Starch source, screw configuration and injection of steam into the barrel affect the physical quality of extruded fish feed

This experiment investigated the physical qualities ('Holmen' durability; 'tumbling box' durability; and pellet strength, length and diameter) of extruded fishmeal-based feeds produced using different starch sources, screw configurations, with or without steam injected in the barrel. The starch sources tested were native or pre-gelatinized potato starch (PGS), wheat starch (WS), whole wheat (WW) or a combination of WS and WW. Screw configurations were a polygon element, followed by two left-pitched segments, Polygon_2L; a polygon, followed by one left-pitched and one right-pitched element, Polygon_LR; and no polygon section, alternating left- and right-pitched elements, LRLR. Starch source and screw configuration affected the physical quality the most. Pellets produced with LRLR had the highest 'Holmen' durability (76%), hardness (30 N) and the least diameter (5.3 mm). The lowest durability (37%) and hardness (22 N) were obtained with Polygon_LR. Potato starch yielded higher expansion than cereal starch. The highest 'Holmen' durability was seen for PGS (79%), while WS yielded the lowest value (44%). Injection of steam reduced hardness and 'Holmen' durability, but provided a minor overall contribution to physical quality compared with the starch source and screw configuration. 'Holmen' durability appeared to be suitable to unveil variation (37-79%) in physical quality, while 'Tumbling box' durability (98.4-98.8%) did not prove efficient.