Hydrogenation is a process where a given compound, such as a fatty acid or sugar, has hydrogen atoms to its structure. This alters its structure and properties. In fatty acids hydrogenation saturates them and renders them more rigid and solid at room temperature. It is also used to create sugar alcohols from a related saccharide, such as glucose to sorbitol.
Hydrogenation (particularly partial hydrogenation) of oils is likely to introduce trans fats to the end product. See: Trans fats.