Extrusion of Cereal Based-Legume Blend: Process and Product Characteristics

Abstract

The evolution of extrusion into cereal processing has widened the available assortment of puffed snack foods and ready-to-eat (RTE) breakfast cereal formulations. In producing these nutritious products, cereals can be fortified with pulse proteins, as these legumes are important sources of food proteins and other nutrients. Response Surface Methodology (RSM) was used in investigating the effects of 3 process variables (viz. Cowpea level, groundnut level and feed moisture) on product indices o f extruded rice-cowpea-groundnut and sorghum-cowpea-groundnut blend systems in a modified oil-expeller. The use of the RSM helped in relating product indices by regression equations to describe the interrelations between the input parameters and the product indices. A Central Composite Rotatable design for K=3 examined the combined effect of groundnut (0 - 10%), cowpea (0 - 20%) and feed moisture(14 - 48.01% for rice blends; 12 - 44.06% for sorghum blend) on product indices such as product moisture, expansion ratio, bulk density, protein , fat, ash (minerals content- calcium, iron and phosphorus), crude fibre, water absorption capacity (at both 27°C and 70°C), swelling capacity, and psychrometric colour terms (L,a,b). The extrusion process was carried out at 165°C. Statistical analysis, development of models and response surface plots were performed using Statgraphics statistical package (STSC, version 4.1). The models developed from the data was used to predict some product indices such as product’s moisture, expansion ratio, bulk density, protein, fat, swelling capacity, water absorption capacity, swelling capacity and the psychrometric colour terms (of L ,a, b) of both rice and sorghum extrudates. Their R2 values ranged between 52.03% - 86.49% and 42.43% - 87.83% for rice and sorghum systems respectively. Lack o f fit test showed no significance hence models developed adequately fitted the data. The physical indices of rice and sorghum blend extrudates such as the moisture content (which ranged between 8.43% - 13.67% and 7.90% - 10. 64% for rice blend and sorghum blend respectively), expansion ratio and bulk density, were affected by feed moisture and cowpea levels. Lower levels o f feed moisture and cowpea addition resulted in good expansion, less bulk density and lower extrudate moisture content in both types of blend at the maximum concentration of groundnut (10%). Increasing the levels of cowpea and groundnut also resulted in increasing levels of the protein (15.76% 19.54% for rice blend and 16.18% - 21.24% for sorghum blend); fat (0.76% - 4.52% for rice blend and 2.22% - 5.97% for sorghum blend); crude fibre (0.694% 1.5% for rice blend and 2.264 - 3.568% for sorghum blend); and ash (0.83% - 1.30% for rice blend and 2.03% 3.66% for sorghum blend; thus increased contents of minerals elements such as calcium, iron and phosphorus). The most remarkable observation was the protein content which increased by as much as between 53.81% - 62.74% and 37.58% - 52.45% in the rice-blend and sorghum-blend extrudates respectively when compared to the extrudates from only the cereals. The water absorption capacity and swelling capacity of flours from both blends were most affected by the feed moisture level. Increasing cowpea addition only caused a slight increase and decrease in the water absorption capacity and swelling capacity respectively. The increasing addition of cowpea increased the brightness or lightness of sorghum blend extrudates and the overall (total) colour change; but decreased both the redness and yellowness of sorghum bend extrudates. However, increasing groundnut addition increased the redness and decreased the yellowness in rice blend extrudates. In the end the models developed, showed that extrusion condition optimal to produce puffed or direct expanded extrudates with spongy structure should be at low feed moisture. Thus low feed moisture together with increasing cowpea(up to 20%) and groundnut(up to 10%) likely to produce ready-to-eat puffed snack with enhanced nutrition from ricecowpea- groundnut and sorghum-cowpea-groundnut blend extrudates (as shown in Figs. 3 -12.