Hydrogenated oil is developed when hydrogen molecules are added to highly unsaturated oils, such as vegetable oils. During this process, called hydrogenation, partially hydrogenated oil is formed containing trans fats, which must be listed on a product's ingredients list.
Hydrogenation helps to make oils more stable and solid when stored at room temperature, aiding the baking characteristics of the liquid oil as well as the taste and texture of the end product. Many manufacturers used partially hydrogenated oil for years as an alternative to lard, beef tallow and butter when making baked and fried foods. While hydrogenated oils provided similar taste, texture and stability as animal fats in food products, scientists discovered that they contributed to heart disease and "bad" cholesterol levels, prompting many manufacturers to replace them with safer alternatives.