Natural antioxidants in fresh and dried vegetables

Abstract

The antioxidant properties of standard phenolic compounds and extracts from plant materials (fresh, freeze-dried, 100°C hot-air-dried, and 200°C hot-air-dried carrot, pumpkin, corn and peas), as well as two commercial extracts (Pycnogenol® and Enzogenol®) were evaluated by measuring free radical scavenging ability (ABTS and DPPH assays), reducing power (FRAP assay), and pro-oxidant property (deoxyribose assay). The derivation of a Trolox equivalent antioxidant capacity (TEAC) value using Trolox as standard antioxidant provides a parameter for the comparison of antioxidant activity among groups of compounds or natural extracts. This concept was applied to ABTS, DPPH and FRAP assays and the results were expressed as TEACABTS, TEACDPPH and TEACFRAP, respectively. For almost all measurements in the present study, the antioxidant activity determined by the different methods followed the order of TEACABTS > TEACFRAP > TEACDPPH. The result of deoxyribose assay confirmed that flavonoids and other phenolic compounds cannot simply be classified as antioxidants, under specific circumstance, they act as pro-oxidants. Among the four fresh plant materials tested, fresh peas exhibited the highest TEAC values (TEACABTS, TEACDPPH, and TEACFRAP values). The Folin-Ciocalteu method was used to measure the total phenolic contents of plant extracts and commercial extracts. There was no significant correlation found between the total phenolic content and antioxidant activity of plant extracts in terms of ABTS and DPPH assays. However, the ferric reducing powers of plant materials are strong related to their total phenolic contents. The results in the present study concluded that the freeze-drying did not significantly change the total phenolic content and antioxidant activities of carrot, corn, pumpkin and peas. The loss of natural antioxidants in heated plant materials might be minimized or compensated by the formation of new antioxidant compounds such as the Maillard reaction products. The comparisons of antioxidant behaviours of phenolic compounds with their chemical structures are examined in terms of TEACABTS, TEACDPPH, TEACFRAP values. The results of deoxyribose assay were confirmed that the pro-oxidant properties of flavonoids and phenolic acids were also related to their chemical structures. Polyphenolic compounds in fresh, freeze-dried, 100°C hot-air-dried, and 200°C hot-air-dried plant materials (carrot, pumpkin, corn, and peas) and two commercial extracts (i.e. Pycnogenol® and Enzogenol®) were analyzed by RP-HPLC on C18-modified silica column. Gradient elution with acetic acid-acetonitrile mixtures gave complete separation of nearly all polyphenolic compounds within 60 minutes at room temperature. The results of HPLC analysis clearly indicated the changes in the composition of phenolic compounds during drying.