The influence of screw configuration on the in vitro digestibility and protein solubility of soybean and rapeseed meals

Abstract

Toasted soybean meal (TSBM), untoasted soybean meal (USBM), and toasted rapeseed meal (TRSM) have been extruded in a single-screw extruder at various shear and mixing levels. Extrusion experiments were performed with different torpedo elements and twin lead slotted screws. Protein dispersibility index (PDI), nitrogen solubility index (NSI) in diluted potassium hydroxide, and in vitro digestibility, measured with the pH-STAT method, have been used as parameter to characterise the extrudates. It followed that PDI was not a suitable parameter to differentiate between extrusion conditions. The NSI was shown to be a better indicator to evaluate the effect on protein solubility with various torpedo elements. Extrusion significantly increased the in vitro digestibility of all extruded samples. However, even higher values were obtained if TSBM and USBM were extruded with a torpedo mixing element provided with four rows of flights. Longer torpedo elements, however, resulted in decreasing in vitro digestibilities. In practice, twin lead slotted screws are commonly used. Extrusion of TSBM and TRSM with these screws resulted in an increasing in vitro digestibility. In these cases no optimum was noticed. NSI values remained unaffected as a result of extrusion with twin lead slotted screws.