Chemical, physical and nutritional changes in soybean meal as a result of toasting and extrusion cooking

The effect of soybean meal extrusion and the development of shear forces during single-screw extrusion was compared with the toasting process of soybean meal. Attention was focused on chemical, physical and nutritional changes during these thermo-mechanical treatments.Monitoring target parameters were tested for their usefulness during processing. It appeared that the nitrogen solubility index in potassium hydroxide, pH-STAT in vitro protein digestibility, trypsin inhibitor activity, differential scanning calorimetry and specific mechanical energy were parameters which can be used to evaluate the protein quality between the different extrusion conditions. On the other hand, the protein dispersibility index and the urease activity were of no use in monitoring the extrusion process.Extrusion significantly increased the in vitro protein digestibility and the apparent ileal digestibility of crude protein in broiler chickens when compared with toasted soybean meal. Also, the feed conversion ratio was significantly improved after extrusion. Thermal studies revealed that toasted soybean meal was only partly denatured, while after extrusion a completely denaturation of the main storage proteins in soybean meal was achieved. This was explained by the fact that during toasting mainly non covalent interactions were involved in protein structure formation, while after low shear extrusion both non covalent and disulfide bonds were broken during extrusion.The development of shear forces during extrusion resulted in a maximum in vitro protein digestibility as well as the apparent digestibility of crude protein (P<0.1). At high shear extrusion, the nutritional value of the soybean meal started to deteriorate again. This was seen in vitro as well as in vivo . Under these high energetic conditions, also covalent cross linking reactions may occur. Also, the amount of soluble non starch polysaccharides and the water holding capacity in the chyme was increased after high shear extrusion, resulting in a significant higher chyme viscosity. It is shown, theoretically and experimentally, that shear forces may have an additional effect on protein denaturation during extrusion cooking at different shear levels.The use of hydrolytic enzyme preparations on toasted and extruded soybean meals resulted in a higher apparent ileal digestibility of crude protein and non starch polysaccharides. An in vitro study showed that after extrusion both b-conglycinin as well as glycinin were rapidly and completely degraded by proteases (Neutrase) when compared with toasted soybean meal. The hemicellulase preparation (Energex) was able to release up to two thirds of the water unextractable solids in extruded soybean meal. However, these results were not translated to a better growth performance in broiler chickens.It was concluded that the trypsin inhibitor activity after extrusion of commercial available toasted soybean meal is only a minor factor in explaining the obtained increased nutritional value of this soybean meal. The development of shear forces may be responsible for an additional inactivation effect of trypsin inhibitors