Nutritional, biochemical and physicochemical properties of Pearl millet and moringa oleifera composite food products

Abstract

Pearl millet (Pennisetum glaucum), contains substantial quantities of protein, minerals and vitamins and is widely cultivated in dry regions. Pearl millet (PM) flour is mostly used for making porridge or beverages for all age groups in the underdeveloped areas of Sub-Saharan Africa and Asia. To curb protein-energy malnutrition, pearl millet has been combined with legumes, to produce a more nutrient-balanced product. However, legumes present challenges such as anti-nutritional factors, poor digestibility, and toxic components. Legumes also do not address pearl millet’s documented micronutrient deficiencies. This study sought to improve pearl millet's nutritional, rheological and organoleptic properties by employing malting and fermentation. Moreover, Moringa oleifera leaf powder (MLP) was used as a fortificant to improve the protein content and profile and the overall nutritional quality. MLP was the chosen fortificant as it is high in protein and lysine (which are deficient in pearl millet), accompanied by its exceptional overall nutrient balance. For optimum results in compositing, response surface methodology employing mixture design was applied to find optimal proportions for each of the components that yield a highly desirable protein content and or minimal saturated fat content. Twelve mixtures with varying ratios of fermented pearl millet flour (FPMF), malted pearl millet flour (MPMF) ranging between 30–65%, and MLP ranging between 5–15% were generated through I-Optimal mixture design. The mixtures were wet-cooked, freeze-dried and analysed for protein, fat, and total phenolic content (TPC). Across the twelve mixtures, the following ranges were acquired: protein (7.3–14.2%), total fat (2.7–3.5%), saturated fat (1.3–1.6%), monounsaturated fat (0.8–1.0%), poly-unsaturated (0.8–1.0%) and TPC (129–790 mg.g-1). The data was fitted to a linear mixture model and the search for the optimum done using Numerical Optimisation of Design-Expert (10) for maximising protein and minimising saturated fat. The linear model was suitable for explaining variation for total protein and saturated fat with R2 of 0.50 and 0.51, respectively. Increasing MLP correlated to an increase in protein content. Two final formulations were generated through the optimisation process, (1) 15:30:55 MLP, MPMF and FPMF respectively, with 12.41% protein and 1.49% saturated fat for maximising protein with the desirability of 0.865 and (2) 15:55:30 MLP, MPMF and FPMF respectively, having 11.84% protein and 1.25% saturated fat for maximising protein and minimising saturated fat with the desirability of 0.625. The two final formulations OS1 and OS2 were then blended, wet-cooked and freeze-dried before nutritional, biochemical and physicochemical analysis. The formulations yielded up to 22% protein and lysine content increases with ranges of 12.60–13.51 g/100 g and 0.45–0.55 g/100 g, respectively. Optimisation yielded up to a 13% reduction in saturated fat content with a resultant range of 3.89–6.59 g/100 g. Ash content was at 2.93% for both formulations translating to a 75% increase as calcium, iron and magnesium increased by over 1200%, 100% and 50%, respectively. TPC increased by up to 80% consequently effecting increases of over 25% in both oxygen radical absorbance capacity and ferric-reducing antioxidant power. Final viscosity and peak viscosity were reduced by up to 95%, respectively, a trend that translated into increased nutrient density in the cooking of gruels by reducing cooking water demand. The water solubility index increased by over 300%, while water activity decreased by up to 35%, improving reconstitutability and the keeping quality of the formulations, respectively. Overall, fermentation increased the protein content of pearl millet whilst malting improved pasting properties such as final and peak viscosity. Mixture design and numerical optimisation were effective in determining the optimum recipe for maximising protein and or minimising saturated fat. Compositing with MLP yielded improvement in protein and mineral content, as well as phytochemical content.