Effect of Heat on Antioxidant Activity of Some Tropical Leafy Vegetables

Abstract

The present study examined the effect of different cooking times on total phenolics, total flavonoids, ascorbic acid content and antioxidant properties of seven tropical leafy vegetables: Pterocarpus mildbraedii, Gongronema latifolium, Ocimum gratissimum, Solanecio biafrae, Piper guineense, Celosia argentea and Solanum melongena. The thermal treatments employed were both long simmer (20 minutes at 80^oC) and short simmer (5 minutes at 80^oC). The result show that both short and long simmer results to a significant decrease (P< 0.05) in the ascorbic acid content in all the vegetables. There was a significant increase in the total phenol, total flavonoid, DPPH radical scavenging ability, reducing property, Fe²⁺ chelating ability and hydroxyl radical scavenging ability. In like manner, short simmer treatment enhances the total antioxidant activities of all the vegetables. These results suggest that although thermal treatments enhances the nutritional value of vegetables by increasing the bio-accessible phenolic content and total antioxidant activity, long simmer treatment results in the disruption of the phytochemicals constituents present. The study therefore concludes that short time simmer treatment may result to optimum yield of antioxidant activities in leafy vegetables.

Keywords: Antioxidant activity, Ascorbic acid, Leafy vegetables, Total flavonoids, Total phenolics