Hydrogenation creates trans fats. It is the process of heating an oil and passing hydrogen bubbles through it. The fatty acids in the oil then acquire some of the hydrogen, which makes it more dense. A fully hydrogenated oil creates a solid fat; a semi-solid partially hydrogenated oil that has a consistency like butter. These are added to foods to add richness and flavor without the cost of adding butter, as well as shelf-stability. Some trans fats occur naturally in lamb, beef, butterfat and dairy.