Method for quantification of antioxidant capacity of processed fruit juices exploring the formation of the Fe(II)/3-hydroxy-4-nitroso-2,7-naphthalenedisulfonic complex

Abstract

The formation of the Fe(NRS)34− complex was used in a modified spectrophotometric method to quantify the total antioxidant capacity (TAC) of processed fruit juice samples. The procedure is based on the reduction of Fe(III) to Fe(II) in aqueous buffered solution (Tris; pH 8.0) containing the 3-hydroxy-4-nitroso-2,7-naphthalenedisulfonic acid (H2NRS). The absorbance values at 730 nm (A730 nm) of the Fe(NRS)34− complex obtained with juice samples were compared with A730 nm values obtained with a standard ascorbic acid solution and then subsequently used to quantify and express the TAC of the samples. Good positive relationship between TAC values in the analysed samples (grape, orange, passion fruit, cashew, peach, and strawberry) was found for both the Fe(NRS)34− complex method (proposed method) and ABTS•+ radical-free method (used as reference method). The TAC values obtained with Fe(NRS)34− and ABTS•+ also showed a positive correlation with the Folin–Ciocalteu index values. Fructose, glucose, BHT, and BHA did not interfere in the TAC quantification of the processed fruit juice samples with the proposed method. However, ascorbic acid (natural or added as additive, E300) and its derivatives (E301, E302, E303, E315, and E316) did interfere positively. As a 10-fold dilution is required and the absorbance measurements are performed at 730 nm, the proposed method was not subject to colour interference from the tested samples. The procedure suggested here is simple, easy to perform, reproducible, does not use organic solvents, and does not need expensive equipment.