Fishmeal physicochemical properties. Impact on the fish feed extrusion process, phase transitions and physical pellet quality
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Norway is the world largest producer of Atlantic salmon (Salmo salar). In 2013 the Norwegian aquaculture industry produced 1.2 million metric tons salmon with the use of 1.5 million metric tons of commercial feeds. The feeds are delivered to the farms in big bags or in bulk and are conveyed pneumatically to the sea cages. Such harsh treatments expose pellets to stress that may give product loss due to abrasion and fragmentation. Feed loss in pneumatic feeding systems is estimated to be in the range of 0.3% to 1.5%. This equals a yearly additional expense of around 40 to 200 million NOK for the Norwegian salmon industry.
To minimize product loss the feed has to be of a consistent and high physical quality. Extrusion processing is a technology that enables production of such feed quality. Physical pellet quality is normally improved by the addition of starch and other binders, but recent research has shown that the protein ingredients in the feed mix also impact the physical quality of extruded feed products. During the last decade several new plant derived protein ingredients has been introduced and partly replaced fishmeal. This has introduced new challenges in fish feed extrusion and stressed the importance to improve the knowledge related to the technical properties of the individual ingredients.
The main objectives of this work have been to quantify fishmeal physicochemical properties with significant effects on the extrusion cooking process and physical pellet quality, and to study the plasticization effect of water solubles in fishmeal. Various multivariate analytical techniques have been applied in the studies, such as principal component analysis, partial least squares -and multiple linear regression. In Paper I and II, the impact of variation in fishmeal physicochemical properties were assessed based on standardized extrusion, drying and coating conditions. In Paper III the effect of water-soluble protein level and moisture content on the extrusion process, extrudate phase transitions and physical quality of feed were studied. In Paper IV the influence of fishmeal water solubles and added moisture on glass transition and flow-starting temperature were quantified.
The studies in Paper I and II document the complexity of fishmeal as a protein ingredient with significant impact on the extrusion process, starch gelatinization and physical pellet quality. Large differences in technical quality within and between the studied fishmeal types (i.e. herring and sand eel) were observed. The research quantifies a positive effect of increased levels of water-soluble protein on pellet durability and hardness. This can be explained by two different mechanisms: a crosslinking effect of large polypeptides and a plasticizing effect of smaller peptides and amino acids. Differences in peptide size distribution between the two studied groups were identified with the highest level of large polypeptides for herring meal. At an equal level of water-soluble protein, extruded feed containing fishmeal from sand eel had significantly lower physical quality than feed containing herring meal. This can be attributed to differences in thermal and rheological properties between the two studied groups, and improper cooking in the extruder barrel for sand eel based feed mixes. Incomplete cooking or transformation may result in increased levels of particles in the extrudates and poor physical feed quality. The studies also document that fishmeal specifications normally used on the world commodity market inadequately describe the technical properties of fishmeal.
In Paper III the effects of water-soluble protein level in fishmeal on extrusion behaviour, phase transitions and physical feed quality were studied. The plasticizing effect of water-soluble protein was comparable to that of moisture. However, in contrast to moisture, addition of water-soluble protein had a positive effect on specific mechanical energy and physical pellet quality. No loss of water-soluble protein during the extrusion process could be observed, confirming that the amino acids and peptides do not form any new covalent bonds in the extrusion process. A non-volatile plasticizer like water-soluble protein will not be removed in the drying process. It will therefore influence the viscoelastic properties of the final product and have a positive effect on physical pellet quality by establishment of an intermolecular binding network through hydrogen-, ionic bond, and hydrophobic interactions. It can be concluded from the study that water-soluble protein can be used as a processing aid for the fish feed industry, serving multiple purposes as nutrient, plasticizer and binder in extruded fish feed.
In Paper IV significant effects of fishmeal water solubles and moisture level on the glass transition and flow-starting temperatures have been documented. The effect of solubles level on the glass transition temperature could be modelled based on the Gordon-Taylor equation. The documented plasticizing effect of water solubles was lower than the effect of moister addition per unit mass, but higher on a molar basis. The plasticization effect can be attributed to the content of low molecular nitrogencompounds. The studied fishmeal model system showed a large composition region of water solubles and moisture with a higher difference between the flow-starting and glass transition temperature than for other reported protein components (i.e. casein, gluten and soya protein isolate). This indicates a reduced temperature effect on viscosity reduction in the rubbery phase for fishmeal in this region. Combined with significantly lower glass transition temperatures, such differences in physicochemical properties may contribute to explain the unique functional properties of fishmeal compared to plant based proteins and casein. This will have positive impact on physical pellet quality and open up the possibility to obtain a satisfactory thermomechanical transformation in the extrusion process at reduced moisture level.