Page 178 - Small Animal Clinical Nutrition 5th Edition

180 Small Animal Clinical Nutrition

VetBooks.ir Box 8-7. Baking and Pelleting Technology.

The first commercial pet foods were produced using baking
technology. Currently, the baking process is used primarily for
the manufacture of treats. The baking process uses equipment
similar to that used for extrusion in the initial phases. Raw mate-
rials need to be finely ground and mixed before they are com-
bined with water to form a “dough.” Rollers then flatten the
dough as it proceeds to the forming stage called a rotary mold-
ing. The dough flows along a conveyor belt where it passes
underneath a cylindrical rotor that contains the desired product
shape outlines, similar to a large multi-shape cookie cutter. As
the dough is “molded” into shapes, the rotor turns and releases
the product now cut into shapes. An oven then bakes the prod-
uct and in the final stages cools it so that it can be packaged.
Shape and product definition in baking are almost always
superior to that of extrusion. Because the rotor can be tooled to
make any detailed outline with little product expansion, the
baked treat can have detail that extruded foods cannot achieve.
The baking process, however, is less flexible than extrusion.
High-wheat formulations (and, therefore high gluten) are neces-
sary to provide product shape and rigidity. Fat levels must be
contained in a relatively small range.Too much fat will inhibit the
molding process and too little will cause the newly formed
pieces to stick in the rotor. Leavening agents must be used to
expand the product and maintain texture.
The pelleting process, which is mainly used to make livestock
feeds, is similar to extrusion, but has notable differences. The
dry mix must be even more finely ground because the pellet mill
has less flexibility in particle size than an extruder. The precon-
ditioning phase is also longer in pelleting because most of the
starch gelatinization must occur before product enters the pel-
let mill. Pressure from pressing the dry mix into the multi-hole
die translates to cooking the mix and forming pellets.
The pelleting process is markedly slower and has lower out-
put than extrusion. Ingredient selection and process flexibility
are limited in pelleting; the extruder can generate much more Figure 8-15. Pictures of single- (top) and twin-screw (bottom)
cooking energy, thereby allowing addition of internal fat and liq- extruders. The head of the extruder barrel has been removed
uid that is incompatible with pelleting. Because pelleting uses showing the screw assemblies. Steam and liquid can be injected
less cooking time than extrusion (two to 16 seconds at 71 to directly into the extruder barrel to modify the cooking process.
82ºC vs. 10 to 270 seconds at 80 to 200ºC), there is a greater
risk for microbial growth and production of a non-commercially
sterile product. In addition, extruded products are more age of internal fat (25%) can be made in the twin-screw extrud-
digestible than pelleted products. The benefit of the pelleting er, but not in the single-screw extruder because of the lubricating
process is its lower cost.
effects of fat. Other products that are difficult to extrude, such as
very sticky mixtures or formulas containing fresh meat, are more
readily processed in twin-screw extruders because of their
increased pumping action and increased cooking ability.
screws may be co-rotating or counter-rotating, intermeshing or
non-intermeshing. The most common twin-screw extruder is DIE AND KNIFE
the co-rotating, intermesh design (Figure 8-15). The final stage in the extrusion process occurs when the
The twin-screw design offers benefits over the single-screw gelatinized dough-like material is forced through the openings
extruder because its intermeshing screws mix more thoroughly, of the die. The die is a removable plate with one or more holes
transfer more frictional energy and convey the dough forward of the desired shape that is bolted to the head of the extruder
more effectively.This allows a shorter residence time distribution barrel. Die configuration is an important piece of the process
(i.e., the amount of time the dough is in the extruder barrel) and because the shape of the openings in the die contributes to the
more uniform cooking. Certain product formulations benefit shape of the finished product (Figure 8-16).In addition,the die
from twin-screw extrusion.Products that contain a large percent- provides the backpressure required for developing shear. The