Abattoir Rendering Processes
The by-products rendering process simultaneously dries the material and separates the fat from the bone and protein while yielding a fat commodity (tallow) and protein meal (meat meal).
The two main raw material rendering processes are wet and dry rendering.
WET RENDERING:
Wet rendering is a process where tallow is separated from the solids while the material is still wet.
In this raw material rendering process, the material is usually cut up into small pieces before being cooked through a reactor so that heat transfer gets to the centre of each particle as quickly as possible. Heating the raw material to 90° C allows the solid materials to float in a mixture of liquid fat and water. From this point on, in the animal rendering process, the solids can be separated from the water/tallow by pressing (twin screw press) OR centrifuging with the water and tallow combined called the liquid phase. The solids are dried to produce meat meal and the liquid becomes known as tallow and stick water where separators (polishers) separate the tallow and water.
DRY RENDERING:
There are two types of dry rendering processes known as batch cooking (cooked in batches) and continuous cooking (product is fed in and discharged continuously).
In this raw material rendering process, heating the material beyond 100°C basically means evaporating the moisture from raw material to a point where the solids start frying in fat instead of boiling in water. As the water evaporates, the temperature of the material (crax) rises to a point where discharge screening separates the crax material from free flowing liquid tallow. The crax material is pressed to produce a dry cake (meat meal) with the pressed tallow combined with cooker discharge tallow being further refined through centrifuging and polishing.
This animal rendering process is called dry rendering because the tallow and solids stay in contact with each other until the crax material is squeezed (dried).