Common bean (Phaseolus vulgaris L.) extrusion cooking: process optimization and product evaluation
Common beans, especially biofortified beans, have potential to contribute to nutrition improvement among populations. Value addition to common beans may lead to increased consumption, thereby enhancing their contribution to nutrient intake. The study was undertaken to optimize the extrusion cooking process for improved value addition and utilization of common beans. The physicochemical properties of common beans (Bishaz, K131, NABE19, Roba1 and RWR2245) and their relationship to extrudate physical and functional properties (water absorption, water solubility, expansion, bulk density, pasting properties) were determined. Effects of extrusion processing conditions on bean extrudate properties were assessed. In addition, optimal extrusion conditions for desirable and acceptable bean snack extrudate were determined and later validated. Protein, fibre, total ash, iron and zinc contents of beans showed significant (positive or negative) relationship with the extrudate physical and functional properties. Beans with high fibre content produced high density extrudates. Positive relationships were found between dietary fibre and pasting viscosities due to the tendency for fibre to hold water thus increasing viscosity characteristics. Positive relationships between protein and water solubility, and negative relationship with expansion of extrudates that could be explained by the hydrophilic properties of protein and the protein-starch interactions hindering expansion of bean extrudate, respectively, were found. Extrusion feed moisture, extruder die temperature and screw speed had influence on the waters absorption, expansion, bulk density, water solubility, hardness, crunchiness and protein digestibility of extrudates, both independently and interactively. High expansion of extrudates was observed at low feed moisture. Increase in expansion of extrudates resulted in low bulk density for extrudates, which is a desirable characteristic for expanded snacks. The expansion of extrudates at low feed moisture was due to the complete vaporization of moisture in the starch cells. High water solubility of extrudates observed at low feed moisture was explained by protein denaturation, starch gelatinization and molecular degradation. Hard extrudates at high feed moisture were attributed to limited expansion of extrudates. High die temperature, high screw speed and low feed moisture produced soft and crunchy extrudates, possibly due to the increased shear and pressure in the extruder causing full expansion of starch. It is likely that protein-starch interactions and the formation of numerous small-sized air cells resulted in improved texture for the bean extrudates. The study revealed that optimal extrusion conditions that could produce soft crunchy high expanded bean extrudates with high protein digestibility and desirable consumer attributes were low feed moisture (15 %), high die temperature (142 °C) and high screw speed (45 Hz). Acceptable protein-rich composite extruded snack formulations with high bean content (82.03 %) and optimal desirability of 0.97 were produced. The snack extrudates exhibited high protein digestibility (81.3 %). Organoleptic evaluation of the snack extrudates revealed moderate taste, flavour and overall acceptability (scores > 5) on a hedonic rating of 1 to 9. The optimal product obtained could provide at least 20 g protein, 4.8 mg iron and 1.5 mg zinc per 100 g serving, and thus a nutrient contribution of at least 107.3 % (for protein, 48 % for iron and 30 % for zinc of the recommended daily nutrient intake for children aged 6 to 12 years. Results from this study show that production of nutritious and acceptable bean-based snack foods with high protein, fibre, iron and zinc content is possible through extrusion cooking. Ingredient complementation with beans can strategically be applied in the development of nutritive extruded snacks to boost nutritional quality and sensory properties, and subsequently improve nutrient intake by consumers. The findings are important for application in the food industry.