Residence Time Distribution of Soy Protein Isolate and Corn Flour Feed Mix in a Twin-Screw Extruder

Abstract

The residence time distribution (RTD) of soy protein isolate (SPI) and corn flour mixture through a twin-screw extruder under different conditions was evaluated. A full-factorial design of experiments was used to select the variables at different levels. The feed mixture was prepared by blending corn flour and SPI in the ratio of 4:1 (20% SPI content). The effect of screw speed (75, 100 and 125 rpm), feed mixture moisture content (25, 30 and 35%) and die diameter (3 and 5 mm) were investigated. All factors were found to have a significant (P < 0.05) effect on the mean resident time and its variance (square of standard deviation). Higher screw speed, higher initial moisture content and larger die diameter resulted in a shorter mean residence time. Two conventional flow models were used to represent the RTD profile in the extruder – the frequency model (F-distribution) and the cumulative RTD model (E distribution). The parameters of these models – the half-concentration internal age and particle accumulation rate – were determined by a nonlinear regression. The parameters of these models were responsive and related to process variables and both F- and E-distributions were well predicted. Practical Applications Residence time distribution (RTD) is an important parameter that describes the time spent by particles through any continuous processing system. RTD study is important with respect to extrusion processing because the properties of extruded product depend on the shear, pressure, time and temperature history of the product in the extruder. The study is aimed at evaluating RTD of a protein-rich formulation obtained by different blends of soy protein isolates and corn flour which are prepared for producing protein-rich soy-based products. The study is important because it examines the influence of selected process and product variables on the residence time of particles in the twin-screw extrusion system.