Cooking quality and nutritional properties of extruded maize pasta with orange-fleshed sweet potato

Abstract

Vitamin A deficiency is prevalent in sub-Saharan Africa and is considered a serious problem of public health significance. The use of bio-fortified staple foods is an intervention strategy to address vitamin A deficiency and one example is orange-fleshed sweet potato (OFSP) cultivar with increased beta-carotene content. Replacing white-fleshed potato cultivars with orange-fleshed sweet potato can improve the vitamin A status of school-aged children. Taking this information into consideration, the diversification of diet through inclusion of orange-fleshed sweet potato could be a useful approach for alleviating vitamin A deficiency. This presents an opportunity whereby white maize flour can be composited with orange flesh sweet potato flour to produce gluten-free pasta with enhanced β-carotene content. This product will also be suitable for celiac patients and other consumers who want to abstain from gluten due to some health related issues, and contribute to the RDA requirements for vitamin A for various groups. This study therefore investigates the effect of extrusion processing on cooking quality, β-carotene content and antioxidant properties of gluten free maize – orange-fleshed sweet potato pasta. Maize and orange-fleshed sweet potato composite (100:0, 80:20, 70:30, 50:50 w/w) flours were extruded into pasta using a twin-screw extruder. Raw flours and raw pasta samples were analysed for proximate composition, β-carotene content and in-vitro radical scavenging properties. Cooked pasta samples were analysed for cooking quality (cooking time, cooking loss and water absorption capacity), textural properties (firmness, stickiness and fracturability), in-vitro starch digestibility and in-vitro protein digestibility. Raw and cooked pasta samples were analysed for thermal properties. Gluten free commercial pasta made from corn and rice was used as a reference. Increase in addition of OFSP flour increased the cooking loss and decreased cooking time and water absorption capacity of pasta. There was an increase in insoluble and soluble dietary fibre as the proportion of OFSP flour in the composite decreased. The fibre content in the OFSP flour caused a loosening of the compact structure of the pasta disrupting the compact protein-starch matrix resulting in higher cooking loss and sticky pasta. This also led to lower hydration of starch as well as protein hydration resulting in lower water absorption capacity. However, compared to the commercial pasta which was conventionally made, extruded pasta samples showed indistinct starch granules as observed with scanning electron microscopy (SEM) indicating higher amount of disrupted starch thereby leading to faster absorption of water during cooking resulting in lower optimum cooking time. Extruded pasta samples exhibited lower in-vitro protein digestibility as compared to the commercial pasta that was conventionally made. Higher temperature used during extrusion cooking leads to the disruption of protein structures by unfolding, re-organization and polymerization through the formation of disulphide bonds. Disulphide bonds can result in reduction in protein solubility resulting in lower in-vitro protein digestibility. Extruded pasta samples showed lower in-vitro starch digestibility. Differential scanning calorimetry showed that amylose-lipid complexes were formed which reduced starch digestibility. Increasing proportion of OFSP in the composites increased β-carotene content and antioxidant properties of both raw flour and pasta samples. After extrusion, β-carotene content of pasta samples decreased but antioxidant properties increased. The high temperature and shear rate conditions used during extrusion processing may have caused losses of β-carotene through cis-trans isomerization, fragmentation and oxidative decomposition. The possible formation of Maillard reaction and caramelization products with reducing properties as a result of extrusion could have contributed to the observed increased antioxidant properties of the pasta samples. In conclusion, addition of 20 % OFSP to maize flour produces pasta by extrusion that have similar characteristics in terms of its good cooking quality and textural properties to the commercial pasta and 100 % maize pasta as evidenced by low cooking time, less cooking loss, less stickiness and more firmness and required greatest amount of force to break. The cooking quality of extruded pasta seems to be related to the microstructure and dietary fibre seems to play an important role in contributing to negative cooking qualities. The addition of OFSP flour in cereal-based pasta could potentially meet an appreciable amount of the recommended daily amount of vitamin A for various groups and exerting good antioxidant properties.