Fatty oils, selective hydrogenation

Polyunsaturated fatty acids in vegetable oils, particularly finolenic esters in soybean oil, are especially sensitive to oxidation. Even a slight degree of oxidation, commonly referred to as flavor reversion, results in undesirable flavors, eg, beany, grassy, painty, or fishy. Oxidation is controlled by the exclusion of metal contaminants, eg, iron and copper addition of metal inactivators such as citric acid minimum exposure to air, protection from light, and selective hydrogenation to decrease the finolenate content to ca 3% (74). Careful quality control is essential for the production of acceptable edible soybean oil products (75). 

Cocoa butter substitutes and equivalents differ greatly with respect to their method of manufacture, source of fats, and functionaHty they are produced by several physical and chemical processes (17,18). Cocoa butter substitutes are produced from lauric acid fats such as coconut, palm, and palm kernel oils by fractionation and hydrogenation from domestic fats such as soy, com, and cotton seed oils by selective hydrogenation or from palm kernel stearines by fractionation. Cocoa butter equivalents can be produced from palm kernel oil and other specialty fats such as shea and ilHpe by fractional crystallization from glycerol and selected fatty acids by direct chemical synthesis or from edible beef tallow by acetone crystallization. 

In this chapter, we report just a few selected examples of heterogeneous catalytic systems for the esterification of fatty acids and for the simultaneous esterification and transesterification of acidic oils and fats, and we discuss the use of selective hydrogenation as a tool for the production of high-quality biodiesel from non-edible raw materials. 

We have demonstrated that vegetable oils and fatty acid esters can be selectively hardened in liquid, near-critical, or supercritical C02 or propane and in mixtures thereof at temperatures between 60 °C and 120 °C and at a total pressure up to 20.0 MPa. Table 14.2 summarizes the results for the selective hydrogenation of vegetable oils in supercritical C02 in comparison with hydrogenation reactions performed in a discontinuous (i.e., batch or semibatch) stirred-tank reactor and in a continuous trickle-bed reactor. 

Fatty alcohols are obtained by direct hydrogenation of fatty acids or by hydrogenation of fatty acid esters. Typically, this is performed over copper catalysts at elevated temperature (170°C-270°C) and pressure (40-300 bar hydrogen) [26], By this route, completely saturated fatty alcohols are produced. In the past, unsaturated fatty alcohols were produced via hydrolysis of whale oil (a natural wax occurring in whale blubber) or by reduction of waxes with sodium (Bouveault-Blanc reduction). Today, they can be obtained by selective hydrogenation at even higher temperatures (250°C-280°C), but lower pressure up to 25 bar over metal oxides (zinc oxide, chromium oxide, iron oxide, or cadmium oxide) or partially deactivated copper chromite catalysts [26],... 

Jung, M. Y., and Ha, Y. L. 1999. Conjugated linoleic acid fatty acids in partially hydrogenated soybean oil obtained during nonselective and selective hydrogenation processes. J. Agric. Food Chem., 47, 704-708. 

Tagawa,T.,Nishiguchi,T., and Fukuzumi, K. 1978. Transfer hydrogenation and transfer hydrogenolysis XII. Selective hydrogenation of fatty acid methyl esters by various hydrogen donors. J. Am. Oil Chem. Soc., 55, 332-336. 

The use of chemical aids and technologies to stabilize lipids also represents a need to evaluate the balance between positive attributes that may reduce the risk of exposure to dietary oxidized lipids, or alternatively, negative consesquences, such as generation of tran -fatty acids derived from selective hydrogenation of vegetable oils. This chapter is intended to update the information on topics of toxicity and safety of fats and oils described earlier (6), as they relate to (1) natural consitutents of fats and oils (2) derived products of oxidation and hydrogenation (3) occurance of natural and pollutant contaminants and (4) additives used to preserve the stability, functionality, and nutritional quality of many constituents present in fats and oils. 

Non-lauric CBS consists of fractions of hydrogenated oils soybean, cotton, corn, peanut, safflower, and sunflower oils. These oils are hydrogenated under selective conditions to promote the formation of trans-fatty acids, thereby increasing the solid contents considerably. The melting point of oleic acid—the cw-configura-tion—is 14°C, whereas the isomer elaidic acid melts at 51.5°C. 

J. W. E. Coenan, The Mechanism of the Selective Hydrogenation of Fatty Oils, in J. H. deBoer (ed.), The Mechanism of Heterogeneous Catalysis, p. 126, Elsevier Publishing Company, New York, 1960. 

Fig. 1 Kinetics of the selective hydrogenation of sunflower oil fatty acid methyl esters (T = 25°C, solvent propene carbonate Pd/ester ratio 1 5000).

Selective hydrogenation is the tool by which partial hydrogenation is accomplished in a controlled manner. Selectivity is the saturation with hydrogen of the double bonds in the most unsaturated fatty acid before that of a less unsaturated fatty acid. In a theoretical sense, an oil hardened with perfect preferential selectivity would first have all of its linolenic fatty acids (C-18 3) reduced to linoleic fatty acids (C-18 2) before any linoleic was reduced to oleic (C-18.T) then, all linoleic fatty acids would be reduced to oleic before any oleic was saturated to stearic (C-18 0). Unfortunately, this does not happen in actual practice, but it is possible to vary the hydrogenation rate of linoleic to that of oleic from the very selective conditions of 50 to 1 to the less selective conditions of 4 linoleic to 1 oleic. The latter is generally described as nonselective. 

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