The document discusses how extrusion processing affects various ingredients used in fish feeds, including starch, proteins, oils, and fibers. Starch is gelatinized during extrusion which improves its digestibility. Proteins are denatured through heating and shear forces, increasing their digestibility while maintaining amino acid levels. Oils are not significantly affected by extrusion processing and act as lubricants. Higher levels of plant-based proteins and oils will be needed in fish feeds as fishmeal production does not expand, and extrusion can improve the nutritional value of these alternative ingredients.

1. July | August 2013
Maintaining ingredient quality
in extruded feeds
The International magazine for the aquaculture feed industry
International Aquafeed is published six times a year by Perendale Publishers Ltd of the United Kingdom.
All data is published in good faith, based on information received, and while every care is taken to prevent inaccuracies,
the publishers accept no liability for any errors or omissions or for the consequences of action taken on the basis of
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INCORPORATING
f ish farming technolog y

3. E
xtrusion processing using a com-
bination of moisture, pressure,
temperature and mechanical shear,
is used in the feed industry. It results
in physical and chemical changes such as
ingredient particle size reduction, starch
gelatinization and inactivation of enzymes.
Mild extrusion processing usually enhances
the digestibility of plant proteins.
Fishmeal production is not expanding
worldwide; therefore, more plant meals will
have to be formulated into fish feeds to
accommodate expected increases in fish feed
production. Plant meals contain starch, which
must be cooked to make it digestible to fish.
Extrusion processing gelatinizes starch and
improves the digestion of starch.
Extrusion processing can increase the
nutritional value of canola meal, rapeseed,
peas and soybean meal. As higher amounts
of plant meals are formulated into fish feeds,
the bioavailability of nutrients, especially bio-
availability of minerals, will be of increasing
concern because plant meals contain lower
amounts of minerals compared to fishmeal.
The effect of extrusion processing on
mineral availability for fish is not known. Thus,
the raw ingredient formulation, selection of
process equipment, and processing condi-
tions are independent regions of control that
may be exercised in the extrusion cooking
of aquafeed. Although the control regions
are independent, they are interrelated to the
point that discussion of one must include the
other.
Raw material utilization and cost effective
formulation are key operational factors. The
ability to alter processing conditions and raw
material formulations to keep formulation
costs at a minimum while maintaining high
quality standards and minimum operating
costs is a challenge for every processor.
Within certain limits set by a nutritionist,
the extrusion cooking process can produce a
wide range of products. In general, during the
extrusion cooking of cereal grain and protein
blends, the moistened granular or floury
materials are converted into dough. The
starchy components gelatinize, resulting in a
substantial uptake of moisture and an increase
in dough viscosity. Some protein constituents
may impact elasticity properties that are
characteristic of hydrated and developed glu-
tinous dough. Other proteinaceous materials,
those with low protein solubility such as meat
meal or fishmeal, may contribute less to the
adhesive and stretchable functional properties.
Impact of starch, protein, fat
and fibre quality on extrusion
Raw materials
Ingredient selection has a tremendous
impact on final product texture, uniformity,
extrudability, nutritional quality, economic via-
bility and ability to accept oil during coating or
flavoring process. The common components
of a recipe include starch, protein, fat, and
fibre. An understanding of each component
and how the extrusion process is affected is
critical to forming an approach for effective
diagnostics and troubleshooting.
Starch
One of the main categories of compo-
nents found within many of the extruded
products is carbohydrates. The primary type
is starch, a complex carbohydrate. The starch
is usually sourced from either the cereal grains
or from tubers. The cooking process has a
pronounced effect upon the starch. In the
raw state, starch has a granular nature and
exists as a distinct particle (or granule) with
very strong internal attractions between the
various portions of the starch molecule within
the granule. This is also the condition of the
starch as it enters the extruder barrel. This
condition is commonly referred to as ‘raw’ or
‘native’ starch.
Starch may be gelatinized at different mois-
ture levels. For example, when boiling starch,
moisture levels as high as 90 percent w/w
are used, but in the extruder barrel, much
lower moisture levels are used. However, the
extrusion cooking process is seldom operated
with an excess of water. The total operating
moisture is typically w ≈ 15-30 percent w/w.
In the extruder, the complete rupture of
the starch granule is brought about by the
combination of the moisture, the heat, the
pressure and most importantly the mechanical
shear. This process will typically take between
10-15 seconds.
Within the extruder, moisture is required
to allow starch to gelatinize into a fluid mass,
permitting it to pass through the die opening
at the discharge of the extruder. As the mate-
rial discharges from the die, the moisture level
should be sufficiently high to retain its fluidity,
but low enough to ensure that the starch will
stiffen up (as a result of the inherent moisture
and temperature losses, which occur at the
die). As a result, moisture levels in the range
of 15-30 percent w/w are typical. Excessively
low moisture limits the lubricating effect as the
product is conveyed along the barrel, causing
high energy consumption. Water content
also allows the expanded product to remain
soft, permitting the cell structure to puff (and
subsequently collapse).
The gelatinization of starch is affected by
the conditions of heat and moisture during
cooking. Additional cooking of gelatinized
starch increases the viscosity and the surface
tension of the gel sufficiently to cause the
material to become so thick it cannot be
poured from an open container. This condi-
tion is referred to as retrogradation.
Maintaining
ingredient
quality in
extruded feeds
by Mian N Riaz, PhD, head of extrusion
technology program, Texas A&M University
System, USA
12 | InternAtIonAl AquAFeed | July-August 2013
FEATURE

