Food Lipids: Chemistry, Nutrition, and Biotechnology

Figure 21 Hydrogenation reaction mechanism. (Modified from Ref. 61; photographs from
Ref. 62.)
good activity, selectivity, filterability, reusability, and economical use. Copper has
been occasionally used, but it must be completely removed to prevent accelerating
oxidation. Since catalytic activity is a surface phenomenon, inducing pore formation
increases surface area and thus activity [61]. However, the size and shape of pores
in the catalyst are important: pores must be at least 10 nm in diameter in order to
accommodate the transfer of triglycerides into them [60].
Catalyst promoters are substances that enhance the activities of catalysts without
having catalytic activity themselves. It is believed that the promoter function is
structural; that is, these substances somehow permit a larger number of active sites
on the catalyst particle. For this reason, hydrogenation catalysts are commonly supported
on siliceous materials, aluminum oxide, chromium oxide, cobalt oxide, or
copper oxide.
Catalysts are expensive, but they can be reused numerous times. Hydrogenation
catalysts can also catalyze oxidation. For these reasons, it is important to efficiently
remove the catalyst from the hydrogenated oil and then recover it, which is accomplished
by filtration. While surface area per unit weight increases as the size of the
catalyst particle decreases, so does the relative difficulty of removing the catalyst by
filtration. For that reason, the catalyst is often incorporated onto a support, such as
17–25% nickel fixed onto kieselguhr [60]. The catalyst is removed with leaf filters,
and the filtered, hydrogenated oil is bleached to assure removal of all catalyst before
deodorizing. This process, though termed postbleaching, is carried out in the same
manner as normal bleaching.
2. Selectivity
Selectivity refers to the relative rates of hydrogenation of specific fatty acids; selective
hydrogenation is the opposite of random hydrogenation. Ideally, the most un-
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