Aquafeed twin-screw extrusion processing - Versatile and ideal for aquafeed

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					                                                             May | June 2011
Feature title: Aquafeed twin-screw extrusion processing - Versatile and ideal for aquafeed

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                        or by any means without prior permission of the copyright owner. Printed by Perendale Publishers Ltd. ISSN: 1464-0058

            The International magazine for the aquaculture feed industry
 F: Extrusion                                                                                                                                                                                                                                                    F: Extrusion
                                                                                                                                                                           Mr Daniel                 mainly by screw speed and screw                 and consequently ensures efficient control
                                                                                                                                                                             Durand                  configuration, which can be varied              of the expansion of the melt at the die.
                                                                                                                                                                                                     extensively to modulate this energy                 In comparison with single-screw extrud-
                                                                                                                                                                                                  - thermal energy input determined by               er, twin-screw is more responsive. By vary-
                                                                                                                                                                                                     direct steam heating and indirect bar-          ing the cooking parameters it is possible to
                                                                                                                                                                                                     rel heating                                     maintain more precise limits on product
                                                                                                                                                                                                    Twin-screw extruders are able to process         characteristics such as density to achieve
                                                                                                                                                                                                a large range of raw materials consistently          floating, low sinking and sinking pellets.
                                                                                                                                                                                                with high levels of flexibility and pumping              Expansion can be further enhanced by
                                                                                                                                                                                                efficiency. The
                                                                                                                                                                                                screws allow
                                                                                                                                                                                                handling         of
                                                                                                                                                                                                viscous, oily,
                                                                                                                                                                                                sticky or very
                                                                                                                                                                                                wet materials
                                                                                                                                                                                                and provide a
                                                                                                                                                                                                very intense
                                                                                                                                                                                                                              MADE IN HOLLAND
                                                                                                                                                                                                mixing, where
                                                                                                                                                                                                m a c ro m i x i n g
                                                                                                                                                                                                and micromix-
                                                                                                                                                  ing double shafts adjusting paddles           ing result in a
                                                                                                                                                  assembly maintains the feed particles at      very      homo-
                                                                                                                                                  the optimum moisture between 20–23            geneous melt
                                                                                                                                                  percent and temperature around 90°C           with excellent
                                                                                                                                                  during two to three minutes average           lipid binding.
                                                                                                                                                  retention time. Preconditioning helps to          In a co-
                                                                                                                                                  maintain starch and nutriment quality         rotating twin-
  Aquafeed twin -screw extrusion processing                                                                                                       and allows increased extrusion capacity,
                                                                                                                                                  while reducing extruder screw wear and
                                                                                                                                                                                                screw extruder,

         Versatile and ideal for aquafeed                                                                                                         mechanical energy requirements.               and        screw
                                                                                                                                                                                                speed are not
                                                                                                                                                  Twin-screw Extruder / Thermo-                 interdependent;
                                   By Mr Daniel Durand, Senior Extrusion Expert at Clextral                                                       mechanical cooking                            for a given               High capacity extruders and expanders.
                                                                                                                                                      The preconditioned feed mix is submit-    formulated feed

                                                                                                                                                  ted to the controlled thermo-mechanical       mix, the mul-
           he aquafeed manufactur-               combination of ingredients with the proper     Mixing                                            cooking which is the main stage in extrud-    tiple operating
           ing industry is widely                levels of essential nutriments required for        Mixing accuracy depends on the proper-        ing aquafeeds.                                points combine
           recognised as one of the              the animal species. The formulation is also    ties of the components, which should be               Thermo-mechanical cooking of the feed     with a high con-
           fastest expanding food                based on cost, availability and chemical       similar in density and particle size. Additives   mix in an extruder requires two energy        trol efficiency           Almex b.v., Verlengde Ooyerhoekseweg 29, 7207 BJ Zutphen
industries in the world. Fishmeal is             composition of the raw materials. Ingredient   or micro ingredients are added at this step.      inputs:                                       of the barrel             The Netherlands, tel. +31 (0)575 572666, fax +31 (0)575 572727
the main protein source in aquafeed,             selection has a direct impact on final prod-   The required mixing time depends on the             - mechanical energy input defined           temperature               E-mail,
but supply is limited, which means               uct characteristics.                           type of mixer technology used, as well as
that alternative sources must be                     According to their functions, these        dry ingredient mixing time before liquids
used.                                            ingredients can be divided in three groups:    are added.
                                                   - Nutriments: to meet the require-
    From a processing point of view, extru-          ments of fish                              Extrusion cooking                                                                                      The twin screw extrusion expert
sion is the most efficient way of turning          - Functional product: binders, expan-           This step of the aquafeed processing line
plant protein into fish feed. Twin-screw             sion, hardness                             can be divided into three stages: precondi-
extruders offer a definite advantage in this.      - Palatants and attractants                  tioning, thermo-mechanical cooking and die
    The goal for production of aquatic feed                                                     texturization-shaping
                                                 Pre- grinding / grinding
                                                                                                                                                  HIGH QUALITY
is to manufacture a nutritionally complete
product that achieves the desired product            It is essential to decrease the particle   Preconditioning
characteristics. While all aspects of the        size to a powder state before mixing the          The primary objective of precondi-
process are important, a number of the unit      ingredients. Post grinding achieves the best   tioning in an extrusion cooking process           Aquatic Feed
operations of the manufacturing process          final performance.                             is to initiate the hydration and the
are critical to achieve this goal.                   In addition, particle size is dependent    cooking of the feed mix. The dry feed
    These operations are organized along         on the final size of the pellets. For die      mix and the liquid parts (slurry, oil                                                                  From 300kg/h to 30 000kg/h
the process as follows:                          openings up to 3mm, the largest particle       …) are separately introduced into the
                                                 size should not be larger than 1/3 of the      preconditioner where they are continu-
Selecting raw materials                          die opening. Smaller particles improve pel-    ously mixed, heated and moisturised by
    The first step of the aquafeed process       let durability, water stability and decrease   the injection of water and steam. The
line is feed mix preparation–selecting a         pellet friability.                             intense mixing created by the rotat-

