Rendering is a process by which animal fats are purified. There are important differences between kitchen rendering and industrial rendering.
In the kitchen, rendering can refer 'clarifying' butter into ghee, suet into tallow and bacon fat into lard.
Rendering involves melting and extended simmering, followed by filtering and cooling. The entire process is then repeated in a clean vessel.
Unlike raw animal fats, rendered animal fats can be stored for extended periods without refrigeration, provided they are kept in airtight containers to prevent oxidation.
Industrial rendering is factory-scale process that uses slaughterhouse waste as its raw material. This material includes the heads, bones, offal, and other waste animal parts. The rendering process separates the fat from the bone and protein in the material. The fat can be used in animal feed, in soap-making, in candles, as a raw material for biodiesel production, and as a feed-stock for the oleo-chemical industry. The bone and protein becomes a dry particles known as meat and bone meal. For many years meat and bone meal were fed to cattle. This practice is now prohibited in developed countries because it was the main way in which BSE (mad-cow disease) was spread.