<P> Only unsaturated fats can be trans or cis fat, since only a double bond can be locked to these orientations . Saturated fatty acids are never called trans fats because they have no double bonds . Therefore, all their bonds are freely rotatable . Other types of fatty acids, such as crepenynic acid, which contains a triple bond, are rare and of no nutritional significance . </P> <P> Carbon atoms are tetravalent, forming four covalent bonds with other atoms, whereas hydrogen atoms bond with only one other atom . In saturated fatty acids, each carbon atom (besides the last) is connected to its two neighbour carbon atoms as well as two hydrogen atoms . In unsaturated fatty acids, the carbon atoms that are missing a hydrogen atom are joined by double bonds rather than single bonds so that each carbon atom participates in four bonds . </P> <P> Hydrogenation of an unsaturated fatty acid refers to the addition of hydrogen atoms to the acid, causing double bonds to become single ones, as carbon atoms acquire new hydrogen partners (to maintain four bonds per carbon atom). Full hydrogenation results in a molecule containing the maximum amount of hydrogen (in other words, the conversion of an unsaturated fatty acid into a saturated one). Partial hydrogenation results in the addition of hydrogen atoms at some of the empty positions, with a corresponding reduction in the number of double bonds . Typical commercial hydrogenation is partial in order to obtain a malleable mixture of fats that is solid at room temperature, but melts upon baking (or consumption). </P> <P> In most naturally occurring unsaturated fatty acids, the hydrogen atoms are on the same side of the double bonds of the carbon chain (cis configuration--from the Latin, meaning "on the same side"). However, partial hydrogenation reconfigures most of the double bonds that do not become chemically saturated, twisting them so that the hydrogen atoms end up on different sides of the chain . This type of configuration is called trans, from the Latin, meaning "across". The trans configuration is the lower energy form, and is favored when catalytically equilibrated as a side reaction in hydrogenation . </P>

How does one convert poly-unsaturated fats into saturated fat