by Alain Brisset, Clextral, France

Today"s worldwide consumed sea food can be split between aquaculture production and wild capture: this represents respectively 80 million metric tons (53%) for breaded animals and 71.2 million metric tons (47%) for the captured fishes. 
These aquatic animals include finfish, crustacean and mollusks. Among the cultured animals in 2016, some are fed manufactured feed (56.6 million metric tonnes), whilst some are not fed manufactured feed (23.4 million metric tonnes such as oysters, mussels, ..) Finally, it is estimated that 35 to 40 million metric tonnes per year of feed is needed for the aquaculture industry.


Floating and sinking feed
In order to meet this demand, feed may be distributed as food wastes or/and as prepared feed. Different technologies are available to prepare the feed such as mixers, pellet-presses, expanders and extruders.
Extrusion technology is a relatively recent technology used in the aquaculture industry, originating approximately 40 years ago. Extrusion is a thermomechanical process consisting in forcing a product through a small size hole, under pressure and temperature thanks to a mechanical device named an archimede screw. The functions of an extruder are generally considered to be for feeding, conveying, compressing, cooking and shaping continuously.


Twin-screw extrusion
A fundamental difference between single and twin-screw technology is the mixing ability of a twin-screw extruder (TSE). This unit operation is generated by the two intermeshing screws co-rotating in a closed cylinder (the barrel) while a single screw extruder (SSE) works with only one archimede- type screw.
Conveying in an SSE relies on friction between the material being processed and the inner surface of the barrel, while a TSE can transfer any mixture from pure water to high viscous doughs even containing high levels of fat, similar to a positive pump.
A TSE is not sensitive to "slip-inducers" such as water and fats. The mixing properties of TSE allows a very homogeneous transfer of mechanical shear and temperature in the processed dough, giving it a homogenous form of cooking and viscosity. It is also possible to mix into a TSE liquid such as water and fat, in order to fine-tune the final expansion degree.
Additionally, the lower availability of fish flour and fish oil have led the manufacturers to use alternative raw materials in their recipes: plant proteins, processed animal proteins, mixtures of different oils and new materials such as insect meals, krill meals, single cell proteins. We need a very flexible machine to handle these ever-changing recipes and possible raw material compositions as well.


Floating and sinking feed
A TSE gives an extreme high flexibility because of its mixing ability; additionally, to the process of different raw materials, it offers independence of parameters, such as feeding rate and screw speed, and very fine temperature and shear control in the different modular zones of the extruder: this is a perfect tool to easy master the sinking and floating parameters. 
The extruded feed must comply with the nutritional needs of the animal, but also with the physical requirements and behavior of the animal: fast sinking feed for the benthic or demersal species, slow sinking for the pelagic or surface species. The salt content (marine, fresh and brackish waters) also plays a role with this request.


How does one control this?
First, the preconditioning must be efficient with optimised cooking of the starch and protein denaturation. Because some recipes present a low starch contain (8-10% for many), it is important to start the gelatinisation with a good device to mix equally water and steam with the powder. It is also crucial to maintain the right holding time to let the steam condense and give its latent energy. Other aspects must be considered, such as the safety, cleaning and automated parameter-control which help in switching quickly from one recipe to another.
Then a key process takes place in the extruder itself: for sinking pellets, we need a high apparent density, relatively hard, but digestible and water stable granulate. We will obtain a water stable pellet when the starch and proteins are well cooked; the extruder must have the right screw configuration to control the shear and the temperatures along the barrel.
If we want to adjust the right apparent density for example to reach values between 350 and 580 g/L, we have to control the expansion degree of the granulates.
 We therefore have to increase/decrease the internal viscosity of the processed material in the extruder.
This will be achieved with a corresponding temperature profile and an efficient internal cooling of the modular barrels; above 580 g/l, we can use a venting system what recovers the vapors/energy into the preconditioner.
Due to this system, one can immediately decrease the temperature of the mass and extrude a high viscous cooked dough which will show a limited expansion. With such a process it is possible to reach densities up to 750 g/l, which is a good value for shrimp feed.  The water stability must obviously remain correct (for example: 3-10h depending on recipe for shrimps).
One can finally extrude the cooked and viscous dough through the die. The die-design and extrusion surface contribute as well to the expansion degree (and therefore the floating/sinking properties). For high expanded floating pellets, you can choose relatively short channels for the die: 1.0 to 1.5 in diameter, and with a relatively small extrusion surface (MM2/extruded kg/h).


Conclusion 
A TSE machine offers the highest flexibility to accurately control the process parameters and adapt itself quickly to recipe and raw-material composition changes. It enables the production of floating and sinking fish feed pellets in range of 350 to 750 g/l, according to the recipes and product size. A TSE will perfectly cook, absorb some variations, and control the temperatures, shear and residence time of the processed material. 
Using a density-control device combined to the right screw and die-design will definitively give technical and economic advantages to the manufacturer, with the fine control of the expansion degree insuring a consistent quality.

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