By Harold McGee
For more than a century now, manufacturers have been making solid, fat-like shortenings and margarines from liquid seed oils to obtain both the desired texture and improved keeping qualities. There are several ways to do this, the simplest and most common being to saturate the unsaturated fatty acids artificially. This process is called hydrogenation, because it adds hydrogen atoms to the unsaturated chains. A small amount of nickel is added to the oil as a catalyst, and the mixture is then exposed to hydrogen gas at high temperature and pressure. After the fat has absorbed the desired amount of hydrogen, the nickel is filtered out.