Nutritional content, phenolic compounds composition and antioxidant activities of selected indigenous vegetables of Zimbabwe

Abstract

Foods of plant origin may contain many phytochemical compounds such as phenolic compounds in addition to commonly identified nutrients. Interest in phenolic compounds has greatly increased recently because these phytochemicals have been implicated in suppressing rates of degenerative processes such as cardiovascular disorders and cancer. The pharmacological properties of the plants may be related to their antioxidant activities and hence there was need to investigate the antioxidant potential of some indigenous vegetables in Zimbabwe. The focus of this study was to investigate the indigenous vegetables consumed in Buhera district of Zimbabwe, their nutritional content, phenolic compounds composition and antioxidant activities and the effects of processing on the nutritional composition, phenolic content and antioxidant activities. The commonly consumed vegetables, Amaranthus hybridus, Cleome gynandra, Bidens pilosa, Corchorus olitorius, Adansonia digitata and two exotic vegetables i.e. lettuce (Lactuva sativa) and rape (Brassica napus) were analyzed for their nutritional content, phenolic compounds composition and antioxidant activities. A questionnaire was used to gather information on the preparation, preservation methods and medicinal uses of the indigenous vegetables. Phenolic compounds were extracted from the vegetables using 50% aqueous methanol. Proximate and micronutrient analysis were done using standard analytical methods. Phenolic concentrations were determined using the vanillin, butanol, tannin binding and Folin Ciocalteau assays. The antioxidant potentials of the extracts were determined using 1, 1-diphenyl-2 picrylhydrazyl radical (DPPH), reducing power, β-carotene and inhibition of phospholipid peroxidation methods. The identification of phenolics acids and flavonoids was done using reversed phase HPLC. People in Buhera district had knowledge on a variety of edible indigenous vegetables. The mentioned vegetables are used as relish and almost all respondents came up with some medicinal uses of the indigenous vegetables. Drying was found to be the common method of preserving the vegetables. Protein content was the highest in Cleome gynandra (6.0 g/100 g) and the lowest in Adansonia digitata (4.2 g/100 g). All the leafy indigenous vegetables were found to be poor sources of carbohydrate and fat, which ranged between 8.7 to 18.0 g/100 g and 0.3 to 0.5 g/100 g respectively. The insoluble dietary fibre in the vegetables was in the range of 1.6 to 4.2 g/100 g. The Fe content of the indigenous vegetables ranged from 8.7 mg/100 g for Corchorus olitorius to 23.0 mg/100 g for Adansonia digitata. The Zn content of the vegetables ranged from 2.9 mg/100 g for Cleome gynandra to 22.0 mg/100 g for Bidens pilosa. The Cu content of the vegetables ranged from 1.8 mg/100 g for Corchorus olitorius to 23.7 mg/100 g for Adansonia digitata. The Vitamin C content varied from 18.0 mg/100 g for Cleome gynandra to 78.0 mg/100 g for Corchorus olitorius. Potassium content was the highest in Adansonia digitata (1090.0 mg/100 g) and lowest in Cleome gynandra (129.0 mg/100 g). Calcium values of the plants ranged from 120.0 mg/100 g in Cleome gynandra to 798.0 mg/100 g in Amaranthus hybridus. Corchorus olitorius had the highest P content (623.0 mg/100 g) and Adansonia digitata the lowest (14.0 mg/100 g). The total content of phenolics varied from 4.9 mg/g in lettuce to 57.5 mg/g in B. pilosa. The contents of flavonoids varied from 1.2 mg/g for C. gynandra to 8.0 mg/g for B. pilosa. The levels of proanthocyanidins were ranging from 1.9 mg/g for lettuce to 11.2 mg/g for Bidens pilosa. The tannin contents of the vegetables ranged from 5.7 mg/g for C. gynandra to 8.3 mg/g for Bidens pilosa. Bidens pilosa had the highest DPPH radical scavenging activity while Amaranthus hybridus had the lowest. All the vegetable extracts exhibited higher capacity in reducing ferric ions. Generally the extracts had the ability to quench peroxyl radicals formed in the β-carotene assay. The extracts showed strong activity in the phospholipid peroxidation assay. Common phenolic acids of the analysed vegetables were gallic acid and protochatechuic acid. Cooking caused significant changes in the nutritional and phenolic compounds composition of the vegetables. The present study showed that the ii vegetables are valuable sources of nutrients and phenolic compounds as compared to exotic species.