Page 177 - Small Animal Clinical Nutrition 5th Edition

Commercial Pet Foods 179

Compounding and Mixing
VetBooks.ir Good compounding (physically combining all of the dry mix
ingredients) and mixing are essential to delivering a consistent,
nutritionally adequate product. Improper compounding will
result in unequal distribution of essential nutrients or lack of
key ingredients in a product.
Ground bulk ingredients are usually stored in large bins.
From here, they are blown, via negative airflow, into mixing
hoppers. Ingredients that form a smaller percentage of the
product can be added by hand and combined with bulk ingre-
dients. Computer-controlled scales weigh dry mix ingredients
to ensure they are present in desired quantities.
After a “batch” of dry mix is compounded it must be proper-
ly mixed. Mixer types and times are carefully selected based on
the flow characteristics of the ingredients that compose the
mixture. Because particle size, density and shape influence flow
characteristics, a ribbon blender is often used for thorough mix-
Figure 8-14. Extruder preconditioner used in commercial pet food
ing (Harnby, 1993). A ribbon blender is a horizontal blending production. The mixing and conveying paddles shown here initiate
machine that has a shaft running through the middle of the starch gelatinization and move the dry mix toward the extruder for
mixer. A ribbon of steel attached to the shaft is the mixing ele- cooking.
ment. A ribbon blender is able to thoroughly mix large quanti-
ties of dry ingredients (up to 5,000 kg) and is the most com- through a multi-holed die where it is cut to the desired length
monly used mixing device. After the finished dry mix is ground by a rotating knife (Figures 8-15 through 8-17). This type of
for the last time, it is transferred to the extrusion process area extruder operates on the principle of friction.The desired cook-
and stored in a bin. ing of the dough is achieved when the material comes in con-
tact with the barrel wall (Johnson, 1993). The most common
Extrusion type of single-screw extruder used in the pet food industry is
Extrusion is the primary processing method for commercial- called a high shear cooker extruder (Dziezak, 1989).
ly prepared dry and semi-moist pet foods. The process itself The process conditions of the single-screw extruder can be
can be thought of in terms of bread making: mixing, knead- adjusted to produce different types of products. For example,
ing, proofing (rising of the dough), shaping, rising and slic- a high-fiber, low-fat formula produces significantly more fric-
ing (Figure 8-13).The first step in extrusion cooking is prop- tion, and is therefore more prone to overcooking than more
er preconditioning of the dry mix to start gelatinization of conventional formulas. Reducing the screw speed, screw pro-
the starches. file, feed rate or amount of preconditioning, increasing the fat,
The dry mix is fed into the preconditioner by a weighing sys- which helps lubricate the extruder barrel or a combination of
tem that accurately maintains a constant feed rate.The precon- these factors will compensate for this processing challenge by
ditioner contains mixing/conveying paddles (Figure 8-14) that reducing the cook time (residence time). Residence times can
mix the dry mix and liquid additions (fat, meat slurries, etc.), be varied from 10 to 270 seconds with temperatures ranging
º
along with water and steam to increase the moisture content from 80 to 200 C (Dziezak, 1989). Because of the ingredient
and prepare the mixture for cooking and forming. Generally, matrix used in pet foods, a high-temperature, short residence
the dry mix and any liquid additions are retained in the precon- time (HTST) approach is used.The benefits of HTST extru-
ditioner for about 45 seconds.The preconditioner achieves a 20 sion include: 1) complete cooking, 2) destruction of microor-
to 25% cook of the starch (a measure of the completeness of ganisms and 3) denaturing of anti-nutritional factors such as
extrusion cooking). Extrusion provides much more flexibility in trypsin inhibitor and hydrolytic enzymes that lead to rancid-
processing different formulations than do pelleting or baking ity (Dziezak, 1989). Nutrient losses, however, are also
technologies (Box 8-7). incurred because of the high temperatures and shear forces
(Box 8-8). Vitamin A and E losses of 21 to 26% and thiamin
SINGLE-SCREW EXTRUDERS losses of 12% may occur (Frye, 1995). Adding higher levels of
Single-screw extruders were originally used in the plastics these vitamins compensates for losses during extrusion.
industry and later converted to all types of food applications
and are used by approximately 90% of pet food manufacturers TWIN-SCREW EXTRUDERS
(Rokey, 1995). Extruders come in a variety of sizes from pilot Similar to the single-screw extruder, the twin-screw extruder
plant prototype machines to large volume machines capable of was borrowed from the plastics industry and introduced to the
extruding 12 to 15 tons of food per hour. Single-screw extrud- food industry in the early 1970s (Dziezak,1989).As their name
ers consist of a cylindrical multi-segmented barrel with a single implies, twin-screw extruders have components similar to those
screw that propels, mixes and cooks the material and forces it of single-screw extruders, except there are two screws. These