Nowadays we often hear startling news such as, ‘seven billionth baby born’ or ‘world population may reach 9.2 billion by 2050’ (world news, msnbc.com). Hunger is the world’s number one health risk, it kills more people than AIDS every year, one in seven people in the world will go to bed hungry tonight.

1. November | December 2012
Extrusion technology for the production of
micro-aquatic feeds and shrimp feeds
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The International magazine for the aquaculture feed industry

2. FEATURE
Extrusion technology for the
production of micro-aquatic
feeds and shrimp feeds
Image courtesy of Wenger
Manufacturing, USA
by Mian N. Riaz, Ph.D, Head of Extrusion Technology Program,
Food Protein R&D Center, Texas A&M University, USA
N
owadays we often hear startling and commercial-scale feed, which is the key production of micro-aquatic feeds. The raw
news such as, ‘seven billionth for growth of aquaculture industry. The fun- recipe is very finely ground or pulverized
baby born’ or ‘world population damental components of extrusion systems and does not possess good flow properties.
may reach 9.2 billion by 2050’ have consisted of the following items for a The feed system must be able to handle
(world news, msnbc.com). Hunger is the number of years: these finely ground formulations and avoid
world’s number one health risk, it kills more 1) Feed delivery system bridging and non-uniform metering of the
people than AIDS every year, one in seven 2) Preconditioning feed.
people in the world will go to bed hungry 3) Extruder Automated feed delivery systems with
tonight. 4) Die and knife assemblies PLC control are the norm. Slurry tanks and
Although existing extrusion systems were liquid feeding devices (pumps) are utilized to
To overcome these issues, farmers must able to produce a wide range of good quality accomplish uniform metering of liquid ingre-
produce 70 percent more food by 2050 to aquatic feeds (both floating and sinking), small dients. The slurry tanks are often jacketed
feed the population. But the impending crisis diameter pellet sizes were difficult to produce for heating or cooking and are equipped with
is that the earth may run out of food by 2050. at reasonable or cost-effective throughputs. agitators as required. Positive displacement
2.4 billion extra people, no more land, how Recent innovations in the basic hardware metering pumps deliver metered liquids at
will we feed the world in 2050? components permit smaller diameter feeds at constant rates by varying length of stroke or
At the same time we also hear in the news attractive production throughputs. speed of rotation. Slurries or liquids can be
that global fish consumption has hit a record premixed with dry ingredients but are prefer-
high. We have seen the commercial fishing Feed delivery system ably injected into preconditioning devices or
trend is declining whereas aquaculture farming Hoppers or bins are an integral part of the extruder barrel. The nutrient profile of
is growing rapidly all over the world. a feeding device and are used to hold the larval feeds is critical and the precise metering
Is this supply enough to feed the future dry ingredients above the feeders. The feed ensures correct formulations.
population? May be not, but fish demand delivery system must be able to uniformly
is growing every day all over the world. To feed both a dry and/or liquid ingredient or a Preconditioning
maintain baseline consumption in every coun- blend of ingredients. The dry portion of the feed and the
try, 159 million tons of fish is needed to feed Generally, when the added fat content of a liquid portion are separately introduced into
the world population in 2030. This demand raw formulation exceeds 12 percent, the por- a preconditioning device where they are
is driven by population and income growth. tion of fat above the 12 percent level should continuously mixed, heated, and moisturised
If a country’s aquaculture production follows be introduced into the extrusion system in by the injection of hot water and/or steam.
the recent trend, the expected aquaculture a separate ingredient stream. The dry feed The intense mixing of water and steam added
growth rate will need a four percent increase portion is delivered to the extrusion system to the dry feed and the ability to extend the
annually. To feed a growing world population, through a specialised metering device capable retention time during the preconditioning
the required aquaculture growth rate is 5.6 of providing uniform flow at any desired phase allows the moisture level to be main-
percent annually. extrusion rate. tained at an optimum.
Some of the main challenges to achieve Dry ingredients are usually free flowing, This ability to maintain optimum moisture
these goals are proper and large-scale feed and there are a number of capable feeding distribution not only initiates proper cooking
production systems for micro aquatic feed. devices which vary in their relative cost but also is reported as a significant factor in
Recently extruder manufacturers came up and complexity. However, gravimetric or the reduction of extruder barrel wear and
with new technologies which can solve some loss-in-weight systems are necessary for the extruder shaft power per ton of product
of the aquaculture issues related to large stable, precise metering of dry feed for the processed. The higher mixing intensity of new
20 | InternatIonal AquAFeed | november-December 2012

