The effect of shear forces and addition of a mixture of a protease and a hemicellulase on chemical, physical and physiological parameters during extrusion of soybean meal

The influence of shear forces during extrusion of toasted soybean meal (TSBM) and the addition of enzymes before and after extrusion on some chemical, physical and physiological parameters were studied. Shear forces were introduced by the use of 0, 1 and 2 twin lead slotted screws (TLSSs) during extrusion of TSBM with a single-screw extruder. The experiment included 6 treatments: 1. unextruded TSBM, 2–4. TSBM extruded with 0, 1, and 2 TLSSs, respectively, 5. addition of a mixture of a protease and a hemicellulase before extrusion of TSBM with 1 TLSS and 6. the same amount of enzymes added after extrusion of TSBM with 1 TLSS. Chemical analysis showed a decrease of 25–41% in trypsin inhibitor activity (TIA) and a decrease in lectin content below the detectable level as a result of extrusion. The in-vitro protein digestibility increased from 60.7% in TSBM to 81.1% after extrusion with 2 TLSS and seemed negatively correlated with TIA. Different shear levels during extrusion gave no correlation between animal performance and TIA. The protein dispersibility index (PDI) and nitrogen solubility index (NSI) decreased as a result of extrusion but both failed to differentiate between the different shear levels. Using NSI as a quality parameter of soybean meal extrusion, it was concluded that NSI levels of 50–55% improved in-vitro protein digestibility and feed conversion of broiler chickens. The content of lysine which reacted with 1-fluor-2,4-dinitrobenzene at 435 nm (FDNB-reactive lysine) did not change as a result of extrusion at different shear levels. A growth experiment with 578 female and 528 male 1 day-old broiler chickens showed that extrusion significantly improved feed conversion (P < 0.001). Also the addition of enzymes before and after extrusion improved feed conversion compared with the unextruded TSBM (P < 0.001). Enzyme addition after extrusion also increased weight gain of mainly the male chickens (P < 0.0256). No significant differences in animal performance between the different shear levels was obtained nor between the addition of enzymes before and after extrusion. A digestibility experiment with 30 rats fed with diets containing TSBM and TSBM extruded with 0, 1, and 2 TLSSs gave no significant differences in total tract digestibility of nitrogen as a result of extrusion at different shear levels.