Page 180 - Small Animal Clinical Nutrition 5th Edition

182 Small Animal Clinical Nutrition

Drying/Cooling
VetBooks.ir The high-moisture, soft, spongy kibbles are conveyed by air to
the dryer. During this conveyance, the kibbles lose another 2 to
3% moisture. The primary purpose of the dryer is to remove
another 10 to 15% moisture from the kibbles. Heated air draws
moisture out of the center of the kibble through the external
surface where the moisture evaporates into the atmosphere.
Reducing product moisture to between 8 and 10% inhibits
mold and bacterial growth,thereby improving product shelf life.
Two types of dryers are commonly used in the production of
pet foods: horizontal bed dryers and vertical bed dryers.
Horizontal bed dryers are usually long rectangular machines
that contain an inlet with a swing feeder, a slow-moving con-
veyor belt and an exit (Figure 8-18). Horizontal bed dryers can
be single or multiple pass, meaning that the product makes one
pass through or, in the case of a double-pass dryer, the product
Figure 8-16. Various die inserts can be fitted into the plate at the
tumbles from one level and doubles back before exiting.
end of the extruder barrel. The shape of the die will determine the
Vertical dryers are typically tall box-shaped machines that
shape of the dry kibble pieces. The dies shown here will produce
contain several levels or “decks” to support the newly extruded
oval, round, triangular or fish-shaped pieces.
pet food as it cascades from the top to the bottom of the unit.
These decks separate the product so that the drying profile can
be varied to meet the needs of a specific product. Each deck
uses a controlled discharge grid that can be cycled to allow all
or part of the product on that deck to flow down to the next
level using gravity.
Various zones of either type of dryer can be heated to differ-
ent temperatures to optimize drying. The first zones are usual-
º
ly heated to about 80 to 100 C where the product is warmed
and then moved into primary drying zones with temperatures
º
º
of 120 to 150 C. Finally, the kibbles are cooled to 80 to 100 C
before they exit the dryer. The typical retention time in a dou-
ble-pass dryer with a cooling zone is 15 minutes for drying and
seven minutes for cooling (Colonna et al, 1989).
Bed depth (height of the product on the conveyor belt or
dryer deck), zone temperature, retention time and humidity all
influence drying of pet food products.If the initial zones are too
hot, the external surface of the kibble will dry too quickly seal-
Figure 8-17. Rotating knives fit flush onto the plate at the end of
ing in moisture, which is known as “case-hardening.” As the
the extruder barrel. The speed of rotation determines the length of
moisture migrates through the external surface, microfissures
the individual food pieces.
can form that will make the kibble more fragile, leading to
º
The temperature of the extrudate is between 100 to 200 C. breakage and excess fine particles (fines) during and after pack-
It contains approximately 25 to 27% moisture and is under aging. If the product is too hot as it exits the dryer and is pack-
tremendous pressure (i.e., 34 to 37 atmospheres) (Dziezak, aged without proper cooling, condensation will occur, creating
1989). When the extrudate encounters ambient pressure and an environment that encourages mold and bacterial growth.
temperature at the die, moisture is flashed off, expanding the
product (i.e., at least 50% greater than the die diameter) and Enrobing
creating the characteristic porous texture of dry pet food. The Enrobing is the process by which coatings, either liquid or dry,
loss of between 3 to 5% moisture markedly cools the product are applied to the kibbles in the final step before packaging. For
and helps retain its shape (Colonna et al, 1989). example, coatings may be applied in a rotating drum (Figure 8-
The extrudate resembles a rope as it leaves the die. A knife 19) to which liquids (usually fat, flavor enhancers or both) are
assembly on the face of the die is used to cut the extrudate to added in predetermined amounts. Fat is usually added in the
the desired size (Figure 8-17).The design and sharpness of the enrobing step because it disrupts starch gelatinization and
knife are critical to product appearance and size. The multi- adversely affects expansion of the product. Flavor enhancers
bladed knife assembly rotates at high speed shearing off the have their greatest effect upon palatability when they are
“rope,” creating kibbles. The speed the knife assembly rotates applied topically. Three to four times the amount of enhancer
determines the length of the kibbles. may be needed if applied internally to achieve the same effect if