Trans-fatty acids are formed by converting (through hydrogenation) liquid vegetable oils into shortening or margarine which are solid or semi-solid at room temperature.
The hydrogenation process employs 1) high heat, 2) a metal catalyst such as nickel, zinc, copper, or other reactive metal, and 3) hydrogen gas. The metals are used to react with the hydrogen gas, which is bubbled up through the mixture. The metals catalyze the hydrogen and carbon atoms and convert the fatty acids by flipping one of the attached hydrogen molecules and rotating it half the diameter of the carbon chain. This effectively creates a new molecular shape resulting in a stiffer or more rigid material, hence the change from a liquid to a semi solid or solid substance. This new shape stiffens with the hydrogenation process making the oil behave more like a saturated fat (such as coconut fat which is 92% saturated and solid at room temperature). Trans-fats are the result of this reaction.
Partial hydrogenation, or "brush hydrogenation," is a minimal conversion step which only offers a small degree of reaction by hydrogenation. Brush hydrogenation increases stability for volatile fatty acids like Omega-3 (Alpha-Linolenic) and Omega-6 (Linoleic) polyunsaturated oils. Most commercial salad dressing oils, such as soybean oil, have been brush hydrogenated. Hydrogenation raises the melting point of the fat and retards rancidity.
Recent studies have found that health problems can ensue when consuming large amounts of trans fats from hydrogenated products. The FDA has determined that the safe level of trans fats in the diet is ZERO and has mandated that all food products be labeled with trans fat content by 2006. Spectrum products do not contain trans fats.
You can also learn about Hydrogenation and Trans-Fatty Acids in our Process section.