraction and precise quantitative analysis, lipids so obtained may be used for analysis of other lipid characteristics and properties provided that nondestructive and mild extraction procedures are employed that retain the integrity of lipids. Thus, determination of lipid classes, fatty acid composition (unit du), and oxidative state of lipids (Chapter D2), amongst others, may be pursued following the extraction process. 

The application of several chromatographic procedures to the separation and identification of milk lipids was mainly responsible for these endeavors. The first gas-liquid chromatographic (GLC) analysis of milk fatty acids was published by James and Martin (1956). By 1960, many laboratories were using GLC for routine analysis of fatty acids. For example, Jensen et al. (1962) reported the fatty acid compositions of 106 milk samples taken during 1 year. In comparison, Hansen and Shorland (1952) analyzed only six samples in a year, using distillation of methyl esters. 

Sample preparation is probably the most important step in any analytical procedure. Poor preparation of lipid samples will only yield inferior or questionable results. Some commonly performed sample-preparation procedures for gas-liquid chromatographic (GC) analysis of fatty acids in food samples are introduced in this unit. Since the introduction of gas chromatography in the 1950s, significant progress has been made in fatty acid analysis of lipids however, fatty acid methyl esters (FAMEs) are still the most commonly used fatty acid derivative for routine analysis of food fatty acid composition. 

Low-wavelength UV detection (200-210 nm) is more sensitive and permits the use of gradients but precludes the use of certain common lipid solvents, such as chloroform and acetone, which are opaque in the UV region of interest. With low-wavelength UV detection, the response will also be somewhat dependent on fatty acid composition. For these reasons the mobile phases used in lipid analysis by HPLC may seem rather strange to workers familiar with the Thin Layer Chromatography (TLC) or open column separations. 

Sato, T. (1997) Application of near infra-red spectroscopy for the analysis of fatty acid composition. Lipid Technol., 9, 46-49. 

Figure 2-6 Chromatogram of the Fatty Acid Composition of Fish Oil (Menhaden). Analysis of methyl esters on a 30-m capillary column. Source Reprinted from R.G. Ackman, Animal and Marine Lipids, in Improved and Technological Advances in Alternative Sources of Lipids, B. Kamel and Y. Kakuda, eds., p. 308, 1994, Aspen Publishers, Inc.

Kallio, H. and Currie, G. (1993) Analysis of lowerucic acid turnip rapeseed oil by negative ion chemical ionization tandem mass spectrometry. A method giving information on the fatty acid composition in positions sn-2 and sn-lB of triglycerols. Lipids, 28, 207-215. 

Four different strains of transformed F. lipolytica have been established in this study. Table 12.2 shows comparison of fatty acid composition in their lipids. The fungal A12-desaturase gene was introduced into yeast cells, generating a new yeast strain, Polg(pYLdl2). Fatty acid analysis showed that the percentage of LA in the cellular... 

Fatty Acid Composition Analysis. Lipid extraction followed the procedures described by Bellenger et al. (9). Fatty acid methyl esters were prepared by transmethylation using 14% boron trifluoride in methanol (10), and analyzed by GLC (Hewlett-Packard model 417 GLC Hewlett-Packard, Downers Grove, IL) equipped with a flame ionization detector and a 30-m capillary fused silica column coated with Carbowax 20M (Applied Science Labs, State College, PA). Fatty acids were identified by their relative retention times in comparison with a fatty acid standard (Nu-Chek-Prep) (11). 

In studies pertaining to rapeseed oil, various types of data are presented as relative measurements. Some measurements, such as organ weights relative to body weight, represent only a small part of the whole. Others, such as percent fatty acid composition of lipids, constitute almost the whole, that is, the percentages sum to nearly 100%. These types of data present particular problems in statistical analysis, several of which will be discussed below. 

The neutral lipid fraction extracted from biological materials contains among other components all the triglyceride fats. The fatty acid composition of these is sometimes of great significance, and methods for the establishment of this composition are based on the quantitative analysis of fatty acid methyl esters by gas chromatography. Initially, alkaline hydrolysis released the fatty acids, which were then extracted, concentrated and esterified, some kind of sulphuric acid/methanol procedure being widely used. However, this sequence of separate procedures has been superseded because losses may occur due to incomplete hydrolysis or side reactions such as polymerization or the alteration of unsaturated fatty adds. The preferred method is transesterification in either alkaline or add media [134]. The most recent methods use methanolic HQ, because in alkaline conditions some hydrolysis may occur as a side reaction and cause lowered yields [19, 135, 136]. There are of course effective quantitative methods for the... 

The comprehensive analyses of the mitochondrial inner membrane by McCarty et al. (1973) raise several interesting points. Here PI is again found to be a highly saturated lipid. Again, PC and PE have similar fatty acid compositions, but again PC is more unsaturated than PE. The analysis for lyso-... 

---