World Aquaculture 65 Development of aquatic feeds by extrusion cooking1 Joseph P. Kearns2 What would my response be when asked by neighbors what is it that I do for a living? “Extrusion cooking of course, which is the process where expandable biopolymers, such as protein and starch, are plasticized in a tube by a combination of heat, shear and pressure resulting in the denaturation of the protein, the gelatinization of the starch and the exothermic expansion of the extrudate.” In any case, an extruder is a machine where the sum of the inputs results in the final output, which happens to be, in this case, aquatic feeds. These inputs or dry raw materials, such as cereal grains, fishmeals, vegetable proteins, minerals, water, steam and other liquid and/or dry ingredients, all need to be introduced in the process in an even manner so that continuous and predictable results are obtained. Exacting density, expansion rates, shape and other product characteristics of the desired feeds require precise inputs that will lead to predictable results. Let’s review the aquatic feed process and how the tools have changed to accomplish the task at hand. The preparation of the dry raw formula requires grinding the to a fine even particle size suitable for the diameter aquatic feed being produced. The dry feed is normally metered into a preconditioner by a feeder screw and live bottom bin that yields an accurate and even flow of that portion of the mix. The preconditioner function gives the final preparation to the feed mix prior to extrusion. The device homogenizes the dry and liquid ingredients typically into a free flowing moistened and heated powder that flows into the extruder barrel. Additional liquids can be introduced into the extruder barrel as well, if desired. The extruder barrel can manage the variety of characteristics required for a full range of aquatic feeds by extrusion cooking. Extruder manufacturers are required to constantly stretch the limits of what can be made with their equipment. Levels and sources of proteins in the formula, starch levels, final fat levels, stability in water, floatability and densities are all typical characteristics that are defined for each product. These may require not only formulation changes but also system changes for different style aquatic feeds. In addition, in some aquatic feed plants the changes take place hourly. Control of the variables and the need to manage them easily, coupled with reduced down time for equipment changes, can justify a review of the possibilities available for a multiple purpose plant or for improvements for specific defined processes. Feeds made by extrusion cooking Table 1. Vegetable Proteins in Salmon, Trout and Shrimp Diets. Vegetable Protein Max. Substitution for Disadvantages Source Fish Meal (%) Maize Gluten Meal 40 Yellow Pigmentation of Flesh Wheat Gluten Meal 25 High Cost Soybean Meal 50 Palatability and Growth Inhibitors Soy Concentrate 75 High Cost Canola Meal 67 Low Protein Content (Hardy 1999) include a range of diameters from 0.8 mm sinking and 0.8 mm floating feeds up to 30 mm and sometimes larger diameter feeds. Feeds in the 300 micron range are produced in the Extru-Tech Sphere-izer Agglomeration System™. Semi-moist feeds are also produced and seem to be gaining use in selected aquatic feed areas, such as for sea urchins and abalone. There is a tremendous variation in feed ingredients around the world, but the pointed decline or flattening in fishmeal availability is considered a global problem. Hardy (1999) pointed out the possible substitution levels of various vegetable protein sources and the possible disadvantages of their use in salmon, trout and shrimp diets as seen in Table 1. Equipment manufacturers need to point out the needed changes in the equipment for the reduction of fishmeal and increases in vegetable protein levels inasmuch as the equipment does require processing those ingredients differently. The variety of aquatic feed characteristics vary greatly, the possible ingredients are
RkJQdWJsaXNoZXIy MjExNDY=