                                                30 InternatIonal AquAFeed May-June 2011
                                                30 || InternatIonal AquAFeed || May-June 2011                                                                                                  May-June 2011 InternatIonal AquAFeed 31
                                                                                                                                                                                               May-June 2011 || InternatIonal AquAFeed || 31
 F: Extrusion                                                                                                                                                                                                                                                                    F: Extrusion
                                                                                                                                                               The drying parameters must be applied              must be lowered to approximately              •    reduction of anti-nutriments
                                                                                                                                                           to remove the moisture while maintaining               50°C otherwise, the water in the              •    flavouring improvement
                                                                                                                                                           pellet quality. (that is, limiting fine gen-           pellets will start boiling because of         •    protein/lipid complexes
                                                                                                                                                           eration, maintaining palability and pigment            the vacuum applied (0.2 bar absolute          •    increase digestibility
                                                                                                                                                           ingredients, minimum energy losses and                 pressure).                                    •    texturization, shaping
                                                                                                                                                           moisture variance)                                    The vacuum coating process allows the          •    expansion, density
                                                                                                                                                               Floating and sinking product characteris-     oil to be drawn into the pellets by capillary      •    hygienic, salmonella-free pellets
                                                                                                                                                           tics can be influenced by the drying condi-       forces ensuring perfect control of the pellet
                                                                                                                                                           tions. Elevated temperatures can lower            oil absorption and the addition of more           Challenges ahead
                                                                                                                                                           residual moisture and improve floatability.       oil into the product while ensuring a dry             The major challenge is to expand sus-
                                                                                                                                                           Sinking aquatic feeds are preferably dried        surface and limiting the oil discharge in the     tainable aquaculture to achieve enhanced
                                                                                                                                                           at moderate temperatures until the storage        natural environment.                              food security and economic development
                                                                                                                                                           stability of the pellets is obtained.                 With this technology it is possible to        for the global population as a whole. In the
                                                                                                                                                               In aquafeed processing lines, extruded        achieve more than 40 percent total product        context of substitution of fish meal with
                                                                                                                                                           pellets are generally dried on one of the         fat content.                                      plant-derived feedstuffs, extrusion technol-
                                                                                                                                                           following dryer designs:

