Relationships between Extrusion Conditions and System Parameters of Extrusion Cooking of Cassava and Soybean Blends : Application of Response Surface Analysis

Blends of cassava flour and partially defatted soybean meal were processed in a singlescrew extruder. Experimental design with feed moisture (16, 20, 24 gwater/100g flour), amount of soybean (10, 20, 30 gsoybean/100g flour) and barrel temperature (120, 145, 170 o C) as independent variables leading to 17 combinations that were studied using Box-Behnken Design of response surface methodology to investigate the effect of these input variables on extruder system parameters, namely: product temperature, residence time, machine throughput and specific mechanical energy. The recorded values for all responses varied from 121 to 175 o C, 42.34 to 65.11 seconds, 3.65 to 4.56 kg/hr and 159.01 to 213.63 kJ/kg, respectively. Second-order polynomials were used to model the extruder responses as a function of process variables. All three variables affected responses significantly especially their linear terms (p<0.05) and all the fitted models were significant (p<0.05) and correlated well with experimental data (R 2  0.934). © 2019 Elixir All rights reserved. Elixir Food Science 134 (2019) 53628-53634 Food Science Available online at www.elixirpublishers. com (Elixir International Journal) Abioye, A.O et al./ Elixir Food Science 134 (2019) 53628-53634 53629 throughput (MT) and specific mechanical energy (SME) using RSM. 2. Materials and Methods 2.1 Materials 12-month old cassava roots (Manihot esculentu Crantz) of local variety Okoyawo as popularly called among farmers and processors in Ogbomoso, Nigeria and its environs were processed with modifications into flour (CF) within 24 hours after harvest according to Badrie and Mellowes [21]. Soybean seeds were processed into partially defatted soybean flour (PDSF) as described by Abioye et al. [6]. The proximate composition of the CF and PDSF were determined by procedures of AOAC [22]. Total carbohydrate was determined by difference. All chemicals were of analytical grade. 2.2 Extrusion process Extrusion was performed on a low-capacity single-screw extrusion facility (Nigerian Design) with the following specifications [6]: The screw was of increasing root diameter and constant pitch with a compression ratio (channel depth in feed zone to that of metering zone) of approximately 2.2:1. The barrel diameter and its length to diameter ratio (L/D) were 32 mm and 15:1, respectively and a die opening of 5 mm with L/D ratio of approximately 2:1. The extruder barrel consisted of 4 zones, with last two zones heated by band electric heaters of 500-W each. The zone at the feed end was cooled by water while the other zone was neither cooled nor heated. The metering section was electrically heated to temperatures according to the experimental design (Table 1) using a temperature control system. A vertical volumetric screw feeder was improvised to force-feed the extruder at approximately 250 cm 3 /min corresponding to speed of 150 rpm. Table 1. Process variables and their levels Process variable Symbol Level Original Coded -1 0 +1 Feed moisture (gwater/100g) FM x1 16 20 24 Amount of soybean (gsoya/100g) AS x2 10 20 30 Barrel temperature (C) BT x3 120 145 170 2.3 Experimental design and statistical analysis The independent variables studied were feed moisture, FM (x1) in gwater/100g, amount of soybean, AS (x2) in gsoybean/100 g and barrel temperature, FM (x3) in o C. The dependent variables or responses were product temperature, PT (YI) in o C; residence time, RT (Y2) in s; machine throughput, MT (Y3) in kg/h and specific mechanical energy, SME (Y4) in kJ/kg were determined on the cassava soybean extrudates. The experimental data were analyzed using multiple regression analysis using Design Expert 6.0 [23] to fit a second order polynomial model expressed in coded variables:             