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FEATURE of the melt. Figure 1 shows some measured results of bulk density vs SME. The SME is influenced by process parameters, most significantly the melt moisture content (used to manipulate the melt rheology), the screw profile design, and the screw speed.

Aquafeed specifications – not the full story

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 affecting 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 nutrition (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 extrusion process – eg drying also affects pellet density. In fact, drying under inappropriate

conditions can result in drying and shrinkage of the outside layers, increasing overall density so the pellet sinks (eg as required). Then, over time in storage, moisture redistributes 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 balance 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 significantly 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 12 | INTERNATIONAL AQUAFEED | March-April 2015

Final product nutrition (and FCR) is not just due to the composition of the formulation 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 nutritional 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 nutritional performance of the feed. Variations in the extrusion process do not only affect physical changes in the product such as density 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)

Mar | Apr 2015 - International Aquafeed magazine  

The March - April 2015 edition of International Aquafeed magazine

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