Hydrogenation is, quite sensibly, the name of the process by which hydrogen is added to something.
Hydrogen is the lightest of all known substances, but paradoxically, hydro-genating an oil makes it thicker and more solid. If we went all out with complete hydrogenation, the oil would become as solid as candle wax, which would make your margarine rather hard to spread.
Oils, whether from plants or petroleum, are made of molecules that can have “bonding gaps” between the atoms, not actual gaps in space, but regions of incomplete chemical-combining power (Techspeak: double bonds).
At these locations in the molecules the yearnings of atoms to join with other atoms are not fully satisfied. The atoms still have some unused bonding power that they could use to grab other atoms, if only the right other atoms would come along. (Techspeak: Such unfulfilled molecules are said to be unsaturated. If there is only one unfulfilled location in the molecule, it is called monounsaturated.)
Hydrogen is the perfect candidate for consummating the bonding desires of these unfulfilled atoms. It is the smallest atom of all, and can snuggle into almost any place in a convoluted molecule where it may be needed, especially if it is forced in under high pressure, which is how they hydrogenate oils. By filling in the gaps, the hydrogen atoms thoroughly satisfy the molecules’ longing to form bonds. (Techspeak: Fully bonded molecules are said to be saturated.)
What does that do to the oil? The saturated molecules are more compact once their bonding gaps are filled in, because they are somewhat more flexible. (Techspeak: Double bonds are more rigid.) They can therefore nestle together more tightly into their solid form, and the substance will stay solid longer when heated. That is, it will melt at a higher temperature. (Whenever an oil happens to be a solid at room temperature, we call it a fat instead of an oil. In fact, the technical term for them all, whether liquid or solid, is fat.)
We want to make vegetable oils semisolid for use as spreadable margarines, for example, but we don’t want to make the margarines too hard. That was no joke about the candle wax. The paraffin in a candle is actually a mixture of completely saturated oils; they come from petroleum, however, rather than from plant seeds.
In general, vegetable oils tend to be mostly unsaturated and are liquid at room temperature, while animal fats tend to be mostly saturated and solid. Vegetable oils contain about 15 percent saturated molecules. To make margarine, they are partially hydrogenated up to about 20 percent. Butter is about 65 percent saturated.
Unsaturated oils tend to break down and smoke at relatively low sauteing temperatures. They also turn rancid rather easily, because oxygen molecules from the air can get into those gaps between the atoms and attack them. Hydro-genation makes the oils more stable because it plugs the gaps with hydrogen atoms.
That’s the good news about hydrogenation. The bad news is that saturated fats appear to raise people’s blood cholesterol and increase their risk of heart disease. Food manufacturers are engaged in a never-ending struggle to keep saturated fats to a minimum so that they can brag about their products’ healthfulness, while at the same time hydro-genating their oils enough to give them desirable properties.