Phytochemistry, Antioxidant Activity and Heavy Metal Content on Precooked and Raw of Celosia argentea L., Launaea taraxacifolia Wild. and Ocimum gratissimum L., Three Leafy Vegetables Eaten in Southern Benin

This study aimed to estimate and compare the antioxidant potential as well as the levels of heavy metal contamination of Celosia argentea (Amaranthaceae), Launaea taraxacifolia (Asteracea) and Ocimum gratissimum (Lamiaceae) collected in four municipalities of South-Benin by taking into accounts the treatment prior to consumption (precooked and raw). A low extraction yield was obtained during the extraction of leafy vegetables subjected to precooking. The phytochemical screening of Launaea taraxacifolia revealed the presence of triterpenes, steroids, flavonoids and tannins. The phytochemical analysis of Ocimum gratissimum revealed the presence of triterpenes, the steroids, tannins, anthraquinones, coumarins and alkaloids. The analysis of Celosia argentea revealed the presence of triterpenes, steroids, flavonoids, tannins, anthraquinones and alkaloids. The antioxidant activity of the various extracts was found to be dose-dependent. The extracts showed significant antioxidant activities with IC50 ranging from 4.03 to 11.42 µg/ml. Ocimum gratissimum had the strongest antioxidant power followed by Celosia argentea and Launaea taraxacifolia respectively. Pre-cooking of leafy vegetables would have an effect on antioxidant activity, which resulted in a slight increase in the DPPH inhibition power of pre-cooked vegetables compared to raw one. The analysis of heavy metals showed the presence of arsenic, mercury and manganese in the different plant extracts. The highest levels were observed in leafy vegetables collect at Cotonou (Houeyiho) followed by those from Sèmè-Podji but all below the thresholds defined by the World Health Organization. The precooking promoted a decrease of the quantity in heavy metals by leaching, causing low concentration in metals of vegetables precooked in contrast with raw ones.