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Grain & Feed Milling Technology is published six times a year by Perendale Publishers Ltd of the United Kingdom.All data is published in good faith, based on information received, and while every care is taken to prevent inaccuracies, the publishers accept no liability for any errors or omissions or for the consequences of action taken on the basis of information published. ©Copyright 2010 Perendale Publishers Ltd. All rights reserved. No part of this publication may be reproduced in any form or by any means without prior permission of the copyright owner. Printed by Perendale Publishers Ltd. ISSN: 1466-3872

Digital Re-print - March | April 2011 Aquafeed twin-screw extrusion processing

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The aquafeed manufacturing industry is widely recognised as one of the fastest expanding food industries

in the world. Fish meal is the main protein source in aquafeed, but supply is limited, which means that alternative sources must be used. From a process-ing point of view, extrusion is the most efficient way of turning plant protein into fish feed. Twin-screw extruders offer a definite advantage in this.

The goal for production of aquatic feed is to manufacture a nutritionally complete product that achieves the desired product characteristics. While all aspects of the proc-ess are important, a number of the unit operations of the manufacturing process are critical to achieve this goal.

These operations are organized along the process as follows:

Selecting Raw materialsThe first step of the aquafeed process line

is feed mix preparation–selecting a combina-tion of ingredients with the proper levels of essential nutriments required for the animal species. The formulation is also based on cost, availability and chemical composition of the raw materials. Ingredient selection has a direct impact on final product characteristics.

According to their functions, these ingre-

dients can be divided in three groups:- Nutriments: to meet the requirements

of fish- Functional product: binders, expansion,

hardness- Palatants and attractants

Pre- grinding / grindingIt is essential to decrease the particle

size to a powder state before mixing the ingredients. Post grinding achieves the best final performance.

In addi-tion, parti-cle size is dependent on the final size of the pellets. For die openings up to 3mm, the largest particle size should not be larger than 1/3 of the die opening. Smaller particles improve pellet durability, water stability and decrease pellet friability.

MixingMixing accuracy depends on the proper-

ties of the components, which should be similar in density and particle size. Additives or micro ingredients are added at this step. The required mixing time depends on the type of mixer technology used, as well as dry ingredi-ent mixing time before liquids are added.

Extrusion cookingThis step of the aquafeed processing line

can be divided into three stages: precondi-tioning, thermo-mechanical cooking and die texturization-shaping

PreconditioningThe primary objective of precondition-

ing in an extrusion cooking process is to initiate the hydration and the cooking of the feed mix. The dry feed mix and the liquid parts (slurry, oil…) are separately

introduced into the preconditioner where they are continuously mixed, heated and moisturized by the injection of water and steam. The intense mixing created by the rotating double shafts adjusting paddles assembly maintains the feed particles at the optimum moisture between 20–23 percent and temperature around 90°C during 2 to 3 minutes average retention time. Preconditioning helps to maintain starch and nutriment quality and allows increased extrusion capacity, while reduc-ing extruder screw wear and mechanical energy requirements.

aquafeed twin-screw extrusion processing

Versatile twin-screw extruder ideal for aquafeed

Table 1: Indication of final pellets bulk density /floating or sinking properties:

Feed characteristics Fast sinking Slow sinking Neutral floatability Floating

Bulk density gr/l >640 540 - 600 480 - 540 <450

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Twin-screw Extruder/Thermo-mechanical cooking

The preconditioned feed mix is submit-ted to the controlled thermo-mechanical cooking which is the main stage in extruding aquafeeds.

Thermo-mechanical cooking of the feed mix in an extruder requires two energy inputs:

- Mechanical energy input defined mainly by screw speed and screw configuration, which can be varied extensively to modulate this energy

- Thermal energy input determined by direct steam heating and indirect barrel heatingTwin-screw extruders are able to process

a large range of raw materials consistently with high levels of flexibility and pumping efficiency. The intermeshing screws allow handling of viscous, oily, sticky or very wet materials and provide a very intense mixing, where macromixing and micromixing result in a very homogeneous melt with excellent lipid binding.

In a co-rotating twin-screw extruder, throughput and screw speed are not inter-dependent; for a given formulated feed mix, the multiple operating points combine with a high control efficiency of the barrel tempera-ture and consequently ensures efficient con-trol of the expansion of the melt at the die.

In comparison with single screw extruder, twin-screw is more responsive. By vary-

ing the cooking parameters it is possible to main-tain more precise limits on product characteristics such as density to achieve floating, low sinking and sinking pellets.

Expansion can be further enhanced by injection of steam into the extruder barrel, which increases thermal energy inputs. Where higher product densities are required for certain feeds, the extruder barrel can be configured to include a vent stuffer to reduce product tem-perature through evaporative cooling. Vacuum regulation can be connected to the vented stuffer barrel to increase

Grain&feed millinG technoloGy march - april 2011 | 35

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temperature must be lowered to approximately 50°C otherwise, the water in the pellets will start boiling because of the vacuum applied (0,2 bar absolute pressure)

The vacuum coating process allows the oil to be drawn into the

pellets by capillary forces ensuring perfect control of the pellet oil absorption and the addition of more oil into the product while ensuring a dry surface and limiting the oil discharge in the natural environment.With this technology it is possible to achieve more than 40 percent total product fat content.

Cooling of pelletsOn completion of the fat coating process,

the pellets are cooled and sieved before the final conditioning. Cooling is required to remove excess heat to prevent condensa-tion from occurring in the storage bins or the final packages. At this stage, the aquafeed pellets’ temperature should be cooled down to a temperature range close to the storage or transportation temperature.

