Enhancement of total phenolics and antioxidant properties of some tropical green leafy vegetables by steam cooking

This study sought to evaluate the effect of steam cooking on the vitamin C, total phenolics, total flavonoid and antioxidant properties (1,1‐diphenyl‐2‐picrylhydrazyl [DPPH] free radical and 2,2‐azinobis(3‐ethylbenzo‐thiazoline‐6‐sulfonate) [ABTS*] radical scavenging abilities, reducing property and Fe2+ chelating ability) of some tropical green leafy vegetables: Talinum triangulare, Ocimum gratissimum, Amaranthus hybridus, Telfairia occidentalis, Ipomea batata, Cnidoscolus aconitifolius, Baselia alba and Senecio biafrae leaves. The results of the study revealed that cooking causes a significant (P < 0.05) decrease in the vitamin C (raw [321.4–842.0], cooked [198.2–638.4 mg/100 g]) content. Conversely, there was a significant (P < 0.05) increase in the total phenol (raw [146.9–693.8], cooked [272.9–1037.5 mg/100 g]), total flavonoid (raw [8.2–53.0], cooked [12.9–57.4 mg/100 g]), DPPH radical scavenging ability (raw [15.7–61.8], cooked [52.8–92.7%]), reducing property (raw: 28.3–61.8; cooked: 43.9–71.6 mg/100 g ascorbic acid equivalent [AAE]), Fe2+ chelating ability (raw [17.4–75.4], cooked [22.8–89.2%]) and ABTS* scavenging ability (raw [17.4–87.3], cooked [57.5–113.2 mmol/100 g trolox equivalent antioxidant capacity]). In view of this, it could be concluded that steam cooking decreases the vitamin C content of all the vegetables, while it increased the phenolic content and antioxidant activities. PRACTICAL APPLICATIONS: Consumption of fruits and vegetables has been known to be a practical approach toward the prevention and management of several degenerative diseases. Most green leafy vegetables are not eaten in their unprocessed form but are rather cooked by steaming before consumption in tropical Africa. It is well reported that vitamin C is lost during the cooking of vegetables, but recently, phenolics have been shown to be the main phytochemicals present in foods and that they play crucial roles in the management of several degenerative diseases. Cooking is an indispensable prerequisite in order to obtain safe and high‐quality food products. Cooked vegetables have much better hygienic quality, and due to chemical reactions during cooking, they become more digestible and have an increased nutritional value. However, steam cooking may affect the antioxidant status of tropical green leafy vegetables due to the release of more phenolic compounds and destruction or creation of redox‐active metabolites.