FEATURE
vs.
Extruded aqua feeds pelleted aqua feeds
T
by Joe Kearns, Extrusion specialist, JPKearns Consulting, LLC and Director of Sales at Meridian Ingredients LLC his is a comparison of two different feed preparation processes. A simple comparison and definition would be cooking versus compressing a feed into pellet form with the percentage of cook being at a higher level with the use of extrusion over pelleting.
the accurate inclusion and evenness of the preconditioned mass headed to the extruder. This is required as higher levels of liquids are used in extrusion. It is not unusual to add water to be in the mid 30% moisture range when extruding some types of aquatic feeds. Pelleting has a liquid or percent water limitation and it is in approximately the 17-18 percent moisture range.
Material preparation
In both cased the preconditioner is above the actual pellet forming system, extruder barrel or ring die and compression rollers. The pellet system as shown in Photo 1 assists in the explanation of the pellet mill chamber. In this case the ring die rotates around the rollers, which create the force to push or compact the feed through the die. Knives are mounted to cut pellets to an acceptable length. The actual feed material is never melted but remains in its ground particle form and is compacted into pellets by pressure; this is assisted by die hole design. Extrusion cooking is based on a rotating shaft with screw elements in a confined tube creating pressure, heat and mechanical shear (friction) that melts the feed material into a viscous mass behind the die. This mass is plasticised and is at such a pressure the product expands upon exiting the die, entrapped air due to the development of a cell structure in each pellet creates floating products. Process control modifications allow for this cell structure to be controlled for floating, slow sinking as well as fast sinking pellets.
Let’s review the processes starting with the material preparation before processing. All is the same in terms of batching and weighing until grinding. The possible need for finer grinding exists for the extrusion process. Finer grinding as the possibility to achieve product diameters as low as 0.5mm exist in the extrusion process. Finer grinding is considered important for good pelleting quality also but typically it is not required to get to the 200-micron raw material size. Extruders can make 0.5mm feeds thus finer grinding for extrusion over pelleting. Ground particles should be 1/3 the die opening size for extrusion thus a 0.5mm feed needs very fine grinding.
Processing differences
This brings us to the actual process differences. The feeding system is similar, as both systems need even raw material feeding with volume or weight deliver control. Weight delivery or gravimetric is common on extrusion cooking as it allows for more accurate control of the process additives such as water and steam and other liquids or solids. Why is more accuracy needed with extrusion, simply put the additions by weight give a more accurate pellet characteristic? Exacting diameters, density, expansion rates for oil uptake, sinking rate and others are controlled by the process and additions. Having stated the above the conditioning systems are usually more advanced on extrusion cookers. Rotational speeds, retention times and mixing effects are controllable for
Photo 1
Pelleting systems
Extrusion cookers
Photo 2 shows the screws in typical extrusion cookers. In most extruders the screws are segmented thus the positions and designs can be changed for friction development effects, control of pressure zones and where the actually cooking occurs in the device. Also seen is an extruder barrel stopped and opened up so
Photo 2
36 | August 2018 - International Aquafeed