Triacylglycerol modification

Rousseau, D., Hill, A.R., Marangoni, A.G. 1996a. Restructuring butterfat through blending and chemical interesterification. 1. Melting behavior and triacylglycerol modifications. J. Am. Oil Chem. Soc. 73, 963-972. 

When heating treatments are applied to obtain stand oils, the following chemical modifications are likely to occur cross-linking of triacylglycerols, isomerization of double bonds, and formation of dimers through Diels-Alder cyclization [50,51]. As a result of double bond isomerization, the amounts of suberic and sebacic acids increase with respect to azelaic acid. Consequently, the ratio of suberic acid to azelaic acid may help to indicate a pre-polymerized oil [52,53]. 

Modification of nascent chylomicron particles The particle released by the intestinal mucosal cell is called a "nascent" chylomicron because it is functionally incomplete. When it reaches the plasma, the particle is rapidly modified, receiving apo E (which is recognized by hepatic receptors) and C apolipoproteins, The latter include apo C-ll, which is necessary for the activation of lipoprotein lipase, the enzyme that degrades the triacylglycerol contained in the chylomicron (see below). The source of these apolipoproteins is circulating HDL (see Figure 18.16). 

The flow of intermediates through metabolic pathways is controlled by 1bir mechanisms 1) the availability of substrates 2) allosteric activation and inhibition of enzymes 3) covalent modification of enzymes and 4) induction-repression of enzyme synthesis. This scheme may at first seem unnecessarily redundant however, each mechanism operates on a different timescale (Figure 24.1), and allows the body to adapt to a wde variety of physiologic situations. In the fed state, these regulatory mechanisms ensure that available nutrients are captured as glycogen, triacylglycerol, and protein. 

Postulated scheme for the synthesis, assembly, and secretion of VLDL by a hepatocyte (liver cell). (1) Synthesis The apoproteins, phospholipid, triacylglycerol, cholesterol, and cholesteryl esters are synthesized in the endoplasmic reticulum. (2) Assembly These components are assembled into a prelipoprotein particle in the lumen of the endoplasmic reticulum. (3) Processing The particle moves to the Golgi apparatus, where modification of the apoproteins occurs. 

Physical modification of milk fat by fractionating milk fat or by blending milk fat or milk fat fractions with other oils and fats results in products with an altered triacylglycerol composition, but one in which the fatty acids in milk fat maintain their original position in the triacylglycerol molecules (Kaylegian, 1999). 

Milk fat may be chemically modified to obtain products with altered functionality. In contrast to physical modification of milk fat where the position and nature of the fatty acid chains of the triacylglycerols are maintained, the use of chemical processes results in modification of the composition of the fatty acid chains or their positions in the triacylglycerol molecule. 

Interesterification involves an exchange of acyl groups within and between triacylglycerol molecules. This re-distribution of the fatty acids results in modification of the physical properties and nutritional properties of the fat (Frede, 1991). The traditional process of interesterification involves the use of chemicals. 

Chemical interesterification randomizes the fatty acid distribution in the triacylglycerol. The extent of modification of the fat depends on the composition of the starting fat and whether a single or a blend of fats is used and the conditions of the chemical interesterification process (Mickle et al., 1963 Huyghebaert et al., 1986 Rousseau and Marangoni, 2002). 

Solvent-free enzymatic interesterification of milk fat alone or with other fats or fatty acids provides the most acceptable route for modification of the triacylglycerol structures in milk fat and further research and development in this field is expected to provide physical and physiological benefits. From a nutritional perspective, it is of interest to examine the effects of randomized milk fat on serum cholesterol. Christophe et al. (1978) reported that substitution of native milk fat with chemically-randomized interester-ified milk fat reduced cholesterol levels in man. However, others found that there was no effect on serum cholesterol levels in man as a result of substitution of ezymatically randomized milk fat (De Greyt and Huyghebaert, 1995). Further studies are required to determine if interesterilied milk fat provides a nutritional benefit. 

Another promising area for adaptation of enzyme bioreactor technology is that of lipid modification. Several examples are a) the interesterification of triacylglycerols to change their composition b) limited lipolysis for production of flavors and c) conversion of cholesterol to forms that are not absorbed. The potential stabilization of enzymes to the presence of organic solvents would provide a definite advantage to enzyme bioreactor technology for the modification of lipid molecules. 

Natural oils and fractionated oils usually have their acyl chains organized in a nonrandom manner, but they become randomized after interesterification with a chemical catalyst. There is no change in fatty acid composition, only in triacylglycerol composition, but this leads to a modification of the physical properties. More selective interesterification can be achieved with enzymic catalysts (Section 8.5). 

Finally, genes required for particular aspects of fatty acid and triacylglycerol biosynthesis can be identified in appropriate sources, cloned, and transferred to other plants. Rapeseed has proved to be particularly flexible in this respect, and its fatty acid composition has been modified in several ways, some of which have now reached or are very close to commercial application (Section 9.4). Genetic modification procedures are also applied to soybean and other oilseed crops. 

There is substantial evidence that indicates that dietary fat can influence significantly not only serum levels of cholesterol and triacylglycerols but also the lipid composition and content of Apoproteins (156-159). Much attention has been placed on the effects of diet on LDL levels, and saturated fatty acid and cholesterol itself have been identified as the major nutritional factors that can raise serum LDL-cholesterol levels. However, LDL cholesterol is only one of the many risk factors for atherosclerosis, and it is not known if oxidative modification of LDL is an equally or more important factor in the pathogenesis of atherosclerosis than total LDL cholesterol per se. More longitudinal studies are needed to answer these questions. If lipid peroxidation is a major risk factor for atherosclerosis, then excess consumption of highly unsaturated fats may not be advisable. 

Modification of the Properties of Fats and Oiis Transesterification reactions (interesterification and acidolysis) of triacylglycerols have been an important tool in fats and oils processing for the modification of their physical and rheological properties by changing the composition and distribution of their fatty acids. 

Yamane, T. Lipase-catalyzed synthesis of structured triacylglycerols containing polyunsaturated fatty acids monitoring the reaction and increasing the yield. In Enzymes in Lipid Modification Bornscheuer, U.T., Ed. Wiley-VCH Weinheim, Germany, 2000 148-169. 

Fig. 5. Pathway depicting how flux through phosphatidylcholine (product of reaction 3) can promote acyl group diversity in plant triacylglycerols. Production of 18 2 (boxed) at the sn-2 position and its transfer to TG is used as a sample modification. Other fatty acid alterations may be substituted. Enzymes 1, glycerol-3-phosphate acyl-CoA acyltransferase and lysophosphatidic acid acyl-CoA acyltransferase 2, phosphatidic acid phosphatase 3, diacylglyceroliCDP-aminoalcohol aminoalcoholphosphotransferase 4, 18 l-desaturase or other fatty acid modifying enzyme 5, phosphlipid diacylglycerol acyltransferase 6, diacylglycerol acyltransferase 7, acyl-CoA phosphatidylcholine acyltransferase or phospholipase plus acyl-CoA synthetase.

---