Feed Strategy - January 2018 - 28
28 ❙ FeedStrategy
Single-screw extruders, like the one pictured, are suitable for feed extrusion capacities up to 30 metric tons
per hour. Wenger Corporation
The extruder shaft grabs the material and pushes it forward
into a sealed barrel where it is subjected to high heat and
pressure. The friction between the extruder and the material
and the direct steam injection provide the heat while the
shaft profile generates increasing pressure on the material
as it travels through the fixed barrel to the discharge.
"The machine is essentially a continuous pressure
cooker with the high-pressure environment accelerating
heat transfer rates to provide a short residence time and
high temperature cook," Stibora says. Temperatures of
240 to 330 F and pressures of 500 to 2,000 psi are common for extruders.
At the discharge, the material is forced through a fixed
or adjustable orifice tailored to the specific application for
product formation and pressure control.
This process can be calibrated for a specific use by
altering the screw, or by adding parallels and barrels that
change the pressure, said Daniel Stoffner, Buhler group
product manager for animal nutrition.
The die can also be adjusted to produce the desired
size and shape of the feed. Extruders may use a predeter-
mined size for the die opening or include several plates
with different size openings. The size and shape of the die
affects the uniformity and expansion of the feed as well as
helps to add pressure inside the extruder by slowing down
the product flow.
This heat and pressure technique used in expanders and
extruders helps feed mills accomplish a variety of goals:
■ De-activate anti-nutritional factors: Some materials
like soybeans contain anti-nutritional factors (e.g.,
trypsin inhibitors, lectins, urease) that prevent them
from being consumed raw. Extrusion breaks down
these anti-nutritional factors to a level that is safe for
■ Improve digestibility: Research shows a significant
increase in livestock digestibility with wet and dry extrusion. This allows feed mills to use lower-quality raw
materials to produce a high-quality feed.
■ Allow for higher fat inclusion rates: Up to 7 percent of
fat can be added to rations produced through extruders
without affecting pellet quality. The higher fat percentage increases the feed energy.
www.WATTAgNet.com ❙ January 2018