Effects of Conditioner Steam, Extruder Water and Screw Speed on Physical Properties of DDGS-Based Extrudates in Twin-Screw Extrusion

Abstract

A fractional factorial design with a replicated central composite point was used to investigate the effects of extrusion processing on physical properties of DDGSbased aquafeeds using a twin screw extruder. Extrusion cooking trials were performed using an ingredient blend for Nile tilapia, with 2 levels of screw speed (350 and 450 rpm), 2 levels of extruder water (0.236 and 0.302 kg/min) and 2 levels of conditioner steam (0.1 and 0.15 kg/min). The central point was 400 rpm screw speed, 0.271 kg/min extruder water, and 0.12 kg/min conditioner steam. Effects of these processing conditions on extrudate characteristics were extensively analyzed, and included moisture content, water activity, thermal properties, expansion ratio, unit density, bulk density, color, water stability, sinking velocity, water absorption and solubility indices and pellet durability index. Increasing the screw speed from 350 to 400 rpm resulted in a decrease in mass flow rate by 5.3%; while increasing the screw speed from 400 to 450 rpm led to a 2.8% increase in mass flow rate. Increasing the extruder water and conditioner steam resulted in a 5.3% decrease and nearly 8.6% rise in mass flow rate, respectively. On the other hand, increasing screw speed from 350 to 400 rpm had no significant effect on some of the extrudate properties such as moisture content, bulk density, water absorption index, and pellet durability. However, as screw speed increased from 350 to 400 rpm, water stability and water activity increased by 13 and 58%, respectively. Increasing screw speed from 400 to 450 rpm decreased water stability and water activity significantly. Increasing extruder water from 0.236 to 0.302 kg/min led to a significant increase in water stability, and a decrease in water absorption index, water activity and expansion ratio by 12.5, 13, 21, and 5.5%, respectively. As conditioner steam increased from 0.1 to 0.15, sinking velocity and water absorption index decreased by 25 and 15%, respectively. Increasing conditioner steam from 0.1 to 0.12 kg/min resulted in a 20%, 5.5%, 10% and 3% decrease in moisture content of the products, brightness(L*), water stability and expansion ratio, respectively. It also increased bulk density by 5.8% and unit density by 4.2%. Overall, all trials produced viable extrudates with properties appropriate for Nile tilapia production.