Trans-fatty acids are unsaturated fatty acids with a special structure created through both natural and food-technology processes. They are created naturally by microorganisms mainly found in the rumen of ruminants, where they convert fatty acids from ingested food into trans-fatty acids. One of the processes in food processing is fat hardening (hydration) to transform the texture and stability of oils into easier to process products with longer storage lives.
This makes it possible to produce more spreadable products, such as margarine, from liquid oils. Trans-fatty acids may be created as a by-product during this process. Their levels can be minimised as part of specific processes during fat hardening (choice of catalyst, optimising temperatures). Trans-fatty acids can also be created during heating or roasting in oil at very high temperatures.