Cowpea flour, whey protein fortification of rice starches: Effects on antioxidant and starch digestibility and starch pasting properties : A dissertation submitted in partial fulfilment of the requirements for the Degree of Master of Science in Food Innovation at Lincoln University

Abstract

Rice contains more starch, less protein and dietary fibre compared with other cereal. Cowpea is one of the important legumes with high nutrition content. It is rich in proteins, complex carbohydrates, dietary fibres, bioactive compounds, vitamins and minerals. Generally, rice flour has a high glycemic index (GI), while legume flour is considered as low GI food due to the high dietary fiber and slowly digestible starch content. Therefore, it is an excellent way to improve the nutrition of the rice starch product and manipulate the starch digestibility by incorporating protein and legume flour (such as cowpea flour) to rice flour. However, the fortification of protein and legume flour also might affect the pasting property of the blended flour due to the synergistic effect of protein, starch and dietary fiber. The objective of this study is to examine the effect of incorporating both legume flour (cowpea flour) and whey protein to rice flour on the antioxidant properties, pasting attributes and starch digestibility of the blended flour composed of different ratio of cowpea flour, whey protein concentrate (WPC) and rice flour. Five formulations were studied. There is a significant positive correlation between mean total phenolic content (TPC) of the samples and the proportion of cowpea flour incorporated (P≤0.05). Also, there is a significant positive correlation between TPC with ABTS radical scavenging capacity (P≤0.05) of the samples. According to the analysis of RVA results, the addition of cowpea flour and whey protein has a significant effect on the pasting properties of the blended flour. The peak, breakdown and final viscosity of samples decreased gradually with the increasing proportion of cowpea four and whey protein concentrate. However, according to ANOVA analysis and Tukey’s comparison test of RVA results, the peak viscosity of Formulation 1 to formulation 3 and cowpea flour, rice flour is significantly different (P≤0.05) while there is no significant difference between Formulation4, 5 samples and cowpea flour in peak viscosity(P>0.05). This means the peak viscosity increased significantly by the incorporation of cowpea flour and whey protein at a low level, while the influence on peak viscosity became not significant at high-level addition. Similarly, the breakdown values also did not significantly differ among Formulation 2-5 samples, which means a low concentration of cowpea flour 10% has a significant effect on breakdown viscosity(P≤0.05), while the effect of higher-level incorporation was not significant(P>0.05). The final viscosity differed significantly among all samples (P≤0.05). Based on the in vitro starch digestion analysis, the incorporation of whey protein and cowpea flour affected the starch digestibility of samples. Overall, the amount of reducing sugar released of the samples decreased during in vitro starch digestion with the increased proportion of whey protein and cowpea flour in the formulations due to the decrease in starch and increasing of slowly digestible starch from cowpea flour, and the synergistic effect of protein, starch and dietary fiber. The effect of cowpea flour added in rice flour on the pasting property and starch digestibility needs to be further studied using a higher proportion of cowpea flour.