Production of triacylglycerols

Enzymatic ester synthesis was generally described by Linfield et al. (1984). A yield of 43% triacylglycerols was realized over 6 weeks of incubation, making the process commercially unattractive. Subsequent improvement was made by Morita et al. (1984) obtaining a yield of 60% in 4 h of incubation in n-hexane. The substrates were 1,2-diacylglycerol and oleic acid. 

Using a modified system as described by Ergan et al. (1990), an 87% conversion of medium chain fatty acids into medium chain triacylglycerols (MCTs) has been observed. However, Kim and Rhee (1991) obtained marginal amounts of tricaprin in their experiments with lipozyme and various methods of removing water from the reaction. The current interest in the use of MCTs in various food applications as fatless fats could trigger more activity in their enzymatic synthesis. Attention might be focused on short-chain specific enzymes such as lipases from Rhizopus delemar (Tahoun and Ali, 1986). 

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The products of triacylglycerol digestion, mainly monoacylglycerol and long-chain fatty acids, interact with bile salts to form micelles, which comprise bile salts/ monoacylglycerols/phospholipids and fatty acids. The micelle aids the absorption of monoacylglycerol and fatty... 

Reversed-phase HPLC has been used to analyze the oxidation products of triacylglycerols in edible oils. The detection is often based on monitoring the conjugated dienes with an ultraviolet detector (234-235 nm). However, the UV detector provides no information about oxidation products without a conjugated diene structure, e.g., products of oleic acid. Information about these compounds is important when oils with a high oleic acid content are studied. The most common universal detector types—refractive index and flame ionization detectors—are not sensitive enough to detect small amounts of oxidation products. 

The lipase-catalyzed interesterification process can be used for the production of triacylglycerols with specific physical properties, and it also opens up possibilities for making so-called structured lipids. An example is a triacylglycerol that carries an essen-... 

Which one of the following products of triacylglycerol breakdown and subsequent P oxidation may undergo gluconeogenesis ... 

The digestion products of triacylglycerols retain their association with bile acids and lipase and combine with other lipids to form small aggregates called micelles that are suspended within the ingesta. The micelles bump into the brush border of the enterocytes (columnar epithelial absorptive cells of the intestinal wall) into which the digestion products pass by diffusion. 

The reaction of Cjg and Cjg higher fatty acids with triacylglycerols of coconut oil results in the release of Cg-Cj4 fatty acids, which can be continuously distilled from the reaction mixture as the product. The reaction can be autocatalysed at higher temperatures or an acidic catalyst (such as 4-toluenesulfonic acid) as well as basic catalysts (such as zinc, calcium or magnesium oxides) may be used. These procedures are now obsolete and fatty acids (for food purposes, for the production of triacylglycerols containing caprylic and capric acids only) are now obtained by hydrolysis and subsequent rectification. 

Triacylglycerols arise not by acylation of glycerol itself but by a sequence of steps m which the first stage is acyl transfer to l glycerol 3 phosphate (from reduction of dihy droxyacetone 3 phosphate formed as described m Section 25 21) The product of this stage IS called a phosphatidic acid... 

Diacylglycerol is glycerol esterified to two fatty acids at the sn-1 and sn-2 positions. It is a membrane-embedded product of phospholipase C action and an activator of protein kinase C. It is also an intermediate in the biosynthesis of triacylglycerol, phosphatidyletha-nolamine and phosphatidylcholine. 

Figure 25-4. Metabolic fate of very low density lipoproteins (VLDL) and production of low-density lipoproteins (LDL). (A, apolipoprotein A B-100, apolipoprotein B-100 , apolipoprotein C E, apolipoprotein E HDL, high-density lipoprotein TG, triacylglycerol IDL, intermediate-density lipoprotein C, cholesterol and cholesteryl ester P, phospholipid.) Only the predominant lipids are shown. It is possible that some IDL is also metabolized via the LRP.

The second type of fatty liver is usually due to a metabolic block in the production of plasma lipoproteins, thus allowing triacylglycerol to accumulate. Theoretically, the lesion may be due to (1) a block in apolipoprotein synthesis, (2) a block in the synthesis of the lipoprotein from lipid and apolipoprotein, (3) a failure in provision of phospholipids that are found in lipoproteins, or (4) a failure in the secretory mechanism itself. 

Phospholipids are digested and absorbed in a similar manner to that of triacylglycerol. Pancreatic lipase has some hydrolytic activity towards phospholipids and removes the fatty acid from the 1-position. The product is a lysophospholipid such as lysolecithin (Figure 4.12). It also acts as a detergent and contributes to the stability of the mixed micelles. 

In developed conntries, fat provides about 40% of the total energy consnmed but in some individuals this percentage may be mnch higher. Almost all of this is in the form of triacylglycerol, containing mainly long-chain bnt also some short-chain fatty acids. The stractnre, digestion, absorption and eventual fate of the products of absorption are described in Chapters 4 and 5 and the metabolism of fat is discussed in Chapter 7. 

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