World Grain - December 2010 - 49

FEATURE

Focusing on
P

ellet presses and many other feed processing technologies result in an agglomerated or pelleted feed.
Recognized advantages of pelleted feed include increased bulk density, less bridging in bins, less dust, reduced
ingredient segregation, less feed waste, increased nutrient
density, improved palatability, increased nutrient availability
and decreased microbiological activity.
While many processing technologies result in an agglomerated feed, only a few have sufficient energy inputs to ensure feed safety. Feed safety is a major factor in choosing
extrusion-based methods over traditional pelleting methods.
Extrusion is a hydro-thermal process where the critical process parameters of retention time, moisture and thermal and
mechanical energy inputs can be varied over a wide range.
The feed industry has considered a provision to mandate
that all feeds are pasteurized, and some form of extrusion will
likely be implemented as the processing method of choice.
The extrusion process can generally be divided into two
basic categories: cooking extrusion and forming extrusion.
Cooking extrusion elevates feed temperature to a level that
often results in an expanded product (final feed bulk density
that is less than the bulk density of the starting raw materials).
Forming extrusion is usually a low-temperature (often called
cold-forming) process that increases product bulk density and
cools the feed, resulting in a feed bulk density that is equal to
or greater than the bulk density of the starting raw materials.
INGREDIENTS
Raw material preparation for extrusion and related processes is very similar to that required for pellet mill installations.
Most applications employ a post-grind step. Individual whole
grains are pre-ground to reduce particle size, mixed with the
balance or the recipe, and then passed through a final postgrind step to achieve the desired particle size distribution. The
correct particle size is important because of:
* improved product appearance;
* reduced incidences of die orifices plugging;
* greater ease of cooking and pasteurization; and
50

by Mian N. Riaz

Studies show extrusion process may
fare better than traditional pelleting
methods in reducing toxins
* improved retention of liquid coatings due to smaller cell
size in final product matrix.
The process flow for an extrusion application differs from
the process flow for a pellet press application in two major
areas.
First, the grinding step for an extruder usually precedes a
sifting operation to remove foreign material and large particles. A magnet is usually installed prior to the grinding step in
all feed mill process flows to remove tramp metal. It is recommended to also include a magnet just prior to the extrusion
process to prevent accidental metal from the grinding operation damaging the extruder components.
Secondly, extruded products usually contain more moisture
than pelleted products. This moisture must be removed in a
drying step if moisture is greater than 12% to 15% by weight
of the extruded product. The higher moisture levels required
for most extrusion processes can lend versatility to the process
and expands the feed manufacturers' product possibilities.
The high temperature/short time extrusion cooking process is
able to accommodate a wide range of raw materials that might
otherwise be discarded as unqualified material.
Although extrusion does not completely eliminate toxins
and other anti-nutritional or anti-growth factors, in many
cases these substances or their activity is reduced to permit
some level of incorporation into the recipe. Research at the
University of Nebraska indicates that certain temperatures of
extrusion are sufficient to reduce fumonisin levels. Studies
have also indicated that viruses, molds and other pathogenic
organisms can be destroyed by the operating parameters emDecember 2010 / World Grain / www.World-Grain.com

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World Grain - December 2010

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