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Drying and cooling during the
Production of Extrudates
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1. Definition – what are Extrudates?
Extrudates are very common in food as well as in petfood and feed
production. For example, in the field of food production, snacks in
various shapes, colors and flavors are created by extrusion. Breakfast
cereals are crunchy extrudate products based on grain that are not only
enjoyed as a breakfast food, but are also loved as a snack.
Co-extrudated products have a stuffing that is covered by the usual
extrudates. This stuffing mass can either be a sweet cream like that
made from chocolate or a spicy cream like that made from cheese.
Raw materials for extrusion are most all of the grains such as corn,
rice, wheat, barley and rye, but other materials like potato flour, soy
and milk protein are also used.
In the field of pet food, extrudates from fish, meat and grains are
known. These extrudates come in various shapes and sizes and are
commonly found in dog and cat food.
After the extrusion process, the extrudates are usually dried, and in
this form they are ready to be sold. Nevertheless, in most cases they
are further processed and coated. Among other things, the coating serves
to add aromatic components and to refine the surface structure of the
extrudate. For this procedure, several raw materials are used: in the
field of food production these are vegetable oils, nuts, chocolate,
herbs and spices, milk powder, honey or cheese; in the field of petfood
in most cases these components are aromatic flavors, fats and oils.
2. Drying and Cooling as Part of the Product Process
2.1. Preparing for extrusion
The processing of the raw materials prior to the extrusion can be done
in several ways and with various automation stages. The main task in
this phase of the product processing is the mixing of the different dry
raw materials while adding water in a well-defined mix ratio.
2.2. Extrusion
The actual extrusion phase is done in the extruder. In order to do so,
several processes have been developed which shall not be mentioned here.
2.3. Drying after the extrusion
For almost all of the products that leave the extruder, the water that
was added in step 2.1 has to be removed or reduced. This is necessary to
achieve the desired consistency and storability.
For this drying task there are several basic processes and devices. The
choice of the optimal process and drier device depends on the product to
be dried and on the quality standard to be achieved.
A simple and very low-cost possibilty that could be used to dry the
product is the so-called countercurrent drier. This drier works in
batches instead of continuously. The extrudates are piled on a
ventilation floor and are exposed to hot air from the top and bottom.
Multiple stage countercurrent driers consist of several ventilation
floors in which the product runs through the drier in a stepwise manner.
After the end of the chosen drying time, the ventilation floor is opened
and the content falls into an emptying device or, as it is in multiple
stage driers, on the next floor underneath.
For products with stricter quality control standards, plate or belt
driers are commonly used. These driers work continuously and, for
multiple stage devices, the extrudates are turned over during drying.
Since there are so many manufacturers to choose from, the general rule
for choosing equipment is the lower the height of the product and the
more constant the air flow, the more constant the drying and the higher
the cost price.
2.4. Coating
Coating is usually done in coating cylinders. For coating of spicy
products, aromatic flavors and spices in suspensions based on fat are
used, while for sweet products and cereals usually sugar solutions based
on water are used. In the field of petfood production, both fat and
oil-based suspensions are commonly used.
Similar to the extruders, several different devices exist to coat the
extrudate which will not be mentioned here.
2.5. Drying / Cooling after Coating
Depending on the product and coating material, an additional drying and
/ or cooling procedure may be necessary after coating.
For this procedure, sugar containing solutions based on water as solvent
are applied to the product. The water is then immediately removed from
the product. For this kind of product, belt driers are used almost
exclusively due to the need of a continuous operation. Because of their
content of sugar, these products tend to become lumpy, they drip syrup,
and they may glue the drier. The optimal drier to be used in this stage
of the process must be able to be easily cleaned while at the same time
produce a product that is dried rapidly and consistently without any
lumping. This is the only way to achieve a continuous and error-free
process with constant product quality. It is recommended to watch for
these aspects very carefully when chosing a manufacturer.
A general rule to follow is the lower the height of the product and the
higher the number of turning stages, the lower the risk of lumps.
Cleaning is very easy with driers that have doors as big as possible to
allow easy access to the inside of the drier. Online cleaning systems
are capable of this. They even allow continuous cleaning during the
drying process. However, such online integrated celaning (CIP) systems
can be expensive, if they shall do their job correctly.
After drying the product to its desired final moisture content, a
cooling process is usually necessary before the product can be further
processed or packed.
In the field of petfood where fats and oils are usually used for coating,
a drying process is not necessary, but cooling is essential. The fat
that is added during coating to the hot dried extrudate is supposed to
soak in quickly and completely before it stiffens in the following
cooling process.
The quality standards in petfood are not as high as in the field of food
processing, but close attention must still be paid to the accessibility
and cleaning of the equipment. As in food processing, incompletely
absorbed fats and oils drip and may cause severe soiling of the cooler.
Even in the field of pet food, hygiene is very important!
Figure 1 shows the possible design of such a process:
Following the extrusion process, the product is dried before it can
enter the coating device. After coating an additional drying and / or
cooling process may be necessary.
In order to save energy, in many cases the exhaust air from the cooler
is not emitted directly into the atmosphere, but it is mixed with the
inlet air of one of the driers. This recycling of the exhaust is very
energy efficient and has the added benefit of being able to incorporate
the escaped aroma back into the product.
Realizing this, a significant effort has to be taken to fit the
necessary air ducts and tubes.
3. Combined Driers and Coolers
Figure 1 shows that great attention must be paid to drying and cooling
in order for them not to be underestimated. In addition, the disposal of
the exhaust air from the three devices, two driers and one cooler, must
be considered.
For that reason STELA has expanded its drier program and now offers a
combined plant containing the two driers and the cooler. The new
combination drier / cooler performs both drying steps – before and after
the coating – and cooling.
Figure 2 shows the integration of the new plant in the whole process.
In addition to the reduced costs and less maintenance, STELA emphasizes
most of all the decreased need for space and the simple guidance of the
air with the new drier/cooler. All three stages of the process can be
operated with a single air inlet and outlet. The system is separated
into zones in which the air flow and temperature is optimally adjusted
to the needs of the system at each particular point.
The exhaust air from the cooling zone is included in the drying process
automatically so that additional air guidance for energy recovering is
unnecessary. The compact and space saving design simplifies the housing.
Considering the lack of space in many companies this is not an
insignificant advantage!
The output of the product can be on the same side as the input or on the
opposite side, according to the needs for an optimal product flow.
Figure 3 shows a sketch of this new plant. It is offered in several
different sizes with performances from 300 t/h up to 7000 t/h. The first
plant of this kind is already operating at a well-known Italian petfood
manufacturing plant and could even do better than its high expectations.
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