A numerical simulation of the extrusion cooking of food materials such as low moisture corn starch, in a single screw extruder is carried out. An effort is made to quantify the degree of conversion, or gelatinization, along the screw channel and, in particular, that of the final product at the die. The mathematical model is based on the transport equations for mass, momentum, and energy, along with equations for chemical kinetics for conversion of the material. The materials considered are non-Newtonian with strongly temperature and moisture dependent viscosity. The conversion process is studied only in the heated, rheological fluid region where the thermally controlled reaction rate is well characterized as a zeroth order reaction rate. The temperature and velocity fields are also computed in this region to indicate the nature of the underlying transport processes. The location where the solid food material converts into a fluid with a continuum viscosity behavior is obtained from the model and compared with the available experimental data, indicating a fair agreement. The temperature and velocity variation in the extruder also follow the expected trends. It is observed that most of conversion occurs in the heated region over the last few turns of the screw. Furthermore, the results show that the degree of conversion depends on the length of the solid conveying zone and the transition region. Therefore, an accurate model for these regions is crucial in the quantification of the conversion process to the final product with the desired qualities.