Assessment of the reducing capacity of processed fruit juices with the copper(I)/4,4′-dicarboxy-2,2′-biquinoline complexes

An alternative method for quantification of the total reducing capacity (TRC) of processed ready-to-drink fruit juices (orange, grape, peach, mango, cashew, strawberry, apple and guava) is suggested. The spectrophotometric procedure is based on the reduction of Cu(II) to Cu(I) by antioxidants (present in the samples) in aqueous buffered solution (pH 7.0), containing 4,4′-dicarboxy-2,2′-biquinoline acid (H₂BCA), yielding the $\text{Cu}{\left(\text{BCA}\right)}_2^{3-}$ complexes. The absorbance values at 558 nm (A_558 nm) of the $\text{Cu}{\left(\text{BCA}\right)}_2^{3-}$ complexes obtained with juice samples were compared with A_558 nm values of the same complexes obtained with a standard ascorbic acid solution and used to quantify and express the reducing capacity of each sample. Regarding orange juices a positive relationship between the TRC values using the $\text{Cu}{\left(\text{BCA}\right)}_2^{3-}$ complexes and the labelled ascorbic acid (AA) content along with the total polyphenol content (TPC) was measured. Grape juices showed the best positive correlation was verified between the TRC (with the $\text{Cu}{\left(\text{BCA}\right)}_2^{3-}$ complexes) and the TPC. While other fruit juices showed good agreement of TRC values with $\text{Cu}{\left(\text{BCA}\right)}_2^{3-}$ complexes and DPPH reagent. The proposed method may be applied to measure the TRC of beers and wines and also for biological samples like serum and follicular fluid.