The process of pelleting consists of forcing a soft feed through holes in a metal die plate to form compacted pellets which are then cut to a pre-determined size. The machinery which has been developed for this purpose is now very diverse in design and there is much controversy between different equipment manufacturers as to which type is the most effective. Pelleting is a key to the production of high quality nutrional feeds as they ensure that the feed formulation is in the correct quantities for all that eat them. Each bite of a pellet will have the same designed formulation ensuring all the stock are feed as intended.
Most pellet mills now have one or more conditioning units mounted above them where liquids such as water and molasses can be added to improve pelletability. The water is sometimes added in the form of steam which softens the feed and partially gelatinises the starch content of the ingredients, resulting in firmer (and for aquafeed more water stable) pellets.
From the conditioner, the feed falls into the centre of the pelleter itself. In the pelleter two or more rollers and feed ploughs push the material through the holes of the die plate (see illustration below). A photograph of a die plate is shown in the photo above right:
Usually the die plate itself rotates, and on its outer-side stationary knives cut the pellets to a pre-set length. From there, they are transferred to a cooler/drier to remove the heat which is generated during the pelleting process or is often added during steam conditioning. The moisture content of the pellets also needs to be reduced for proper storage.
Pelleting and the efficient operation of pellet mills are specialized subjects on which many books have been written. All makers of pelleting equipment provide instruction in their use and some organize training courses in this and related topics.
Die hole sizes usually range from 1.5 mm (shrimp) up to 9.5 mm. The thickness of the die plate helps to determine the compactness and stability of the pellet. Pellet dies can be up to about 90 mm thick.