Hydrogenation of soybean oil triglycerides: Effect of pressure on selectivity

Soybean oil contains five triglycerides that account for over 70% of the to tal. These include LLL, LLO, LLP, LOG, and LOP and their isomers (where L = linoleic, O = oleic, P = palmitic). High-performance liquid chromatographi c analysis of samples taken during a typical hydrogenation run in which the iodine value (IV) was reduced from 130 to about 70 showed that the linolea te-containing triglycerides were reduced at a much faster rate than the lin olenate-containing triglycerides. Results clearly show that hydrogenation p roceeds through definite pathways rather than by random saturation of fatty acids within the triglyceride molecules. Pressure has a significant effect on the course of hydrogenation. At higher pressures (500 psi), the reactio n is truly nonselective, since di- and trisaturated glycerides are formed a t about 70 IV, whereas at 50 psi, the reaction becomes selective. At higher pressures, fat functionality and solid fat result primarily from di- and t risaturated fatty acid triglycerides as well as trans fatty acid triglyceri des. At lower pressures, functionality results from trans acid triglyceride formation. Although the reactivity of linoleate containing triglycerides f ollowed the pattern 6 double bonds > 5 double bonds > 4 double bonds, other factors may be important. For example, LLP is reactive and undergoes hydro genation, while LLS remains unchanged. Triolein, which constitutes less tha n 3% of the total triglyceride in natural soybean oil, is a significant pro duct of hydrogenation, which suggests that LLL and LLO are reduced directly while adsorbed on the catalyst surface.