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The
Manufacture of Sinking Fish Feeds on an Extrusion System
Introduction
The fish farming industry has undergone dramatic world wide
growth in the last ten years. As the industry has matured, a
more sophisticated understanding of efficiency and nutrition
is placed on the fish feed manufacturers. Customers view
critically the total quality package offered by the supplier
of feed, stretching the capability of a poorly designed feed
manufacturing system. Today's expectations are that the fish
feed supplier can produce a product that hits precise quality
targets in terms of size, durability, density and nutritional
composition at a highly competitive price. The following
article describes a few of the ways that Extru-Tech, Inc. has
tackled these issues to keep our customers at the cutting edge
of their market.
Product Consistency
A typical feed system, in an extrusion plant, consists of a
feed bin with bottom discharge into a screw conveyor for
metering of dry feed or mixed grist. This volumetric dry feed
addition system typically varies in dry feed rate by up to 15%
over or under the target rate. This wide swing in feed rate
causes dramatic changes in product quality as the extruder
tries to cope with being starved of feed, then overfed. Degree
of cook, product density and proportion of added materials
such as fish oil can all swing wildly as the feed rate varies.
To address this
issue we have developed a gravimetric feed system which weighs
the feed bin and contents continuously to adjust the feed
screw rate to accurately control feed rate. We have designed,
built and tested the system that can control within 0.5% of
control dry feed rate, this being achieved by a combination of
sound mechanical design and advanced electronics which
continuously learns. Other added ingredients such as fish oil,
water and steam can then be controlled at a proportional rate,
giving a highly consistent product. Our customers enjoy a
definite edge in their field with this feed system.
Product Density
It is hard to cook a product thoroughly and then form a dense
pellet all in the same extruder barrel, especially if the feed
is high in oil and low in starch. We realized some years ago
that it is extremely difficult to ask an extruder to both cook
and then form a dense pellet. This led to the development of
the Product Densifying Unit or PDU (A Patented Process). This
system uses an extruder to cook a rope of material which is
then fed into the PDU. The extruder runs at high power and
high speed (rpm), it’s operating parameters being set to
optimally cook the product. The PDU runs under lower power at
lower speed while precisely controlling the product's
compression and temperature to give the required density. As
the composition of the dry feed changes for different products
or as ingredients vary the extruder and PDU can be
independently adjusted to maintain product quality. The
production rates that can be achieved with this combination
are highly competitive with total capital and energy costs
significantly less than an equivalent twin screw extruder
system.
Pellet Quality
A sinking fish feed pellet should be of uniform size and shape
with minimum deformation of shape and no roughness of surface.
If the surface of the pellet appears to be rough it is highly
likely that there is too much drag through the die and the
product is being cooked and expanded in an uncontrolled way as
it leaves the forming machine. This can reduce pellet density
and create more fines as the pellet is handled. We have
developed a series of dies using a combination of materials
that create the ultimate in pellet appearance. The drag as the
pellet travels through the die is reduced to a minimum and the
pellets travel easily from the machine in a uniform manner.
The way that the
pellets are cut and handled as they exit the machine is very
important for control of breakage and appearance and we are
continually working to improve cutting and handling systems.
Starch Cooking
A fish feed pellet is held together by a matrix of cooked
starch. Water and steam are added to the dry feed to a total
moisture content of between 22% and 32%, depending upon
individual process requirements. As the moisture content rises
it is easier to cook the starch, but other components of the
feed compete with the starch for water, especially protein.
The lower amount of water available to the starch means that
more energy has to be put into the starch to cook it. This
extra energy can have anti-nutritional effects on the rest of
the dry feed and can increase energy costs.
We have
developed the Enhanced Hydration System (A Patented Process)
that first separates the starch component from the dry feed,
then hydrates the starch and finally adds it back to the dry
feed. The starch cooks much more quickly at relatively low
temperatures, allowing not only energy savings and reduction
of anti-nutritional effects, while using lower levels of
starch.
Note that many
fish feed manufacturers try to add as much oil as possible
before extrusion. This added oil can have an
‘encapsulation’ effect on the starch, making it more
difficult to cook the starch and to obtain a durable pellet.
The Enhanced Hydration System allows higher levels of oil to
be added before extrusion because it avoids this encapsulation
problem.
Oil Coating
Some feeds need oil contents as high as 35%. Typical maximum
oil levels before the extruder can rise as high as 18%; but
levels higher than this cause product quality problems with
the cooked matrix not being able to hang together. This means
that a significant proportion of oil has to be added after the
drying system. A coating system that operates at ambient
pressure can add up to 10-12% oil to a product, depending upon
product expansion and density. A vacuum coating system is
required for addition levels higher than this. Note that the
product will need to be cooked in a different way if very high
levels of post-drying oil are to be added, a more open matrix
being needed to soak up and hold the extra oil.
Pigment Loss
Canthaxanthin pigment is fed to salmon species to give the
pink flesh color required by the consumer. This pigment is
extremely expensive and is sensitive to heat. Much higher
quantities of the pigment are added to the feed than are
required by the fish to allow for heat destruction. We are
actively working in this area to reduce the amount of pigment
loss and are looking for industrial partners to take some of
our more exciting developments through to full commercial
application.
Specific Density
A product characterized only in terms of bulk density will not
necessarily predict whether it will sink quickly or slowly in
water or brine. We typically make a sinking fish food with a
wet bulk density off the extruder of about 42 pounds per cubic
foot, knowing the dry bulk density will be 1 to 2 pounds per
cubic foot below this. A more precise way of characterizing
the product is by weight under water, measuring the
displacement volume of water. This allows us to calculate the
specific density of the product, a property that is
independent of product size and shape.
The density of
4% brine is about 1040 grams per liter (64.9 pounds cubic
foot). A sinking fish food should have a finished specific
density of at least 1055 grams per liter (65.9 pounds per
cubic foot). Remember that post-coating with fish oil will
cause displacement of air in the product, thus increasing the
specific density.
Conclusion
It can be seen that Extru-Tech, Inc. is actively working on
many issues in the area of sinking fish feed manufacture. We
haven’t solved all the problems yet but feel we’ve made
significant progress in many areas, as evidenced by some of
our customers who are competing very effectively in the world
market.
We are willing
to work with anyone who can help us all to stride forward in
this very important commercial area and will be pleased to
talk to anybody who either wants more detailed information or
would like to work with us on specific projects.
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