Extrusion technology provides a number of major benefits over the more traditional pellet milling processes commonly used for aquaculture feeds. In particular, extrusion can provide a much higher degree of control over the “cook” achieved, as well as better control of the product density (therefore controlling the floating/sinking characteristics).

1. Extrusion
of aquafeeds
B
ut extrusion is a very complex
process – and we only have
“indirect” control over that
process. That is, we have full
control over some variables – eg screw
speed and amount of water added. But
there are other inputs over which we have
“limited” control. For example, we specify
a formulation, and within limits we control
the specifications of ingredients that go into
it – but ingredients do vary, so our process
needs to cope with “normal variation”. We
set up the extruder and die to a known con-
figuration – but the machine and the die-plate
wears, so that over time the process changes.
In addition, we never get just one param-
eter changing during extrusion – if ingre-
dients vary, it doesn’t just change the final
product composition – it changes the rheol-
ogy of the mix and therefore changes how
the melt moves through the extruder and
the die, which in turn affects the residence
time and temperature developed in the melt,
which changes degree of cook and expan-
sion – and therefore affects digestibility and
floating/sinking characteristics. Within this
complex relationship, we need to achieve
consistency – of nutrition, of digestibility, of
physical characteristics. It is the cumulative
effect of these parameters which ultimately
determine the feed conversion ratio (FCR)
for the product.
So to use extrusion effectively – and take
full advantage of the opportunities offered
by the technology – the extrusion process
needs to be properly understood. This article
discusses just a couple of the issues relating to
successful extrusion of aquafeeds.
Extrusion chemistry &
aquafeed ingredients
In the area of ingredients, the aquafeeds
industry faces a series of contradictions.
Marine meals provide the best nutritional
basis, but are increasingly difficult to obtain
and are therefore expensive. Selected plant
sources of protein can provide required
proteins, but tend to come with fibre and
starch which can interfere with the extru-
sion process and affect digestibility. We
generally require very high fat (energy) con-
tents in the feeds – but high fat levels are a
problem in extrusion. Following are some
basics of “extrusion chemistry” that relate
to the way we design aquafeed extrusion
processes:
by Gordon Young, FoodStream Pty Ltd, Australia and Dennis Forte, Dennis Forte & Associates, Australia
Extrusion technology provides a number of major benefits over the more traditional
pellet milling processes commonly used for aquaculture feeds. In particular, extrusion
can provide a much higher degree of control over the “cook” achieved, as well as better
control of the product density (therefore controlling the floating/sinking characteristics).
Figure 1
10 | INTERNATIONAL AQUAFEED | March-April 2015
FEATURE

