A vacuum coater applies liquid ingredients — typically fats, oils or other functional liquids — to porous feed products such as extruded pet food, aquafeed or some pelleted products by first drawing a vacuum on the product within a sealed chamber, then introducing the liquid. The reduced air pressure inside the product's internal pore structure causes the liquid to be drawn rapidly and uniformly into the product as the vacuum is released, rather than simply coating the outer surface as with conventional drum coating.

This method allows significantly higher fat or liquid inclusion levels than surface coating alone can achieve, since the liquid is absorbed throughout the product rather than sitting only on the exterior, where it could be lost through handling, abrasion or leaching during storage and transport.

The vacuum coating cycle typically involves loading a batch of product into the chamber, drawing down to the target vacuum level and holding briefly to allow air to be evacuated from the product's internal pore structure, introducing the liquid (sometimes in stages for multiple liquid ingredients), and then releasing the vacuum, at which point atmospheric pressure forces the liquid into the now-evacuated pores throughout the product.

Batch size, vacuum level, hold time, and the sequence and quantity of liquids introduced are all adjustable parameters that vacuum coater operators tune to achieve the desired fat or liquid inclusion level and uniformity for a specific product formulation, with different products (depending on their porosity and density) requiring different vacuum coating parameters to achieve consistent results.

Some vacuum coating systems are configured for continuous rather than batch operation, using a rotating or augered chamber design to move product through the vacuum and liquid application zones in a continuous flow, which can offer higher throughput than batch systems at the cost of somewhat less flexibility in adjusting the process for different products run through the same equipment.

Vacuum coating is especially common in premium pet food and some aquafeed applications, where high fat content and palatant inclusion are nutritionally and commercially important, and where the extruded product's porous structure makes it well suited to vacuum infusion compared to denser, less porous pellets that would not benefit as significantly from the vacuum infusion process.