Antioxidant activity of butterhead lettuce: evaluation of significant factors affecting antioxidant extraction and quantification

Abstract

This work presents a standardized methodology for both the extraction of antioxidants from butterhead lettuce and the evaluation of the antioxidant capacity through DPPH methodology. The standardized extraction of antioxidants requires processing the fresh lettuce leaves (1 g) for 1 min, mixing with 10 mL of acidified ethanol (0.5 or 1 % citric acid) maintaining this mixture with continuous shaking during 1 h at 2 °C. After that, a centrifugation at 8,000 rpm for 15 min is necessary to obtain a fist supernatant (antioxidant source). A second extraction step with acetone:water (70:30 v/v) improves the extraction yield. Extracts can be stored for 14 days at −20 °C without changes in its antioxidant capacity. For quantification, a mixture 1:4 of lettuce extract and DPPH (100 μM) must be incubated for 60 min at 2 °C in dark before reading at 517 nm. Expression of results in terms of mg ascorbic acid equivalents/100 g fresh weight is recommended in order to compare results from different studies. A kinetic study of the reaction between lettuce antioxidants and DPPH was adjusted to a biphasic first-order kinetic model and showed evidence of the presence of antioxidants of slow and rapid antioxidants in lettuce extracts.