A consommé is made by adding a mixture of ground meats, together with mirepoix (a combination of carrots, celery, and onion), tomatoes, and egg whites into either bouillon or stock. The key to making a high quality consommé is simmering; the act of simmering, combined with frequent stirring, brings impurities to the surface of the liquid, which are further drawn out due to the presence of acid from the tomatoes. Eventually, the solids begin to congeal at the surface of the liquid, forming a 'raft', which is caused by the proteins in the egg whites. Once the 'raft' begins to form, the heat is reduced, and the consommé is simmered at a lower heat until it reaches the desired flavor, which usually takes anywhere from 45 minutes to over an hour. The resulting concoction is a clear liquid that has either a rich amber colour (for beef or veal consommé) or a very pale yellow colour (for poultry consommé). It is then carefully drawn from the pot and passed again through a filter to ensure its purity, and is then put through a lengthy process where all of the visible fat is skimmed from the surface. To ensure total purification, the consommé can be refrigerated, which draws out remaining fat, which can easily be skimmed off with a cheesecloth. Alternatively, the consommé can be placed in a wide, shallow container such as a sauté pan or large bowl and wide strips of parchment paper can be dragged along the surface; the tiny amounts of remaining fat adhere to the parchment, leaving the consommé perfectly de-greased. Cartilage and tendons should be left on the meat because of the gelatin they contain, which enhances the mouthfeel of the soup. If beef or veal is used, shin meat is ideal because it is very low in fat and very high in gristle, and although it is undesirable for most other purposes, it is near essential for the flavour of the consommé. The meat is best if it is ground very fine, as if for a mousseline.