4. July-August 2013 | InternAtIonAl AquAFeed | 13
FEATURE
Extruder OEE for the Production of Fish FeedExtruder OEE for the Production of Fish Feed
AMANDUS KAHL GmbH & Co. KG, Dieselstrasse 5-9, D-21465 Reinbek / Hamburg,
Phone: +49 40 727 71 0, Fax: +49 40 727 71 100, info@amandus-kahl-group.de www.akahl.de

5. Starch, when cooked, can be puffed or
expanded to a remarkable degree. If a com-
parison between the diameters of the expanded
product to the diameter of the die orifice is used
to express the degree of expansion, then starch
can be expanded by a factor of up to five.
Protein
The next most important category of
components is the proteins. The extrusion
process has been found to provide sufficient
cooking to denature proteins, but because of
the short retention time, does little damage
to the nutritional value of the heat-sensitive
amino acids. The denaturation of protein is a
phenomenon very similar to the gelatinization
of starch. In the presence of heat and mois-
ture the grains hydrate and swell. The action
of the shear encountered within the extruder
barrel leads to the rupture of the membrane
and the disentanglement of the molecules.
The shear also leads to the alignment and
stretching of these molecules.
Due to these changes the formulation
becomes a plasticized, fluid mass. As the mass
begins to cool cross-linking of the molecules
into a three-dimensional structure begins to
occur, leading to a rigid physical form.
As a result of denaturation, protein may
undergo one or more of the following changes:
1. Those proteins which are enzymes lose
their enzymic activity
2. Those proteins that are difficult to digest
become more digestible
3. Those proteins which are soluble in
their native state lose their solubility and
coagulate after being denatured
When more severe cooking takes place,
the protein is not merely denatured, but is
hardened beyond that stage to a very tough,
horn-like condition. Under specific conditions,
severe cooking can damage certain amino
acids, rendering them unavailable to animal
nutrition.
Proteins can be classified as plant and
vegetable sources or as animal and marine
sources. Vegetable or plant proteins are
largely water-soluble and therefore possess
very functional properties during extrusion.
The functionality or water-soluble proper-
ties of plant proteins can be measured with
several laboratory tests. The primary test for
potential functionality is the measurement of
protein dispersibility index (PDI). The PDI is a
means of comparing the solubility of a protein
in water, and is widely used in the soybean
processing industry. A PDI of 100 indicates
total solubility.
During the milling or extraction steps to
refine a plant protein for use as an ingredient
in extruded products, there are often one
or more heating steps which affect the PDI
value. These heating or drying operations are
usually very mild and do not significantly lower
PDI values. A PDI value of greater than 40
will have significant functionality during extru-
sion, reasonable binding, and some expansion
potential. Extremely high PDI values (>80)
may actually be so functional that, at high lev-
els in a recipe, may contribute to a stickiness
or tackiness when hydrated that eventually
results in unstable extrusion conditions.
Proteins of animal or marine origin may
be subjected to higher temperatures during
manufacturing. Higher process temperatures
are employed for many reasons including
improved extraction and separation from fat
and water components, and adequate pas-
teurization. Where high temperatures have
been employed over an extended time peri-
od, the resulting protein solubility is quite low
and these proteins may be essentially inert
during the extrusion process. Inert means that
the protein will not contribute to binding or
expansion, but may actually reduce expan-
sion. This is in part due to the presence of
significant levels of minerals and fat compo-
nents, but mainly due to the denatured (non-
soluble) structure of the protein. The high
temperature processing of ingredients will be
reflected in low PDI values and dark colours.
Animal proteins are supplied to the
extrusion system in a fresh (un-cooked or
lightly cooked) or spray-dried form that will
have significant solubility and functional-
ity. Protein solubility is an indication of the
degree of denaturation of protein ingredients.
Denaturation does not necessarily impact
protein digestibility. Denaturation does impact
extrusion functionality and usually occurs in a
temperature range of 55-70 °C.
Oil and other lipid components
Extrusion does not seem to adversely
affect fats and oils. Studies have shown little or
no changes in the free fatty acid levels, nor any
indication of rancidity due to heat oxidation
of the fat. Proper levels of fat are important in
the cooking process. Fat is a lubricant, allow-
ing product to ease through the screw(s) and
barrel of the extruder with less resistance.
Too much fat retards product expansion and
the degree of cook, making a denser product.
More retention time in the barrel, together
with higher temperatures, in most instances
will tolerate levels of fat in excess of 12-15
percent w/w.
Conversely if the fat is bound, such as in a
coarsely ground or whole oil seed, then sig-
nificantly higher levels of fat may be tolerated.
Almost all ingredients contain some level of oil
or other lipid constituents. Oils or derivatives of
various fats such as lecithin or mono and dig-
lycerides are often added to recipes to impart
specific emulsifying or textural properties. The
presence of oil and similar ingredients will act
as a lubricant in the extruder screw. Fat addi-
tion reduces specific mechanical energy inputs.
At lower inclusion rates, lipids can disrupt cell
structure and texture by affecting plasticity and
viscosity. In most recipes, the addition of lipids
will begin to affect expansion and product
durability at levels of less than 7 percent (total
crude fat). If internal fat levels exceed 12 per-
cent (total crude fat), distinct shapes may not
be possible. At moderate inclusion levels, fats
will tend to yield large cell sizes and thick cell
walls in the extrudate.
Fibre
Materials with a high fibre content show
an increase in bulk density after expan-
sion, when the product densities are based
upon uniform grinds of feed and expanded
product. The presence of the fibre particles
appears to provide a nucleation site for
bubble formation during the puffing proc-
ess. At low inclusion levels (less than 5 %),
fibrous ingredients may not have a noticeable
impact on extruded products. Particle size
of the fibre is important and if smaller than
400 microns, the fibre may actually increase
expansion and reduce bulk density of the
14 | InternAtIonAl AquAFeed | July-August 2013
FEATURE

