Recent developments in extruder hood design can help pet food manufacturers reduce the risk of microbial recontamination after extrusion, Jonathan Thorn, executive director of processing technology at Coperion, said during a Tech Talk session at Petfood Forum 2026.
Thorn centered his presentation on what he called a negative airlift, a lightly loaded, dilute-phase conveying system that uses a fan rather than a high-pressure pump or compressed air as its motive force. Because a fan offers limited static pressure — generally less than 40 inches of water, or about 100 millibars — the system requires a larger pipeline relative to the volume of material, often starting around 8 inches in diameter and ranging up to 14 inches or more, he said.
That larger pipeline increases the volume of air needed to run the system, which in turn calls for sizable air-movement and separation equipment such as cyclones, filters and diverter valves, according to Thorn. He acknowledged the tradeoff candidly.
"It's a very inefficiently designed operation or conveying system," Thorn said. "You don't do it just because it's nice to do. We do it because there's some other benefit to the process that this large airflow facilitates."
When the negative airlift is attached to the discharge of an extruder, the joining point is known as an extruder hood, or negative airlift hood, Thorn explained. Air drawn into the system washes across the face of the extruder die and cutter assembly, carrying away product along with the steam and heat generated during extrusion.
That airflow delivers multiple benefits, Thorn said. It manages the heat and steam produced during extrusion, and it produces a flash-drying effect at the product surface.
"You can see 3% to 5% reductions on its way to the dryer," he said, which lowers dryer duty and leaves the product surface drier and less prone to sticking or clumping. Once inside the pipeline, the material is isolated from outside influences, adding a measure of product safety.
Rethinking a key recontamination risk
The extruder hood has historically used a clamshell design that wraps around the extruder discharge and draws ambient air in from below, Thorn said. Over time, that ambient air has been identified as a key source of recontamination.
"If you consider the extruder to be a kill step when it's operated at a certain pressure and temperature, we generally consider the product coming off that extruder to be free from microbial contamination," Thorn said. "If you then reintroduce microbes to that material, it now has a chance to grow and become a problem further downstream."
That concern led to the sanitary air extruder hood, which brings in air from a remote source and introduces it to the product in an isolated environment before using it to convey the material to its destination, according to Thorn. He outlined several benefits of the design, beginning with isolation that prevents outside contact with product after the kill step.
The incoming air can be conditioned, with filtration ranging from a high MERV rating up to a HEPA filter, and its temperature can be controlled, Thorn said. Operators can also measure air volume directly, improving process control that many systems have historically handled only indirectly. Divert gates that were previously external to the hood are integrated into the sanitary design.
A further benefit involves room balancing, Thorn said. Each system draws roughly 3,000 to 5,000 cubic feet per minute, and multiple extrusion processes in a single room can strain a plant's HVAC system. Supplying air from an external source and then extracting it makes the system nearly neutral from a balancing standpoint. "It's almost immaterial to the HVAC system in terms of balancing," Thorn said. "It's basically one to one."
Building a fully sanitary takeaway system
Combined with other sanitary components, the hood forms what Thorn described as a fully sanitary air takeaway system. Air is drawn through a HEPA or advanced filter by a supply fan operating on a variable frequency drive, allowing active speed control, he said.
A gas-powered line heater can serve three functions, according to Thorn: offline burnouts that raise internal pipe temperatures to sanitize the system, preheating to prevent condensation when hot, wet material enters cold equipment, and added flash drying that reduces downstream dryer load.
At startup, hood gates divert off-grade material out the bottom until good product is achieved, at which point the gates flip to send material through the conveying line, Thorn said. He emphasized that gaps in the system create cleaning concerns.
"Each gap in a convey line is an opportunity for a harborage point," he said, adding that machine couplings creating a tight connection between pipes represent a process improvement.
Separation devices such as cyclones and rotary valves with demountable rotors that allow full access to interior surfaces make thorough cleaning easier, Thorn said.
"The more the equipment facilitates that cleaning, the more likely it is to be done correctly," he said. A fan pulls product from the hood discharge to the cyclone and discharges steam and heat, typically to the atmosphere.
Thorn recommended testing extruded materials before installation. Because wet extruded products cannot be stored or shipped, testing is often done at the use point or with surrogates, he said — materials that "look and act the same shape, same size, same density, but don't have that moisture content," with the results translated to field performance.
