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 CuBCA23-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{Cu}}\left( {\text{BCA}} \right)_{2}^{3 - } $$\end{document} complexes. The absorbance values at 558 nm (A558 nm) of the CuBCA23-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{Cu}}\left( {\text{BCA}} \right)_{2}^{3 - } $$\end{document} complexes obtained with juice samples were compared with A558 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 CuBCA23-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{Cu}}\left( {\text{BCA}} \right)_{2}^{3 - } $$\end{document} 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 CuBCA23-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{Cu}}\left( {\text{BCA}} \right)_{2}^{3 - } $$\end{document} complexes) and the TPC. While other fruit juices showed good agreement of TRC values with CuBCA23-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{Cu}}\left( {\text{BCA}} \right)_{2}^{3 - } $$\end{document} 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.