a) Hydrogenation
Hardening of fats is produced by the addition of hydrogen to double bonds in the chains of fatty acids in triacylglycerols. This process has a vital role in the fats and oils industry because it achieves two main goals. In the first place, it permits the transformation of liquid oils into semisolid fats more indicated for specific applications, as in the cases of margarine and shortenings, and secondly, it results in materials with an improved stability.
For hydrogenation, a mixture of oil with a finely divided transition metal surface acting as a catalyst (usually nickel, but also palladium, platinium or rhodium) is heated to the hydrogenation temperature (140 to 225ºC), exposed to a hydrogen pressure of up to 60 psi whilst ensuring thorough mixing. Mixing permits faster rates of this heterogeneous reaction, helps dissipate, and is especially important given the large density difference between the catalyst and the reactants. Ensuring adequate agitation presents the most intricate technological barrier for it is mandatory that hydrogen-tightness be maintained throughout the hydrogenation device, even at moderately high pressure and when a rotor is installed, powered by an externally located engine. The starting oil must be refined, bleached, low in soap, and dry, or else the catalyst will suffer and become inactive due to preferential adsorption of any of the above (catalyst poisoning). The hydrogen must also be dry and free of sulphur, carbon dioxide and ammonia for exactly the same reason. The catalyst must possess long-term activity, act in the desired manner concerning selectivity of hydrogenation and isomer formation, and be easy to remove by filtration. Refractive index alteration, which is related to the extent of saturation of the oil, is usually used to monitor the course of the hydrogenation reaction. As the expected end point is reached, the hydrogenated oil is cooled and the catalyst filtered off.