3. FEATURE
preconditioner designs improves hydration increase. The mechanism of shear does not desired final product texture, density, color,
and cooking, helping to capture the steam in begin to play a dominant role until the screw and functional properties.
the raw material. Excess steam can escape flow channel is full. This full flow channel Twin-screw systems are preferred for
the preconditioner and create fugitive dust condition begins in the kneading zone. extrusion of aquatic feeds smaller than 2
which creates housekeeping concerns in the The flow channel fills, first, with loose mm diameter due to their positive transport
plant environments. granular material which is compressed and and self-wiping characteristics which prevents
Better cooking with new precondition- worked by shear as it passes through the significant product build-up in the extruder
ers gives lower product viscosities which kneading zone. It is in the kneading zone barrel which could later dislodge and plug the
Image courtesy of Wenger Manufacturing, USA
improves extrudate flow through small where the discrete particles of material begin small die orifices. The C²TX system is a co-
die orifices. The result is smaller pellets to agglomerate because of their temperature rotating system that includes a tapered screw
and more uniform pellet size. The higher increase resulting from conduction, direct diameter which de-aerates the extrudate and
mixing intensities in new preconditioner steam injection, and viscous energy dissipa- makes it easier to create high density feeds for
designs is the result of unique beater tion. Here, the discrete particles begin to form good sinking characteristics without the need
designs and more beater contacts per a more integral flowing dough mass. for vented barrels, pressurized density control
retention time. At the discharge
end of the knead-
Extrusion ing zone, the extru-
Extruders are generally classified as either being date most typically
a single or twin-screw design. In both designs, the reaches its maxi-
impact of final product characteristics are affected mum compaction.
by screw and barrel profile, screw speed, process- The shear in this
ing conditions (temperature, moisture, etc.), raw area of the extruder
material characteristics, and die/knife selection. barrel is moderate
The feeding zone of the extruder is that and the extrudate
area where the low-density discrete particles temperature begins
of raw material are transported into the to increase. The
extruder barrel inlet. This low-density, often final cooking zone
preconditioned, material is then transported is that area where
into the interior of the extrusion processing amorphousizing and
chamber. The flow channel of the screw is texturising occur.
typically not filled in this zone due to the Temperature and
air entrapped in the incoming material. The pressure typically
incoming material is compressed slightly in this increase most rap-
zone with the air being expelled. idly in this region
Water, an excellent plasticizer, is typically as shear rates are
injected into the barrel in the feeding zone highest because of
to facilitate textural development, viscosity the extruder screw
development, and to enhance conductive heat configuration and
transfer. The kneading zone of the cooking maximum compres-
extruder continues the compression started sion of the extru-
in the feeding zone, and the flow channels of date. The pressure,
the extruder screw have a higher degree of fill. temperature, and
As the degree of screw fill increases and resulting fluid viscos-
pressure begins to develop in the extruder ity are such that the
barrel, leakage flow (flow over the outside extrudate will expel
diameter of the screw in a direction toward from the extruder
the extruder inlet) and pressure flow both die to form the
november-December 2012 | InternatIonal AquAFeed | 21