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7. July-August 2013 | InternAtIonAl AquAFeed | 15
FEATURE
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8. extrudate. Large
particles of fibre
in a recipe usually
result in a coarse,
fuzzy product sur-
face appearance
after extrusion. If
the particle size is
less than 50 microns,
there is less effect
on expansion even
at higher levels in
the recipe. Very fine
fibre particles create
an extremely small
cell structure in the
product after extru-
sion. Insoluble fibre
remains nearly inert
during extrusion
and the individual
fibre particulates can
serve as nucleat-
ing sites during the
expansion process
at the die. More
soluble forms of
fibre have less con-
tribution to reduced
expansion even at
high inclusion levels.
Several studies have
indicated that extru-
sion can increase
fibre solubility. The
extent of this con-
version depends on
processing condi-
tions.
Measuring raw
material quality
in relation to
extrusion
The Phase
Transition Analyzer
(PTA) instrument
measures the glass
and melt transi-
tion temperature of
ingredients which
are a complex mix
of biopolymers.
Knowing the glass
and melt transition
temperatures of
the ingredients or
ingredient mix helps
assess the suitability
of the raw materi-
als for extrusion and
how the properties
of that recipe will be
affected by the extrusion temperatures and
moistures.
Grinding and mixing
and extrusion
The particle size of the raw materials will affect
the texture and uniformity of the final product.
The extrusion cooking process can utilize a broad
spectrum of ingredient particle sizes. It is desirable,
but not necessarily essential that particles be of
uniform size and density to prevent segregation
during mixing and transport prior to extru-
sion. Most importantly, a uniform particle size
promotes uniform moisture uptake and cooking
during extrusion which prevents hard, partially
cooked particles in the final product.
When whole grains are received into the
manufacturing facility, they should be pre-ground
to pass through an opening of 1,000 micron or
larger prior to mixing. The final formulation is
then ground just prior to extrusion to achieve
the desired final particle size. When die openings
are 3 mm in diameter or larger, it is common
for this final grinding step to be through a screen
having 1.2 mm openings. With die openings
smaller than 3 mm in diameter, the maximum
particle size should be one-third of the die
opening. Smaller ingredient article size results in
smaller cell structure of the extrudate.
Conclusion
Raw materials are selected primarily based
on their nutritional and functional contribu-
tions. Secondly, economics enters into the
selection process. Many recipes are formu-
lated based on least cost formulation software
programs. Thirdly, the availability of the raw
material becomes a factor.
When purchasing or selecting raw materi-
als, establish a specification range based on
desirable characteristics. This range of specifi-
cations should include the proximate analysis
and other known critical qualities. However,
some desirable characteristics are only vaguely
recognised and no satisfactory test exists as
yet to monitor quality in a reliable manner.
There exist variabilities within a raw material
due to influences such as the variety, growing
season, and post-harvest handling or process-
ing of grains. Different types of grains, legumes,
and variations within animal or marine protein
sources are reflected in the processability of
raw materials. Many problems can be avoided
by developing historical databases that record
raw material characteristics that correlate with
good processing. Establishing a sample library
of acceptable and unacceptable raw materi-
als may be especially useful in maintaining a
smooth running extruder and troubleshooting
future challenges.
More InforMatIon:
Email: mnriaz@tamu.edu
16 | InternAtIonAl AquAFeed | July-August 2013
FEATURE
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ET-233A.indd 1 12/27/12 10:51 AM

10. www.aquafeed.co.uk
LINKS
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Maintaining ingredient
quality in extruded feeds
Fine particle filtration in
aquaculture
Effect of probiotic,
Hydroyeast Aquaculture
– as growth promoter for adult Nile tilapia
Volume 16 Issue 4 2013 - JulY | August
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