A liquid coating system applies fats, oils, flavors or other liquid ingredients onto the surface of pellets, crumbles or extruded feed products after they have been formed, cooled and (where applicable) dried. Coating is generally preferred over adding all liquid ingredients before pelleting because high fat or liquid inclusion in the mash can reduce pellet binding and durability, so applying part of the liquid load as a post-process coating allows higher total fat inclusion while preserving pellet quality.

Common coating system designs include rotating drum coaters, where product tumbles through a liquid spray as it passes through a rotating drum, and vacuum coaters, which use reduced pressure to draw liquid into the pores of porous products such as extruded pet food or aquafeed for more uniform absorption and higher fat loading.

Drum coaters typically use a spray nozzle or manifold positioned above or within the rotating drum to apply liquid as product tumbles past, with drum rotation speed, internal baffle or paddle configuration, and dwell time within the drum all influencing how evenly the liquid is distributed across the product batch — insufficient tumbling action can leave some product pieces over-coated while others receive little or no liquid.

Coating uniformity, liquid temperature and application rate are key operational parameters, since uneven coating can result in inconsistent nutrient delivery between individual pellets or product pieces, and excess surface liquid can cause clumping or reduced shelf stability of the finished feed.

Liquid temperature management is particularly important for fat-based coatings, since many fats and oils must be maintained above their melting point to remain flowable and sprayable through the coating system's delivery lines and nozzles, requiring heated storage tanks and, in many installations, heat-traced or insulated delivery piping to prevent the liquid from solidifying before it reaches the coating equipment.

Coating systems are also commonly used to apply liquid additives beyond simple fats and oils — including enzymes, probiotics, flavors and certain heat-sensitive vitamins that would not survive the temperatures encountered during steam conditioning and pelleting — making post-process liquid coating an important tool for formulators wanting to include heat-labile functional ingredients without degrading them during processing.