                                                                                                                                                             - Horizontal belt dryer
                                                                                                                                                             - Vertical counter flow dryer
                                                                                                                                                             - Fluidized bed dryer
                                                                                                                                                               And recently, rotary dryer tech-
                                                                                                                                                           nology: the Rotante type rotary
                                                                                                                                                           dryer was previously       designed
                                                                                                                                                           to process other cereal based
                                                                                                                                                           products. Through gentle stirring
                                                                                                                                                           of the product to eliminate build-
                                                                                                                                                           up, the “Rotante” design achieves
injection of steam into the extruder bar-             aquafeed processing line as it determines              moisture level is to make the product         excellent heat exchange close to
rel, which increases thermal energy inputs.           the physical quality of the final product.             shelf stable. Most aquatic products are       that obtained in a fluidized bed.
Where higher product densities are required              Micro-aquatic feeds often used as start-            best processed at moisture levels between     Other advantages include a per-
for certain feeds, the extruder barrel can be         ers are products smaller than 2.0mm.                   20-28 percent. Moisture levels as low as      fectly controlled residence time
configured to include a vent stuffer to reduce           Specific die design allows direct extru-            20 percent can be required for some light     with virtually no dispersion, of
product temperature through evaporative               sion of micro-aquatic feeds down to 0.5mm.             density aquafeed pellets. Some moisture is    type FIFO (First In, First Out) and
cooling.Vacuum regulation can be connected            Products are pasteurised and very nicely               lost during flash evaporation as the cooked   precise product moisture homo-
to the vented stuffer barrel to increase the          shaped.Water stability is excellent and floating       product exits the die. Then, the products     geneity at dryer output. To master
product density even further with higher              products are possible. Raw material must be            are conveyed to the dryer to reduce           the drying barema, temperature &
degrees of evaporative cooling.                       carefully ground and sifted to achieve proper          the moisture content from 18–24 percent       moisture are precisely regulated,
                                                      particle size before extrusion.                        down to 8–10 percent, this corresponds        which helps to prevent product
Texturisation-shaping                                    Macro-aquatic feeds with sizes up to                to a water activity (aW) around 0.5–0.4, in   cracks. Clextral’s Research and
of aquafeed pellets                                   30mm emphasize the physical quality of                 order to obtain satisfactory water stabil-    Test Centre in France is equipped
   The end of the last barrel of the extru-           the pellet and the related process history.            ity.                                          with a ‘Rotante’ and tests have proven this       Cooling pellets                                   ogy has a role to play in reducing the level
sion chamber is capped with a final die,              A combination of twin-screw mixing and                      Several factors control the water        dryer to be particularly efficient for fish          On completion of the fat coating proc-         of anti-nutriments. (Kaushik 2006)
which serves two major functions. First, the          cooking, special Rotante drying technol-               removal from the aquafeed pellets:            feed, ensuring complete homogeneity of            ess, the pellets are cooled and sieved before        Twin-screw extrusion technology has
die restricts product flow thereby causing            ogy and dedicated die design will produce                - Air related factors: depending on the     drying with lower energy consumption.             the final conditioning. Cooling is required to    contributed and will carry on the tre-
the extruder to develop the required pres-            pellets with sufficient resistance to avoid                  air flow characteristics around the                                                       remove excess heat to prevent condensation        mendous improvements, both in terms of
sure and shear; and second, the die shapes            breakage and dust, yet porous enough to                      pellets, temperature and humidity of    Fat coating                                       from occurring in the storage bins or the final   nutritional value and in terms of physical
the extrudate.                                        deliver all the nutrients to the digestive                   the drying air                             Fat addition is commonly done after            packages. At this stage, the aquafeed pellets’    quality characteristics of the aquafeeds.
   A face cutter is used in conjunction with          system of the fish.                                      - Pellet related factors: depending         dryer, while the dried extrudates are still       temperature should be cooled down to a               With the continued economic devel-
the die, which consists of cutting knives                                                                          on the initial moisture content and     warm. For that, the oil is sprayed directly       temperature range close to the storage or         opment of aquaculture, each venture,
revolving in a plane parallel to the face of         Pellet drying                                                 temperature, porosity and size of the   on pellets. During this stage, fats, pigments,    transportation temperature.                       and species will be subject to important
the die. The relative speed of the knives and           The primary purpose of reducing pellet                     pellet                                  attractants and even powdered ingredients                                                           research efforts so as to obtain, under
the linear speed of the extru-                                                                                                                             can be added.                                     Conclusion                                        acceptable economic conditions, efficient
date result in the desired          Table 1: Indication of final pellets bulk density /floating or sinking properties:                                        This processing stage can be imple-                Clextral systems can easily produce high      feeds delivered at the right time, which
product length.                          Feed characteristics       Fast sinking          Slow sinking          Neutral floatability      Floating         mented two ways:                                  energy feed, allowing a delicate balance of       are non-polluting and which care for
   Die      design     –cutter                                                                                                                              • under atmospheric pressure: the                proteins, oils and carbohydrates, processed       the health of the fish as well as the
assembly is one of the most                                                                                                                                    added oil, temperature around 40°C,           for total digestibility with no waste. It’s the   consumers.
                                           Bulk density gr/l            >640               540 - 600                 480 - 540             <450
important sub-units of the                                                                                                                                     is diffused from the surface of the           right formula for high quality products that
                                                                                                                                                               product to the centre and is stored in        “turn feed into flesh.” Output range from
            Species                    Salmon             Trout             Cod              Carp             Tilapia          Cat fish     Shrimp             cavities in the pellets’ structure.           25 to 30,000kg/h.                                      More   inforMation:

                                                                                                                                                            • under vacuum pressure: this consists               Thanks to twin-screw extrusion, high               Clextral
                                                                                                                                                                                                                                                                    1 Rue du Colonel Riez
                                                                                                                                                               of a closed system with a mixing              quality aquafeed pellets are achieved:
           Final pellet total fat %      >35             15 - 35          15 - 25           5 - 15            5 - 10            5 - 10        <5                                                                                                                    Firminy, F-42700 France
                                                                                                                                                               shaft or screw and spray system in              • denaturing of proteins                             Website:
              Texture bulk density Slow sinking       Slow sinking      Slow sinking        Floating         Floating          Floating   Fast sinking         the top. The product temperature                • gelatinization of starch

                                                    32 InternatIonal AquAFeed May-June 2011
                                                    32 || InternatIonal AquAFeed || May-June 2011                                                                                                           May-June 2011 InternatIonal AquAFeed 33
                                                                                                                                                                                                            May-June 2011 || InternatIonal AquAFeed || 33
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Description: The aquafeed manufacturing industry is widely recognised as one of the fastest expanding food industries in the world. Fishmeal is the main protein source in aquafeed, but supply is limited, which means that alternative sources must be used.