2. Starch
Many fish species do not tolerate starch –
especially uncooked starch. But starch comes
with many of the alternative plant-based
protein ingredients. Also starch can be an
important “binder” to help achieve pellet
durability.
Firstly, realise that starch does not truly
“gelatinise” during extrusion – there is not
enough water present for the swelling and
unravelling of granules that characterise the
gelatinisation reaction. In addition, the starch
is easily damaged by excessive “shear” during
extrusion. So we promote conditions that
will “cook” the starch – achieving sufficient
temperature with sufficient water, and with
sufficient time, but without excessive shear
– therefore use of a preconditioner can be a
major advantage.
Another option – if we require starch for
pellet binding – is to choose a starch that
cooks and binds more effectively at a lower
temperature – such as use of tuber starches
instead of grain starches. Therefore effective
selection of both the source and the amount
of starch can be used to optimise the pro-
cessing and nutritional characteristics of the
product formulation.
Protein
During extrusion, the protein dena-
turation reaction is not unlike that of
starch – that is, with respect to good
“functional” protein (here referring to
protein functionality from a physical, rather
than nutritional, perspective). The globu-
lar proteins unravel and, under the right
conditions (optimal moisture content and
temperature), can cross-link. Therefore
“functional” protein contributes to binding
and pellet durability. But many of the tra-
ditional fish meals, while good nutritionally,
contain denatured protein, and contribute
little to the “binding” function.
So while the scarcity of traditional
marine proteins (eg fish meals) is an
issue for nutritional balance, the sub-
stitution of “functional” plant proteins
can have the added benefit of assisting
pellet durability – as long as the process
promotes rather than destroys that
functionality (temperature and shear not
excessive).
Figure 2
March-April 2015 | INTERNATIONAL AQUAFEED | 11
FEATURE
Effect of Density on Sink / Float
Pellet Behaviour Sea Water (3% Salt) Fresh Water
Fast Sinking 640 g/L > 600 g/L
Slow Sinking 580 to 600 g/L 540 to560 g/L
Neutral Bouyancy 520 to 540 g/L 480 to 520 g/L
Floating < 480 g/L < 440 g/L
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3. Fat
Fat under normal extrusion conditions
is stable – it does not itself change and
“cook” as starches and proteins do. But it
does have a major effect on the extrusion
process – increasing “slip” and effectively
lowering viscosity, and therefore affect-
ing expansion/density, extruder backfill/
degree of cook, and pellet durability. An
increase in fat content of the mix of only
two percent (which can happen due to
raw material variations) has a similar effect
on the effective viscosity of the melt as a
five percent increase in moisture content.
Therefore control of fat content can be a
major issue.
At the same time, for most species, as high
a fat content as possible is required for nutri-
tion (feed energy). This is where one of the
tensions lie – the challenge of achieving good
stable pellets at sufficiently high fat content.
Strategies to achieve this balance include
the use of appropriate extrusion systems
(twin screw extruders can operate in a more
stable manner to higher fat levels than single
screw extruders), and methods to maximise
post-process addition of oil, such as vacuum
infusion.
There are of course many other aspects of
ingredients that also need to be balanced in
successful feed extrusion.
Density control
The finished product bulk density is one
of the key product quality attributes, as it
directly influences the sink / float behaviour
of the finished product. Note, however,
that this does not only relate to the extru-
sion process – eg drying also affects pellet
density. In fact, drying under inappropriate
conditions can result in drying and shrink-
age of the outside layers, increasing overall
density so the pellet sinks (eg as required).
Then, over time in storage, moisture redis-
tributes within the pellet, the outside layers
relax and expand, and the feed turns from
floating to sinking.
The bulk density is the result of the bal-
ance between the expansion (influenced by
the process conditions and the amount and
type of starch) and elastic collapse (influenced
by the amount and type of protein). That is,
again there is a complexity in the extrusion
process which makes it inherently difficult
to predict and control. Under the correct
conditions:
• Higher amylose starch content makes
the extrudate expand more at the time
of exiting the die
• Higher “functional” protein makes the
melt more elastic, so it recoils after the
initial expansion and starts to collapse
• Higher amylopectin starch increases the
“solidification” temperature of the melt,
which determines how far the melt
recoils before the structure hardens.
So the final pellet size – and therefore
density – is the result of the interaction of all
these effects (Figure 2).
In addition:
Degree of Expansion = f { Melt
Temperature (TM), Die Pressure Drop (DPd)
}, with the Melt Rheology and the Die
Geometry greatly affecting DPd.
The product bulk density is also signifi-
cantly influenced by the Specific Mechanical
Energy (SME), since the SME directly
affects TM and also affects the molecular
degradation of the starch and the proteins
– which changes the viscosity and elasticity
of the melt. Figure 1 shows some meas-
ured results of bulk density vs SME.
The SME is influenced by process param-
eters, most significantly the melt moisture
content (used to manipulate the melt rheol-
ogy), the screw profile design, and the screw
speed.
Aquafeed specifications
– not the full story
Final product nutrition (and FCR) is not
just due to the composition of the formula-
tion ingredients – though that is what feed
specifications almost exclusively (along with
density) often dictate.
The manufacture of a feed should be
considered as
Formulation + Process ----> Product
The ingredient source (and the order of
addition) have a significant impact upon the
product nutrition. For example, the effect of
oil added during the process is not the same
as oil added via external coating – complexes
formed during extrusion can change the
nutritional effect of the oil.
Energy inputs also have a major role in final
nutrition of the feed. Energy may be added via
Convective Energy (steam injection), Thermal
Energy (barrel heating) and Mechanical Energy
(viscous dissipation or SME). The comparative
balance of these energy inputs affect nutrition
by changing the conversion and break-down
of proteins and starches, changing their nutri-
tional contribution. In extreme cases, it can
even form fat complexes that are toxic to fish.
When considering Convective Energy, the
role of process time is also critical. This is
the basis of preconditioning, which allows
extended time to initiate the cooking of
starches and proteins.
Conclusion
Extrusion is not a simple process. The
interactions that occur in process parameters,
along with variations in raw materials and
changes over time due to extruder and die
wear, makes it a difficult process to control.
And these variations can have “invisible”
effects – because nutritional content of the
formulation does not fully define the nutri-
tional performance of the feed. Variations
in the extrusion process do not only affect
physical changes in the product such as den-
sity and size/shape, it also affects the way in
which the fish will digest the ingredients. A
good understanding of the extrusion process
– and well-defined process parameters to
guide operators – is required for reliable and
consistent feed production.
The authors are presenting a short
course on Aquafeed Extrusion Technology
at Centre for Feed Technology, FôrTek,
Norway, from 25 to 27 March 2015
(www.foodstream.com.au/events)
12 | INTERNATIONAL AQUAFEED | March-April 2015
FEATURE

4. March-April 2015 | INTERNATIONAL AQUAFEED | 13
FEATURE
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