Twin screw extrusion processing in aqua feed- Benefits of twin-screw extrusion

by Clextral, France


The Aqua Feed Extrusion Conference, co-organised by International Aquafeed, Dr Mian Riaz of Texas A&M University and VIV is taking place again in March 2020!
Two renditions of the conference will be taking place:

  • The Aqua Feed Extrusion Conference on March 10th at VIV MEA, Abu Dhabi
  • The Aqua Feed Extrusion Conference on March 23rd at VICTAM and Animal Health and Nutrition Asia, Bangkok

These two conferences will feature a variety of industry expert speakers delivering innovative presentations on how users can make the best use of their extrusion machinery and aqua feed systems. Currently confirmed speakers include Andritz, Clextral, Jefo Nutrition, Bühler, Amandus Kahl and Wenger, to name a few.
In this series, our conference speakers will be providing articles about the latest innovations in extrusion and nutrition processing technologies.
To find out more about these two conferences please email Rebecca Sherratt (

Fish is known as a high value food for humans, with health benefits that include long chain unsaturated fatty acids, vitamins and minerals and a well-balanced protein supply. 
Therefore, fish feed processing technology plays a particularly important role to this industry. It requires:

  • Easy adaptation to any change in raw-material composition: moisture content, lipid content, particle size distribution, de-mixing of powdered materials. These are due to various sources of raw materials (for example, soy flour can be purchased in many parts of the world), transport and storage conditions, grinding process
  • Flexibility to adjust the sinking/floating properties of the granulates to follow as closely as possible the food habits of each animal family
  • Processing a wide range of recipes to respond to industry demand for foods with low or high amounts of lipids, vegetable proteins, or various sources of protein to meet the specific nutritional fish requests and adapt to rapid environmental fluctuations
  • Flexibility to adjust the shear, cooking and shaping conditions in the extruder and apply precise drying and coating parameters during the entire production process
  • Insuring a high hygienic standard, to avoid any contamination during the feed manufacturing process.

Taking these considerations into account, combined with good manufacturing/breeding practices, scientific education and adapted legislation, we can nurture high quality aquatic animals that offer health benefits to people around the world–using local production to ensure low carbon footprints and maintain reasonable sales prices.

Co-rotating twin-screw extruder technology
Single screw extrusion technology has offered a simplified method of continuous cooking of doughs under controlled processing conditions for many decades.
60 years ago, some pioneers developed an alternative process: co-rotating twin-screw technology applied to the food industry, which offered much greater flexibility than single screw machines due to its intensive mixing ability with precise shear and temperature control.
With the evolution of electronic PLC's, mechanical features, gauges, drives, metallurgy, together with heavy R&D investment, these pioneers offered the fish feed industry more sophisticated systems, allowing the industry to move quickly into new areas–such as recipes with high amounts of fat and vegetable protein. 
Clextral, a major player in twin screw technology for food and feed applications has launched innovations which offer even greater possibilities to the fish farming industry to process original recipes and use raw materials such as new pulses, proteins, insects, krill meals and possibly processed animal proteins, seaweeds, etc.

Density control system
Clextral has developed a system for instantly varying the density of the material within the extruder. It is possible for example (depending on the recipe) to quickly pass from an extruded pellet of 350g/l to 750g/l without modifying the process parameters. This fully automated system ensures the control of the pellets density and the ability to produce floating, slow sinking and sinking aquatic feed.
With the density control system, the cooking degree and resulting density of the product can be adjusted according to customer specification. By adding steam or through the generation of vacuum, the density control system ensures the precise density and proper degree of expansion in the end product. 
The main advantages of the process are: 

  • Precise density adjustment and control
  • Reduced product moisture levels 
  • Generate high density product
  • Fines recovery back to the preconditioner.

Ideal for aquatic feed production, this system can be retrofitted on existing machine.

Advanced Thermal Control (ATC)
The Advanced Thermal Control system (ATC) is a self-learning, proprietary software solution that ensures absolute precision in temperature control of the barrel assembly of the Evolum+ extruder.
ATC continuously monitors production parameters to ensure process and product consistency. ATC is proven to enhance process stability up to 70 percent, with energy savings averaging 20 percent by eliminating repeated heating/cooling cycles to maintain process temperature set-points in all circumstances. 
Combined with automatic start up and shut down procedures, this system represents a powerful tool to enhance the productivity of the extrusion line.

Preconditioning: Higher efficiency and improved product texture
Another innovation refers to the preconditioning process, a key operation in fish is feed manufacturing. Preconditioning allows moistening of the powdered mix and pre-gelatinisation of the starch molecules through the addition of water and steam; thereby increasing the pellets' water stability, enhancing production capacity and reducing wear on the extruder.
This patented, innovative Preconditioner+ improves heat and mass transfer to the product due to the Advanced Filling Control device (AFC); it interacts directly with the material inside the mixing chamber and enables the filling ratio to be adjusted.
The AFC system uses an exclusive conveying screw inside the tank and adjusts the flow by a partial and controlled recycling of the material being processed from the outlet to the entry point, thus intensifying the specific preconditioning functions.
Laboratory tests have proven that the final hardness of fish feed pellets increased between seven percent and 29 percent using the same recipe and modifying the preconditioning and extrusion parameters versus adjusting the bottom-screw speed.
During the cleaning procedure, the bottom-screw rotation is reversed to facilitate the cleaning procedure of the preconditioning chamber.  

Finally, today much attention is focused on hygienic extruder design as food security is a key parameter for the fish feed and food industries.
Fish feed manufacturers want to be able to clean their extruder from the outside using hot water and sometimes with cleaning agents. The stainless steel, hygienic Evolum+ frame structure is designed to avoid water stagnation and all the extruder areas are easily accessible. The internal processing assembly of the twin-screw extruder must be cleaned easily as well: the complete quick barrel extraction device is today a paradigm to the industry. It offers access to the screws and barrels in only few minutes and is a state-of-the-art solution that simplifies preventive maintenance, wear monitoring and cleaning processes.

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