Effect of Extrusion Cooking Variables on the Torque and Specific Mechanical Energy of a Single Screw Extruder

Abstract

Aims: The operation of a single screw extruder under the influence of locally available raw materials (African yam bean (AYB) and cassava products were determined using the measures of torque and specific mechanical energy of the extruder. The specific mechanical energy (SME) is a good quantitative description in extrusion processes, since it allows the direct comparison of different combinations such as screw speed, feed rate and torque, while torque is directly correlated with power usage. Study Design: A three- factor five level response surface methodology central composite design (CCD) was adopted to study the effect of feed moisture content (MC), feed composition (FC) and the screw speed (SS) on the energy efficiency of a single screw extruder using torque and specific mechanical energy (SME). Place and Duration of Study: The raw materials for the work were obtained from Abia State Nigeria. The extrusion work was done at the Federal polytechnic Mubi Adamawa State Nigeria, while the statistical analysis was done at Michael Okpara University of Agriculture, Umudike Abia State Nigeria. Methodology: African yam bean (AYB), cassava tubers and Ighu (pre-gelatinized cassava chips or slices) were processed into flours. The flours were sieved to pass 0.75mm laboratory mesh. The moisture content of the flours were determined. AYB flour was added to cassava flour at ratios of 0.54:99.46, 20:80, 50:50, 80:20 and 99.46:0.54. Also AYB was added to ighu flour in the same ratio as cassava flour. The moisture levels of the blends were adjusted by adding a pre-determined amount of water using material balance equation to yield (13.3, 16, 20, 24 and 26.7)% moisture content. The flour samples were then mixed and transferred into polythene bags and allowed to equilibrate for 12 hours at room temperature. Extrusion was carried out using a Brabender laboratory single screw extruder (Duisburg DCE -330 model). The screw speed was adjusted from 109.95 to 125,150,175 and 190.05rpm.Twenty extrusion runs were conducted with the cassava/AYB flour blends and another twenty extrusion runs were carried out for Ighu/AYB flour blends. Torque (T) was determined by reading directly from the extruder during extrusion run, while the specific mechanical energy was calculated from the data of torque and volumetric flow rate. Data obtained were subjected to response surface analysis (RSA) and analysis of variance (ANOVA). Results: Regression analysis for extruded blends of Ighu/AYB showed that specific mechanical energy (SME) had significant (p <0.05) linear, quadratic and interactive effects on the independent variables. Reduction in the moisture content of the feed materials resulted in an increase in the SME of the extruder. The result showed that the lowest torque occurred at the lowest moisture content and feed composition of AYB. The response surface plots showed that the maximum SME occurred at the critical values of screw speed and feed composition of 192.05 rpm and 0.54% ighu/ 99.46% AYB respectively. The responses obtained with cassava/AYB flour varied from the ones obtained using ighu/AYB blends, signifying that the feed composition had effect on the operation of the single screw extruder. Conclusion: Extrusion variables influenced the torque and specific mechanical energy of the single screw extruder. The energy efficiency of the extruder was affected by the operating variables. The independent variables must be considered in the design of an experiment.