Nutritional and functional properties of porridges from extrusion cooked cassava-soy composite with wheat bran or grape pomace

Abstract

Diet related non-communicable diseases such as obesity and type-2 diabetes are on the rise in sub-Saharan Africa. The rise is related to an increasing demand for highly refined and convenience-type foods with a decline in consumption of whole grains and high fibre foods. Dietary fibre can be incorporated into locally available food crops using extrusion cooking to produce instant food products with improved nutritional and functional properties. This study therefore investigated the effect of extrusion cooking and wheat bran or grape pomace addition on the nutritional, functional and sensory properties of a cassava- soy composite. Grape pomace or wheat bran was added at 0, 10 and 20% levels to a cassava- soy composite and extrusion cooked. The composites with and without grape pomace or wheat bran were analysed for proximate composition, water absorption and solubility index, in-vitro starch digestibility (IVSD), soluble and insoluble dietary fibre content, in-vitro protein digestibility (IVPD), nitrogen solubility index, flow properties, total phenolic content and, anti-oxidant property. Descriptive sensory properties, oral processing characteristics and satiety of porridges with and without wheat bran were determined. Extrusion cooking led to a decrease in IVPD and nitrogen solubility index of all composites. The high temperature in the extruder may have facilitated formation of covalent and non-covalent interactions between soy proteins and other food components which led to reduction in nitrogen solubility index and IVPD. Extrusion cooking led to an increase in starch digestibility of the composites compared to unextruded samples. The addition of either grape pomace or wheat bran lowered starch digestibility and estimated glycaemic index of composites. The high viscosity of composites with wheat bran may be responsible for the decrease in starch digestibility observed. High viscosity may retard enzyme diffusion to substrate for digestion and slow down the release and transit of hydrolysis products towards the absorptive surface of the mucosa. The phenolics present in grape pomace may have prevented the formation of an enzyme-substrate complex thus, lowering starch digestibility of composites which contained grape pomace. There was an increase in soluble dietary fibre content and a decrease in insoluble dietary fibre content of all composites after extrusion cooking. High shear in the extruder may have facilitated fragmentation and depolymerization of insoluble dietary fibre. All porridges showed shear thinning behaviour. Composites with 20% wheat bran had a higher apparent viscosity compared to the other composites. This may be due to solubilization of insoluble dietary fibre during extrusion cooking and this could be related to the decrease in IVSD and estimated glycaemic index observed in this composite. In contrast, lower apparent viscosity was observed in the composites with grape pomace upon extrusion cooking. It is possible that despite the increased solubilization of insoluble dietary fibre after extrusion cooking, the molecular weight of the soluble dietary fibre portion was low and could not promote viscosity. Although, grape pomace addition to cassava-soy composite increased the total phenolic content and anti-oxidant activity of composites, extrusion cooking decreased the total phenolic content and anti-oxidant activity of composites. Sensory attributes such as visually perceived viscosity, presence of particles, coarseness and thickness were strongly perceived in the composite with 20% wheat bran. The addition of wheat bran reduced the cassava aroma and glossiness of composite porridges. The composite porridge with 20% wheat bran was eaten with a higher number of bites, required a longer oral processing time and total meal duration compared with porridge without wheat bran. The subjective satiety responses post ingestion of the porridges also showed the composite porridge with 20% wheat bran led to greater reduction in hunger, an increase in fullness, a decreasing desire to eat and lower prospect to consume another meal compared to the other porridges. In conclusion, the results from this research demonstrate that extruded instant composites of cassava-soy flour with grape pomace or wheat bran have lower starch digestibility and estimated glycaemic index compared to composites without wheat bran or grape pomace. Changes in the functional properties of dietary fibre during extrusion cooking is largely responsible for the improved nutritional and sensory properties of extrudates. Thus, grape pomace or wheat bran have great potential to be incorporated as dietary fibre sources in starch-rich foods with the use of extrusion cooking to produce instant food products suitable for the management of diet-related diseases such as type-2 diabetes.