THE INFLUENCE OF SCREW CONFIGURATION ON THE IN-VITRO DIGESTIBILITY AND PROTEIN SOLUBILITY OF SOYBEAN AND RAPESEED MEALS

Toasted soybean meal (TSBM), untoasted soybean meal (USBM), and toaste d rapeseed meal (TRSM) have been extruded in a single-screw extruder a t various shear and mixing levels. Extrusion experiments were performe d with different torpedo elements and twin lead slotted screws. Protei n dispersibility index (PDI), nitrogen solubility index (NSI) in dilut ed potassium hydroxide, and in vitro digestibility, measured with the pH-STAT method, have been used as parameter to characterise the extrud ates. It followed that PDI was not a suitable parameter to differentia te between extrusion conditions. The NSI was shown to be a better indi cator to evaluate the effect on protein solubility with various torped o elements. Extrusion significantly increased the in vitro digestibili ty 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 screw s resulted in an increasing in vitro digestibility. In these cases no optimum was noticed NSI values remained unaffected as a result of extr usion with twin lead slotted screws.