Sustainability of aquafeeds implies the use of ingredients originating from different sources and alternative locations. As a result of this, there is an increasing trend to replace fishery-derived ingredients with a variety of plant sources, especially legumes, cereals and oilseeds. If we look further into the future it is likely that other sources, for example bacterial, yeast and algae, will also become viable. Besides the nutritional quality of feed, it is necessary to acquire knowledge about the functionality of these sources in the production process, before they can be fully used in a commercial context. In other words, how these ingredients behave in terms of their processing, their interaction with other ingredients and the end physical quality of the feed.

1. I N C O R P O R AT I N G
f i s h far m ing t e c h no l og y
January | February 2014
X-ray microtomography: a new tool in
assessing the properties of aquatic feed
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The International magazine for the aquaculture feed industry

2. FEATURE
X-ray microtomography:
a new tool in assessing the properties of aquatic feed
by Vukasin Draganovic, feed production researcher, Skretting Aquaculture Research Centre, Norway
S
ustainability of aquafeeds implies
the use of ingredients originating
from different sources and alternative locations. As a result of this,
there is an increasing trend to replace
fishery-derived ingredients with a variety of
plant sources, especially legumes, cereals and
oilseeds. If we look further into the future
it is likely that other sources, for example
bacterial, yeast and algae, will also become
viable. Besides the nutritional quality of
feed, it is necessary to acquire knowledge
about the functionality of these sources in
the production process, before they can be
fully used in a commercial context. In other
words, how these ingredients behave in
terms of their processing, their interaction
with other ingredients and the end physical
quality of the feed.
The substitution of one ingredient for
another is not simple, as it can alter physical
properties of pellets including their durability
and the absorption of oil. These mechanical properties of feed pellets can be clearly
understood if we can effectively describe their
structure.
The problem lies in finding a suitable
technique that can help in characterising the
internal structure of the pellet. The downsides
of traditional imaging techniques like light or
electron microscopy are that these techniques
are two-dimensional, and that they just give
information about the fracture plane of a product. In
addition, a sample needs to
be prepared. This usually
involves cutting, which can
in turn alter the product
structure. Another option
is pycnometry, where the
pressure is applied for the
penetration of gas into the
porous fish feed pellets.
This can however cause
cracks in the cell walls and
it seems therefore to be a
less suitable technique.
Figure 1: Micro-CT principle (Source: Bruker micro CT)
‘As is’ analysis
X-ray microtomography is an invasive
imaging technique, meaning that no sample
preparation is needed and the structure is
analysed ‘as is’. So, no coating or vacuum
treatment is needed to prepare the sample. Microtomography, like tomography, uses
X-rays to create cross-sections of a threedimensional object (see Figure 1). The term
‘micro’ is used to indicate that the pixel sizes
of the cross-sections are in the micrometer
range. The principle is very similar to that of
a medical CT scanner. The only difference to
a clinical one is that in the case of the micro
CT scanner, the X-ray source and detector
remain in a fixed position (see Figure 2).
This technique allows Skretting techni-
Figure 2: Difference between clinical and micro CT instrument
(Source: University of Leicester)
34 | InternatIonal AquAFeed | January-February 2014
cians to see cross-sections of the inner
pellet, without the usual need to physically
cut the sample with a razor blade. Once
the pellet is scanned, software can generate
three-dimensional images of the sample’s
morphology and internal microstructure with
resolution down to the sub-micron level.
Subsequently, various microstructural features of the porous pellet including the average size of the cells, pore size distribution,
cell wall thickness and its open and closed
porosity can be quantified.
Applications
Skretting Aquaculture Research Centre
has applied this imaging technique to both dry
kernels and pellets coated with oil. Differences
in density between the solid matrix, oil and air
(void cells) can be easily detected by X-rays.
The measurements were limited to an appropriate volume of interest, that is, a cylinder
located in the centre of each pellet (see Figure
3). From the right-hand image it can be seen
that some cells are still not yet filled with
oil, although both products have the same
density, were coated with the same amount
of oil and have the same visual appearance.
These results indicate that the infusion of oil
into the product could be studied further
with this technique. For example, the effect of
different protein sources could be visualised in
future research.
Using X-ray microtomography to visualise
the microstructure of fish feed can also assist

3. FEATURE
Figure 3: Three-dimensional models of two coated products obtained by XMT. The objects presented are the volume of interest,
not the whole pellet. The blue areas represent oil, white areas are void cells, and the continuous solid matrix is represented as
green or red (dense material)
in the further optimisation of the physical
quality of the pellet. Applications include the
reduction of undesired fat migration out of
the product, increasing pellet durability or
controlling the sinking-floating behavior of
aquafeed.
Feed pellets must meet a series of physical specifications. They must be sufficiently
durable to withstand the stresses exerted
during transport and handling, and by pneumatic feeding devices. In addition to this, the
pellets must be consistent in appearance,
size and density, the last of which must be
controlled precisely during the extrusion
process to give the required oil absorption
and sinking speed characteristics. Pellets that
stay afloat will not be eaten, and pellets that
sink too fast may escape being eaten by the
fish altogether.
The X-ray technology can, in addition play
an important role in developing new feeds
with the inclusion of novel raw materials that
meet all required quality criteria. From the
x-ray scans, we can see how one ingredient
affects the structure of the pellet. A more
compact structure from increased cell wall
thickness inside the pellet will in turn result
in a more durable product. The size of the
pores also plays an important role when it
comes to oil absorption. Using this technology, we have defined an optimal range of
pore size distribution and interconnectedness between the pores, which will ensure
good oil infusion and reduced fat leakage
during product storage.
At Skretting we are always screening different ingredients and looking at how they affect
these parameters. Knowing this, we can select
the right ingredients.
Extruder 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
January-February 2014 | InternatIonal AquAFeed | 35
www.akahl.de

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I N C O R P O R AT I N G
f I s h fA R m I N G T e C h N O l O G y
Successful moisture
control in aquatic feeds
Current challenges and opportunities
in amino acid nutrition of salmonids
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Closing the food waste loop:
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