In animal products, the majority of tissue processed comes from slaughterhouses, but also includes restaurant grease, butcher shop trimmings, and expired meat from grocery stores.
This material can include the fatty tissue, bones, and offal, as well as entire carcasses of animals condemned at slaughterhouses and those that have died on farms, in transit, etc.
Edible rendering is generally carried out in a continuous process at low temperature (less than the boiling point of water).
Materials that for aesthetic or sanitary reasons are not suitable for human food are the feedstocks for inedible rendering processes.
The material is heated with added steam and then pressed to remove a water-fat mixture that is then separated into fat, water, and fine solids by stages of centrifuging and/or evaporation.
With the development of steam boilers, it was possible to use steam-jacketed kettles to make a higher grade product, and reduce fire danger.
From at least 1896, yellow grease has referred to lower-quality grades of tallow (cow or sheep fat) from animal rendering plants.
The pressure tank made possible the development of the Chicago meat industry in the United States—with its concentration in one geographic area—because it allowed the economic disposal of byproducts which would otherwise overwhelm the environment in that area.
Upton Sinclair wrote The Jungle (1906), an exposé on the Chicago meat processing industry which created public outrage.
In the 1920s, a batch dry rendering process was invented; the material was cooked in horizontal steam-jacketed cylinders (similar to the fertilizer dryers of the day).
Diversion in these materials into animal feeds soon replaced the lost soap market and eventually became the single largest use for inedible fats.
As an example, the United States annually recycles more than 21 million metric tons of highly perishable and noxious organic matter.
The fat obtained can be used as low-cost raw material in making grease, animal feed, soap, candles, biodiesel, and as a feed-stock for the chemical industry.
Meat and bone meal in animal feed was one route for the late-20th century spread of bovine spongiform encephalopathy (mad-cow disease, BSE), which is also fatal to humans.