Addition of dry ingredients
The most common way to apply dry ingredients after the extrusion process is to meter the dry ingredients onto the particles after the application of liquids. This causes the dry ingredients to stick onto the particles. In order to apply a uniform tack application of dust to a particle, the liquid must first be applied in a uniform fashion. The dry dusting should be applied just after liquid application and before the retention time has given the liquid time to absorb on the particle. This can be done in any of the systems already discussed.
Another method for using dry ingredients is to blend the dry material with the liquids to be applied. Care must be taken that the ingredients are compatible and that the application system is capable of applying the solution or slurry.
The type of system that is employed depends on the required accuracy of the ingredients and how expensive the ingredients are. Even though mass flow system and loss in weigh systems have a greater up-front cost, a very short payback may be realized by eliminating the need to overapply an ingredient due to worst case errors. Regardless of which system is employed, regular cleaning, maintenance, and calibration of the system should be scheduled to insure accurate and trouble free operation.