Relationship between pH and antioxidant capacity of selected locally available vegetables

In recent times, there have been controversies on the consumption of alkaline foods; with some authors suggesting that consistent consumption of acidic foods may lead to long-term health challenges, while consumption of alkaline foods will support health and help the body to remove the stress of an acidic lifestyle. Fruits and vegetables have been proposed to be associated with a greater degree of alkalinity. Thus, there is a need to determine the antioxidant capacities of selected vegetables with a particular focus on understanding how their pH influences antioxidant activity. This study evaluated 15 locally available vegetables: Fluted pumpkin leaves (Ugwu), Jute mallow (Ewedu), Bitter leaf, African basil (Scent leaf), Amaranthus, Cabbage, Lettuce, Okro, Cucumber, Cayenne pepper (Sombo), Habanero pepper (Rodo), Bell pepper (Tatase), Tomato, Carrot and Spring onions. The pH of the fresh and freeze-dried samples, Phytochemical analysis (Total phenolics and Ascorbic acid content) and Antioxidant Analysis-1, 1-diphenyl-2- picryl-hydrazil (DPPH) radical scavenging activity, Ferric reducing antioxidant power (FRAP) assay, Nitric Oxide (NO) scavenging activity and Total antioxidant activity (TAC)-were determined. Correlation analysis was done to establish relationships between pH, phytochemical constituents and antioxidant capacities of the selected vegetables. Results revealed that most of the fresh and freeze-dried vegetables had pH less than or greater than 7 respectively. The vegetables with higher concentrations of phytochemicals-ascorbic acid and total phenolic- had higher antioxidant capacities. While there was no significant correlation between pH and DPPH IC50 there was a significant negative correlation between pH of freeze-dried samples and NO IC50. Additionally, the pH of both fresh and freeze-dried vegetables was significantly negatively correlated with the TAC. This indicated that as the pH of the vegetables increased their antioxidant capacity reduced. Thus, the antioxidant potentials of these vegetables increased with increasing acidity which is contrary to popular belief.