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Ask the Expert Your aquafeed processing questions answered
Q: We have been attempting to produce small diameter fish feeds for hatchery and nursery, but without success. We are not able to keep the extruder running for long and pellets are not uniform. This is an issue I see regularly in plants that are producing standard feeds above 2 mm and then decide to produce smaller feeds. There are multiple process items at stake here, the main one being mash preparation. The key thing to remember, being the rule of thumb, is that all particles must be ground to below one third of die diameter. Die blockage is unforgiving in extrusion processing as there is no mechanical contact with the die surface (unlike pellet milling). If a particle is large enough to partially block a die, this will interrupt flow in that die, reducing the size of pellet emerging from it. Eventually, other particles will build up to block the flow altogether. As this process is occurring, the pressure on the die is increasing, SME is increasing and pellet density starts to decrease. As the process reaches a critical point where dies are almost completely blocked, the SME and shear forces can be so extreme that a high proportion of starches and proteins become damaged, viscosity plummets and density actually increase again, with pellets appearing very ragged and lacking uniformity. The smaller the die sizes, the greater the potential
for blockage, with issues generally increasing significantly below 2 mm die size in a standard extrusion mill. For example, during a production run that lasts 30 minutes and is eventually stopped due to die blockage, the production is not at a steady state and operators will continually make adjustments to account for changes in pellet conformance. Pellet quality from different parts of that run will be highly variable. Due to the tendency to compensate by increasing die open area, there is potential for pellets to be poorly cooked with low SME at early stages of a run and turn out highly destructed or burnt at the end. These two points can be equally damaging to fish health, depending on species. Die pressure is a useful way of determining the pace of die blockage with pressure increasing as the dies block. I would recommend installing a gauge, if you don’t already have one in place, as they are also a useful safety mechanism when integrated into PLC by signaling the bypass gate to open should pressure exceed the manufacturers recommended operating limits. Excess die pressure can result in catastrophic failure. I have seen dies and heads come off with explosive force on occasion with potential for injury. Increasing knife speed continuously due to increasing pellet length is another key indicator
Peter Hutchinson is a Technical Editor at Aquafeed.com, the owner – director of ENH Ltd., New Zealand, and an aquaculture feed consultant. Send your questions to: pete@aquafeed.com
of die blockage, as the same rate of flow is forced to travel through a lesser number of holes. No doubt you have noticed at least some of these issues described, so how do you go about resolving the problem without spending money? You can’t! This problem won’t self-resolve and requires some expenditure. Here is a list of items you will need to work on: Raw material sources are important. Due to fine grind screens and energy required, ingredients that are difficult to grind or have large amounts of fibrous material are less suitable. Fibers which pass grinding and sieving do not fly through the die like an arrow, but
Aquafeed: Advances in Processing & Formulation Vol 11 Issue 4 2019