66 September 2009 also in constant flux for a number of factors. Why then should the machine not have a few additional advantages in its tool box for the just right application? Obviously one of the biggest topics in aquatic feed production is density. To get a better idea of what is meant by this topic, the buoyancy properties chart in Table 2 defines aquatic animals by feed floatation properties. Some of these animals are saltwater species and others freshwater species. There is a difference in buoyancy of feeds in salt and freshwater. In seawater at 20°C at 30 ppt salinity, feeds float right at 480 g/L, whereas they need to be 440 g/L to float in freshwater at 20°C. Conversely, fast sinking feeds need to be 640 g/L in the same salinity saltwater and 600 g/L in freshwater. This would be the density of the finished feeds, not the feed Table 2. Buoyancy Properties of Feeds for Common Aquatic Species. Floating Slow-Sinking Sinking Alligator Flatfish Abalone Carp Mahi Mahi Cod Catfish Salmon Flounder Eel Sea bass/bream Halibut Frog Tilapia River Crab Milkfish Trout Sea bass/bream Tilapia Yellowtail Sea Urchin Trout Shrimp as it exits the extruder. Production is required to make the adjustment to the density needed based on the level of coating to be applied following extrusion. Having mentioned the density or buoyancy of feeds with regard to fat coating levels, Table 3 illustrates the importance for sinking feeds as it indicates the various standard fat ranges seen, as well as the associated densities needed to achieve those levels with additional related standard extrusion parameters. An important factor when coating feeds with elevated oil levels is the ability of the feed to retain the oil. The moisture level during extrusion, as well as the amount of energy imparted into the process in the form of specific mechanical energy (SME), has a big effect on the fat levels applied. The importance of density and SME allows for the control of floating/ sinking properties, pellet durability for feeding or handling and transportation devices, attractive pellet appearance in terms of shape and size, proper fat absorption characteristics and the effect of rapid water absorption while maintaining pellet integrity. In looking at the mechanical devices developed to easily control and simplify the production and maintain the characteristics in a range of feeds by extrusion cooking, preconditioning is an aspect that must be mentioned. This is because the variations in the raw materials used in aquatic feeds has resulted in the need to understand the various moisture levels required for the various protein sources now being used or considered. Water and steam were always added in a controlled fashion, but the effectiveness of the additions was based on the retention time in the conditioner. The development of retention time controlled preconditioning is based on load cells and computer controls. Known inputs into the cylinder, coupled with a controlled discharge device, results in the retention time being adjustable on line without mechanical down time. The extruder barrels for both conventional twin-screw designs, as well as singles, have also received some attention, mainly because of the increased SME requirements and the styles of cook now used for aquatic feed production. Extruder configurations have evolved with new rotating element designs to transmit the additional energy required into the feed mix. Beefed up drives to handle the additional power inputs allows for the higher SME required diets at maximum capacity. Development and acceptance of high horsepower variable frequency drives has also given the process an additional variable allowing speed changes that, when increased, impart higher levels of specific energy and the opposite when the speed is reduced. When used within reason, it allows for expansion or density control. Table 3. Sinking Aquatic Feed Categories. Low Medium High Ultra High Fat Fat Fat Fat Characteristic Total Fat % <15 15-25 25-35 >35 Density before Coating (g/l) >594 594-520 520-480 <480 Coating Atmos. or Atmos. or Vacuum Vacuum Process Vacuum Vacuum Extrusion 25-29 21-25 19-23 20-24 Moisture % SME (kWhr/t) <25 25-30 30-35 >35 Aquatic cod cod yellowtail salmon Species shrimp trout trout yellowtail salmon sea bass (Continued on page 69)
RkJQdWJsaXNoZXIy MjExNDY=