Nutritional composition, antioxidant activity and bioaccessibility of phenolic compounds in selected wild cereal and pseudo-cereal grains found in Zimbabwe
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The nutritional composition of five wild and two domesticated cereal grains was determined using standard analytical methods. It was hypothesised that wild cereal and pseudo cereal grains found in Zimbabwe contained macro and micronutrients which are beneficial to humans.The wild cereal grains that were used in the study were Brachiaria brizantha, Panicum maximum, Rottboellia cochinchinensis, Sorghum arundinaceum and Amaranthus hybridus, a pseudo cereal was studied. The domesticated cereal grains used were Eleusine corocana and a red variety of Sorghum bicolor. Samples were collected from fields in Harare and some in Buhera, a district in Manicaland province of Zimbabwe. Phenolic compounds were extracted from the cereal grains and were quantified. The phenolics were characterised, the antioxidant properties studied and the bioaccessibility of the phenolic compounds was determined using a gastrointestinal model system assay. Macronutrients determined were proteins, carbohydrates and fats. Minerals were determined as well. A. hybridus, a wild pseudo-cereal, had the highest protein and fat content of 21.44 ± 0.05 % and 11.50 ± 0.03 % respectively, compared to all the other cereal grains. B. brizantha had the highest fibre content of 30.43 ± 0.01 % while the red variety of S. bicolor had the least fibre content of 2.51 ± 0.07 %. The values were comparable to those reported elsewhere for traditional cereal grains. Phosphorus was detected in all cereal grains studied. Calcium was detected in all cereal grains except in red variety of S. bicolor. The mineral values obtained were lower than those reported elsewhere for traditional cereal grains like wheat. E. corocana and S. arundinaceum had significantly higher total phenolic compounds than all other cereal grains studied. S. arundinaceum had the highest concentration of total flavanoids while A. hybridus had the lowest. The highest amounts of proanthocyanidins were determined in S. arundinaceum with 12.2 ± 0.08 % followed by S. bicolor with 4.6 ± 0.03 % proanthocyanidins content. The HPLC method was used to tentatively identify the constituent phenolic compounds in the cereal grain extracts. Ferulic acid and p-coumaric acid were detected in all cereal samples. Caffeic acid, catechin, gallic acid, p- hydroxybenzoic acid, p-hydroxybenzaldehyde, protocatechuic acid, qeucertin, syringic acid and vanillic acid were detected but were not common to all the samples. The ability of the cereal grains to quench the DPPH radical was assayed. E. corocana and S. arundinaceum had the highest ability and statistically, there was a positive correlation between the concentration of phenolic compounds and the ability to scavenge for DPPH radicals. The ability of extracts to reduce ferrous ions increased as the amount of extract added was increased. Cereal grain extracts were found to delay/halt lipid peroxidation and the extracts of all the cereal grains were also found to prevent the bleaching of β-carotene to varying extents. The bioaccessibility of phenolic compounds was generally high for all cereal grains. A. hybridus had the highest intestinal bioaccessibility percentage of 95.4 ± 0.01 % while the cereal with the lowest intestinal bioaccessibility was R. cochinchinensis with 81.85 ± 0.03 %. The research work demonstrated the importance of wild cereal and pseudo cereal grains as a potential source of nutrition and industrially utilisable natural products.