WWW.WAS.ORG • WORLD AQUACULTURE • MARCH 2025 29 effect of size non-uniformity is, however, observed during the coating process. Larger pellets may remain unsaturated with oil due to insufficient diffusion time or vacuum strength, while smaller pellets may absorb excess oil, resulting in surface wetness and increased likelihood of oil leakage. This issue becomes evident when a handful of coated pellets is placed in water. Larger, unsaturated pellets may float due to lower density, whereas smaller, oil-saturated pellets will leach oil readily into the water. Beyond processing concerns, pellet size uniformity directly affects feeding performance, particularly for smaller pellets, as it influences sinking velocity and surface tension (Eriegha et al. 2017). Figure 5 illustrates that all runs on the AFX platform had a significantly lower standard variation on diameter than in both TX and SX runs. Figure 6 further shows this effect, zooming in on the runs with low water addition (‘3’) across the three extrusion platforms. These run conditions seem to have caused more strain on TX and SX than for AFX, as seen by the outcome of variation in pellet diameters. Difference in pellet diameters in extrusion can come from pellets spatially expanding to a different degree across the die, and temporally different expansion over time (i.e. extruder is ‘surging’), but also from a single pellet exhibiting a nonuniform expansion. Figure 7 zooms in on this behavior and it is very evident that pellets produced on a SX platform are significantly less circular than on both twinscrew platforms. In this regard, it should also be noted that since pellets will not obtain a circularity larger than 100 %, the circularity variation becomes much less on AFX and TX platforms than on the SX. Circularity of pellets can also be important for downstream bulk handling properties. Pellets that are more uniform in circularity will tend to pack both less densely and more predictably, resulting in consistent bulk density. Pellets more uniform in shape will also experience more evenly distributed forces during handling, leading to minimization of breakage and improved yield. To consider also axial expansion, i.e. pellet length, Figure 8 zooms in on net surface area/volume for all runs on all three platforms. Again, one-way ANOVA across the 3 runs demonstrates SDs to be significantly different (p=0.02), but what is further noted here is that pellets on the AFX runs are much more evenly distributed around its mean than on TX, and particularly than on SX runs. This becomes important in that pellets lying a longer distance away from the mean will be those that are highly prone to become unsaturated with oil exhibiting undesired nutritional profiles as well as unwanted buoyancy. Likewise, but on the opposing end of the scale, would be those pellets that are more prone to leaking fat. SX runs and TX runs both produced pellets with surface area/volume that lie further away from the mean, when compared to AFX runs. Summary Trial results demonstrate that the twin-screw technology (AFX) consistently and significantly produces feed with lower pellet variation (CV), improved circularity, and higher durability when compared with both the traditional twin-screw (TX) and single-screw (SX) systems. Additionally, the AFX system achieved a similar density LEFT, FIGURE 4. Oil uptake in vacuum across the three runs. AFX is superior to TX, where SX runs were generally varying a lot depending on run conditions and corresponding well with density. RIGHT, FIGURE 5. Diameter measured for all runs, each for 30 pellets. AFX pellets have significantly less standard deviation in diameter across all runs, than for pellets in TX and SX runs. LEFT, FIGURE 6. Diameter measured for runs using lower water inclusion, runs ‘3’. RIGHT, FIGURE 7. Circularity of pellets across all runs and extrusion platforms. Pellet from SX runs was significantly less circular and had much higher variation in obtained circularity. (CONTINUED ON PAGE 30)
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