DEVELOPMENT AND PERFORMANCE EVALUATION OF A SINGLE- SCREW STARCH EXTRUDER

Abstract

Graveyards of unserviceable and broken down machines litter the landscape of many developing countries including Nigeria because of poor adaptation. Also, in Nigeria and other tropical countries, significant amount of locally produced agricultural materials including starch producing crops, are lost both in quality and quantity during the peak seasons due to inadequate storage and processing technology. In this study, a single screw starch extruder was designed, fabricated and tested using locally available materials as a means of arresting post harvest losses of starch based crops in Nigeria. The extruder is the dry type. It consists of a hopper, feeding screw with agitator, extruder screw rotating in a closely fitting barrel with surrounding jacket of variable flow rate for cooling, and variable die all made of stainless steel. The extruder has a capacity of 1.06 t/h, a compression ratio of approximately 4.5: 1 and is powered by a 4kW electric motor. A unit of the machine costs W 470, 390.00.00 as at April 2007. It is self cleaning and requires little maintenance. The extruder was used to investigate the process for screw extrusion of the flour and starch of maize and cassava which are grown in Nigeria in large quantity. These were compared with wheat flour which is commonly used for the production of alimentary pastes. Response Surface Methodology, stepwise regression, correlation and Analysis of Variance were employed to a factorial experiment in completely randomized design. The effect of extrusion variables: feed moisture (25, 30, 40%), extruder temperature (40, 70, 100 0C) built up by varying the duration of sampling and screw speed (100, 150, 200 rpm) on system parameters including product temperature, mean residence time (MRT), torque, specific mechanical energy input (SME) and product quality attributes, that is water absorption index (WAI) and water solubility index (WSI), product moisture, transverse expansion, melt viscosity, cold and hot paste viscosity and strength properties of extrudates were studied. A maximum temperature of 150 °C/30 minutes was attained through viscous dissipation and up to a moisture loss of 55% w.b. An increase in temperature and screw speed increased transverse expansion and WAI up to a critical value of 6.46 gig above which it decreased. The melt viscosity reduced with an increase in moisture content and temperature. The extrudates' moisture loss is directly proportional with expansion. Increasing feed moisture content caused a decrease in transverse expansion and increase in extrudate moisture content. A high temperature process produced extrudates with low moisture content. Cold paste viscosity decreased when moisture decreased and temperature increased. An increase in moisture content of maize starch decreased the transverse expansion and the WSI, but increased the WAI and hot paste viscosity. Also, an increase in moisture content decreased the gelatinization of cassava starch and maize flour than that of wheat flour. SME decreased with increasing moisture content and a lower product temperature. The equations relating the various dependent and independent variables were established to predict the performance of the machine and the quality of the products. Generally, the response surface study revealed the range of the extrusion variables for optimum performance. Quadratic coefficients (0.61- 0.86) fitted the extrusion data very well, better than linear models.