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5. FEATURE
which mini- die/knife area. The small diameter feeds
mised nutrient results in spillage in this area and will
destruction. cause sanitation problems.
Floating prod- b) For product separation. Floating feeds
ucts are pos- have a tendency to stick together
sible by simply when wet on belt or HVH conveyors
decreasing die and pneumatic conveying enhances
open area. separation.
c) For separation of ‘tails’ from pellets.
Process Pneumatic systems ‘scrub’ the product
guidelines and remove tails for later separation
Image courtesy of Wenger Manufacturing, USA
Process during sifting.
guidelines 7) Fluid bed dryers are recommended
required for die for products under 1.2 mm diameter
holes smaller in size although horizontal dryers with
than 1.2 mm polyester screens can work with some
diameter: products.
1) Recipe to 8) Final product sifting after dryer and
contain ade- before coating. This sifting operation is
quate starch critical for three reasons:
levels for bind- a) To remove ‘overs’ (large tails and
ing (at least ‘doubles’) for regrind.
25% starch for b) To remove ‘fines’ for regrind. This
floating feeds). prevents a mess during coating step
devices, or double extrusion. By adding a BPV 2) Maximum particle size of the recipe where the fines are also coated and
(Back Pressure Valve) after the extruder, the must be smaller than one third the die cause buildup.
necessary restriction is provided to expand hole sizes. c) To separate good pellets into several
the product if floating pellets are desired. 3) A spring-loaded knife blade is different diameters depending on the
recommended. client criteria for size. The expecta-
Die assemblies 4) All mass flow inputs must be free of tions from the industry will be for
The die is the most critical part of the material that is large enough to block tight specs on pellet size and this can
complete system as it determines prod- or partially block the die openings and easily be controlled at this point by
uct shape and size, but also determines this includes the steam, water, fat, and sifting product and producing several
throughputs and buoyancy properties of other liquid inputs. The water and steam different sizes at the same time and
the final aquatic feed. As pellet diameters lines going to the extruder system need setting the standard for the industry.
became smaller, the die created more to be fitted with screen filters having The primary-sized product can be sent
restriction and drastically reduced through- 30 mesh (0.6 mm) openings and these on through the system for coating and
puts. should be adequate if maintained. The into final product bins. The secondary
One die assembly design that allowed fat line (and fat source) also needs to sizes can either be reworked or saved
an increase in throughputs by increasing die be filtered to remove debris larger than separately in tote bags for coating and
30 mesh (0.6 mm). All strainers bagging later.
or filters must be easy to clean 9) Production procedures. This small diam-
or they will get removed ‘in the eter product requires a dedicated line,
"This micro-aquatic floating heat of a run’ where liquid flows strict startup and shutdown procedures
are interrupted due to plugged to avoid die plugging, and thorough
feed will be the foundation filters. It may be necessary to cleanup techniques. The extruder and
to start fish farming on have a dual filter set up for fish coater areas should be considered as
solubles and fat lines. With this ‘wet areas’ for cleaning. The coater
commercial scale to fulfill the installation, if one filter is plugged may need to be cleaned between each
you can close the valves to the different pellet size to avoid cross
fish demand in the world" primary filter for cleaning and contamination.
open the valves for the second By following these guidelines and using
for continued operation. newly innovative extruder parts micro-aquatic
open area is the OTD (Oblique Tube Die). 5) The dry feed must pass through a vibrat- floating feed can be produced on a large
This die actually increased die open area (the ing sifter after the grinder and before scale basis. This micro-aquatic floating feed
number of orifices) by two to three times the extruder live bin. This sifter must be will be the foundation to start fish farming on
which maintained high throughputs even for sized to remove particles the same size commercial scale to fulfill the fish demand in
small diameter products due to larger die hole or larger than the die openings. High the world.
populations. The tubes created longer reten- fishmeal diets plug vibrating sifter screens
tion times for improved cooking. Pressure very easily and the industry often employs
drop in the tubes created a denser product rotary sifters to avoid this bottleneck.
so that micro-aquatic feeds could be cooked 6) Pneumatic conveying is required from More InforMatIon:
thoroughly but still maintain high densities the extruder die to the dryer inlet for Email: mnriaz@tamu.edu
for sinking characteristics. The process was several reasons:
still a short time/high temperature process, a) For product containment around the
22 | InternatIonal AquAFeed | november-December 2012

6. FEATURE
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november-December 2012 | InternatIonal AquAFeed | 23

7. This digital re-print is part of the November | December 2012 edition of International
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