Transesterification using modified

As discussed in section 3.6, the fatty acids in milk fat are not distributed randomly and the melting point may be modified by randomizing the fatty acid distribution by transesterification using a lipase or chemical catalysts. 

Before fatty acids are analyzed by GC, they must first be converted to the methyl esters. There is now a substantial body of work to confirm that acid-catalyzed transesterification is undesirable in general for the preparation of methyl esters because it can cause geometrical isomerization with an increase in the relative proportions of trans,trans isomers, and other unwanted side reactions (2,5,6). However, there appear to be no significant drawbacks to the use of base-catalyzed transesterification of lipids. For example with sodium methoxide in methanol, glycerolipids are transesterified in 10 min at 50°C. (A longer reaction time is necessary for cholesterol esters containing CLA.) Of course, with samples such as milk fat that contain a high proportion of short-chain fatty acids, it is advisable to use modified methods to minimize the loss of butyric and hexanoic adds, especially (7). 

The polymer is exposed to an extensive heat history in this process. Early work on transesterification technology was troubled by thermal—oxidative limitations of the polymer, especially in the presence of the catalyst. More recent work on catalyst systems, more reactive carbonates, and modified processes have improved the process to the point where color and decomposition can be suppressed. One of the key requirements for the transesterification process is the use of clean starting materials. Methods for purification of both BPA and diphenyl carbonate have been developed. 

The effect of incorporating p-hydroxybenzoic acid (I) into the structures of various unsaturated polyesters synthesised from polyethylene terephthalate (PET) waste depolymerised by glycolysis at three different diethylene glycol (DEG) ratios with Mn acetate as transesterification catalyst, was studied. Copolyesters of PET modified using various I mole ratios showed excellent mechanical and chemical properties because of their liquid crystalline behaviour. The oligoesters obtained from the twelve modified unsaturated polyesters (MUP) were reacted with I and maleic anhydride, with variation of the I ratio with a view to determining the effect on mechanical... 

It was necessary to add over 10% buffer for the transesteiification of phosphatidyl choline by native lipase (5). Hydrolysis occurred as a side reaction in the hydrophobic solvent-water system. Tlie transesterification of phosphatidyl choline and eicosapentaenoic acid (EPA) was carried out in water-saturated n-hexane using palmitic acid-modified lipase. Table II shows the transesterification of phosphatidyl choline and EPA. Modified lipase made it possible for the transesterification of phospholipids in organic solvents. 

Because a substantial excess of the halide is used in the Koenigs-Knorr synthesis of cardenolides, it is usually difficult, if not impossible, to secure the acylated intermediate in crystalline form. Hence, Reichstein nd coworkers18 introduced a procedure whereby the whole of the products arising from the coupling reaction are saponified, employing a solution of potassium hydrogen carbonate in aqueous methanol. This modified transesterification procedure renders all extraneous carbohydrate materials water-soluble the methanol is removed by evaporation, leaving an aqueous solution from which the (deacylated) cardenolide may be obtained by... 

In the food industry, lipases are used in lipid modification processes. In these processes the texture, digestibility, or physical properties of natural lipids are modified by lipase-catalyzed transesterification reactions with lipids other than the original fatty acids. In the baking industry, lipases are used to influence the quality of bread through modification of the wheat flour lipids. Finally lipases are used for flavor enhancement of cheese in the dairy industry. 

Alcoholysis. Methanolysis of triacylglycerols is used to prepare methyl esters for fatty acid analysis, a process frequently referred to as transesterification. This can be acid-or base-catalyzed, the method being chosen to avoid modifying acid-or base-sensitive fatty acids and to minimize reaction times. Sterol esters of fatty acids react more slowly than triacylglycerols, and samples containing them require more vigorous reaction conditions. The preparation of methyl esters from oils and fats for GC and GC-MS analysis has been extensively reviewed (19, 22, 23). 

Polybutylene therephthalate (PBT) has been used as a blend component to provide chemical resistance in various systems, but the most interesting one results from a combination with polycarbonate and, eventually, an Impact modifier of the coreshell type. Polyester blends containing polycarbonate exhibit ester interchange chemical reactions, which add to the complexity of property control of these materials. DEVAUX and co-workers (14) have examined the transesterification reaction catalysed by residual catalysts in PBT which can lead to the formation of block and random copolymers. They have shown that allyl or aryl phosphites inactivate the residual titanium catalyst and minimise the transesterification reaction. HOBBS et al. (15) reported a way of controlling miscibility behaviour, morphology and deformation mechanisms, in order to obtain blends compati-bilisation and excellent mechanical properties. 

Patents on poly(ester-imide) resins where one of the components is ethylene carbonate [54, 55] or an imide modified linear poly(ethylene carbonate) [56] also exist. In another case the carbonate group is introduced into the molecule using diphenylcarbonate via transesterification reaction [57]. 

R = CH2C6H5, Ci4Hi202, Mt 212.25, Z>p2.okPa 170-171 °C, df 1.1121, ng 1.5680, is the main component of Peru balsam oil. It occurs in fairly large amounts in a number of blossom concretes and absolutes (e.g., tuberose and hyacinth). It forms either a viscous liquid or solid flakes (mp 21-22°C) and has a weak, sweet-balsamic odor. It is prepared either by transesterification of technical methyl benzoate with benzyl alcohol, or from benzyl chloride and sodium benzoate. A third process starts with benzaldehyde, which is converted in high yield into benzyl benzoate in the presence of sodium or aluminum benzylate (Tishchenko reaction). Benzyl benzoate is used in perfumery as a fixative and as a modifier in heavy blossom fragrances. 

Homogeneous catalytic system has been used for the transesterification of p-ketoesters (1). From the environmental as well as economical point of view, a heterogeneous catalyst is much more desirable. For this reason the fimctionalized and modified silicas have been tested. These catalysts were obtained by sol-gel method in both synthesis systems. Optimal reaction... 

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