Screw configuration effects during twin-screw extrusion of starchy and proteinaceous materials

Abstract

This study investigated the effects of screw configuration and feed composition during extrusion of starchy and proteinaceous materials. All experiments were carried out in a twin-screw extruder with a length/diameter ratio of 32:1. The screw speed, feed flow rate, and moisture content were 400 rpm, 12 kg/h, and 15%, respectively. Kneading block (KB) and reverse screw elements (RSE) were placed at different locations in the 200 mm experimental zone near the die, where the temperature was maintained at $\rm 150\sp\circ C.$ An on-line method for measurement of residence time distribution (RTD) in a food extruder was developed, tested, and validated. The technique was based on the electrical conductivity of the material in the die, which was altered by addition of an electrolyte tracer at the feed inlet. The change in current flow was measured as a proportional voltage response across a resistor. The on-line method correlated well with the established erythrosine dye method and precisely determined the effects of screw speed, feed flow rate, screw configuration on RTD. The effects of type, length and position of mixing elements and spacing between two elements on energy inputs, RTD, molecular changes of starch, and macroscopic extrudate characteristics were compared. The results showed that the specific mechanical energy (SME), mean residence time, and extent of starch breakdown were higher for screw profiles with RSE than that with KB. These parameters also increased with longer mixing elements, increased distance of the elements from the die, and with increased spacing between two elements. Specific thermal energy (STE) input showed opposite trend to that of SME. Die temperature was highest when the elements were placed at 0 mm from the die. Such a screw profile produced an extrudate with highest overall expansion and lowest apparent density. Radial expansion was highest with KB in the screw profile than with RSE. KB seemed to be the element of choice for maximizing radial expansion. Increasing mixing element length or spacing between two elements decreased product expansion. Hardness of the product decreased linearly with increasing radial expansion as shown by the breaking strength data. Changing feed composition by adding Arrowtooth flounder muscle decreased the SME input, increased STE and mean residence time. Hydrolysis changed the properties of Arrowtooth flounder muscle so much that it enhanced the expansion characteristics of starch in rice flour and improved extrudate texture.