ConclusionClextral systems can easily produce high

energy feed, allowing a delicate balance of proteins, oils and carbohydrates, processed for total digestibility with no waste. It’s the right formula for high quality products that “turn feed into flesh”. Output range from 25 to 30,000kg/h.

Thanks to twin-screw extrusion, high quality aquafeed pellets are achieved:

• denaturing of proteins• gelatinization of starch• reduction of anti-nutriments• flavouring improvement• protein/lipid complexes• increase digestibility • texturization, shaping• expansion, density • hygienic, salmonella-free pellets

Challenges aheadThe major challenge is to expand sustain-

able aquaculture to achieve enhanced food security and economic development for the global population as a whole. In the context of substitution of fishmeal with plant-derived feedstuffs, extrusion technology has a role to play in reducing the level of anti-nutriments. (Kaushik 2006)

Twin-screw extrusion technology has contributed and will carry on the tremen-dous improvements, both in terms of nutri-tional value and in terms of physical quality characteristics of the aquafeeds.

With the continued economic develop-ment of aquaculture, each venture, and species will be subject to important research efforts so as to obtain, under acceptable eco-nomic conditions, efficient feeds delivered at the right time, which are non-polluting and which care for the health of the fish as well as the consumers.

temperature and humidity of the drying air

- Pellet related factors: depending on the initial moisture content and temperature, porosity and size of the pelletThe drying parameters must be applied

to remove the moisture while maintaining pellet quality. (that is, limiting fine generation, maintaining palability and pigment ingredi-ents, minimum energy losses and moisture variance)

Floating and sinking product character-istics can be influenced by the drying con-ditions. Elevated temperatures can lower residual moisture and improve floatability. Sinking aquatic feeds are preferably dried at moderate temperatures until the storage stability of the pellets is obtained. In aquafeed processing lines, extruded pellets are generally dried on one of the following dryer designs:- Horizontal belt dryer - Vertical counter flow dryer- Fluidized bed dryer

and recently, rotary dryer technology: the Rotante type rotary dryer was previ-ously designed to process other cereal based products. Through gentle stirring of the product to eliminate build-up, the “Rotante” design achieves excellent heat exchange close to that obtained in a flu-idized bed. Other advantages include a perfectly controlled residence time with virtually no dispersion, of type FIFO (First In, First Out) and precise product moisture homogeneity at dryer output. To master the drying barema, temperature & moisture are precisely regulated, which helps to prevent product cracks. Clextral’s Research & Test Center in France is equipped with a “Rotante” and tests have proven this dryer to be particularly efficient for fish feed, ensuring complete homogeneity of drying with lower energy consumption.

Fat coatingFat addition is commonly done after

dryer, while the dried extrudates are still warm. For that, the oil is sprayed directly on pellets. During this stage, fats, pigments, attractants and even powdered ingredients can be added.

This processing stage can be implement-ed two ways:*Under atmospheric pressure: the added oil, temperature around 40°C, is diffused from the surface of the product to the centre and is stored in cavities in the pellets’ structure*Under vacuum pressure: this consists of a closed system with a mixing shaft or screw and spray system in the top. The product

the product density even further with higher degrees of evaporative cooling.

Texturisation-shaping of aquafeed pellets

The end of the last barrel of the extrusion chamber is capped with a final die, which serves two major functions. First, the die restricts product flow thereby causing the extruder to develop the required pressure and shear; and second, the die shapes the extrudate.

A face cutter is used in conjunction with the die, which consists of cutting knives revolving in a plane parallel to the face of the die. The relative speed of the knives and the linear speed of the extrudate result in the desired product length.

Die design –cutter assembly is one of the most important sub-units of the aquafeed processing line as it determines the physical quality of the final product.

Micro-aquatic feeds often used as starters are products smaller than 2.0mm.

Specific die design allows direct extrusion of micro-aquatic feeds down to 0.5 mm. Products are pasteurized and very nicely shaped. Water stability is excellent and float-ing products are possible. Raw material must be carefully ground and sifted to achieve proper particle size before extrusion.

Macro-aquatic feeds with sizes up to 30 mm emphasize the physical quality of the pellet and the related process history. A combination of twin-screw mixing and cooking, special Rotante drying technol-ogy and dedicated die design will produce pellets with sufficient resistance to avoid breakage and dust, yet porous enough to deliver all the nutrients to the digestive system of the fish.

Pellet dryingThe primary purpose of reducing pel-

let moisture level is to make the product shelf stable. Most aquatic products are best processed at moisture levels between 20-28 percent. Moisture levels as low as 20 percent can be required for some light density aqua-feed pellets. Some moisture is lost during flash evaporation as the cooked product exits the die. Then, the products are con-veyed to the dryer to reduce the moisture content from 18 – 24 percent down to 8 – 10 percent, this corresponds to a water activity (aW) around 0,5 – 0,4, in order to obtain satisfactory water stability.

Several factors control the water removal from the aquafeed pellets:- Air related factors: depending on the air

flow characteristics around the pellets,

Table 2: Species / product characteristics example:

Species Salmon Trout Cod Carp Tilapia Cat fish Shrimp

Final pellet total fat % >35 15 - 35 15 - 25 5 - 15 5 - 10 5 - 10 <5

Texture bulk density Slow sinking Slow sinking Slow sinking Floating Floating Floating Fast sinking

Grain&feed millinG technoloGy36 | march - april 2011

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Grain&feed millinG technoloGy march - april 2011 | 11

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