How is liver fattened? A fattened liver is not a diseased liver. Migratory birds like ducks or geese in particular, naturally overfeed to endure winter and to fly long distances. They stock fat in their liver. Following this observation, peoples like The Egyptians have copied this natural operation for 4000 years to obtain foie gras. The differents types of Foie Gras and the designations Fresh Foie Gras : Fresh foie gras can be used to prepare preserves but generally it is used directly for warm preparations in gourmet restaurants. Fresh foie gras is generally cut into slices, and simply cooked on a high heat before being served. Semi-cooked Foie Gras : Semi-cooked foie gras is offered in terrines, jars or tins. It is a half-preserve that has been pasteurised, which means through-cooked to between 70° and 85°. Preserved Foie Gras : The most common preparation. Preserved foie gras is sterilised, through-cooked to between 105° and 115°. It may be stored for some years. Foie gras is served at the beginning of the meal, as an entree or appertiser. There is a wide range of preserved foie gras: whole foie gras, mass, parfait, mousse. But regulations are very strict for the designation of "foie gras". -Whole foie gras (foie gras entier) made of one or more entire liver lobes -Foie gras : made of pieces of liver lobe agglomerated together (75% minimum of fois gras). -Foie gras mass (mousse de foie gras) : made of reconstituted foie gras (50% minimum of foie gras).