Background studies in the modeling of extrusion cooking processes for soy flour doughs

Soy flour is processed in single screw extruders to yield textured vegetable protein used as meat extenders and replacements. The fundamental processes which take place in extrusion cooking of soy doughs are poorly understood from an engineering point of view. This paper is concerned with gaining an understanding of extrusion cooking in order to develop a quantitative model of this process in single screw extruders. Rheological and thermodynamic data are obtained over a range of conditions found in the extrusion process and this data is used both in understanding and modeling the extrusion cooking process.In particular, differential scanning calorimetry (DSC) has been used to determine changes in the enthalpy of soy doughs at various moisture levels. It is observed that in general the enthalpy changes are small (e.g. of the order of 1.0 cal/g) and endothermic. However, if the dough is subjected to shear and thermal history, then the enthalpy changes become significant. The viscosity of the dough exhibits a mild increase in the temperature range where the endotherms are observed in the DSC data. Based on the results of this study, it is concluded that cooking does not involve cross-linking of the proteins but more likely a conformation change of the molecules. Also, it is found that for the 40% to 60% added moisture soy flour dough systems, the rheological properties can be modeled using a Bingham model modified with a shear rate dependent viscosity.