Copper Complexing Capacity and Trace Metal Content in Common and Balsamic Vinegars: Impact of Organic Matter

Abstract

Complex formation is among the mechanisms affecting metal bioaccessibility. Hence, the quantification of organic metal complexation in food items is of interest. Organic ligands in solutions of environmental and/or food origin function as buffering agents against small changes in dissolved metal concentrations, being able to maintain free metal ion concentrations below the toxicity threshold. Organic matter in vinegars consists of bioactive compounds, such as polyphenols, Maillard reaction endproducts, etc., capable of complexing metal ions. Furthermore, transition metal ions are considered as micronutrients essential for living organisms exerting a crucial role in metabolic processes. In this study, differential pulse anodic stripping voltammetry (DPASV), a sensitive electrochemical technique considered to be a powerful tool for the study of metal speciation, was applied for the first time in vinegar samples. The concentrations of Cu complexing ligands (L_T) in 43 vinegars retailed in Greece varied between 0.05 and 52 μM, with the highest median concentration determined in balsamic vinegars (14 μM), compared to that of common vinegars (0.86 μM). In 21% of the vinegar samples examined, L_T values were exceeded by the corresponding total Cu concentrations, indicating the presence of free Cu ion and/or bound within labile inorganic/organic complexes. Red grape balsamic vinegars exhibited the highest density of Cu ligands per mass unit of organic matter compared to other foodstuffs such as herbal infusions, coffee brews, and beers. Among the 16 metals determined in vinegars, Pb is of particular importance from a toxicological point of view, whereas further investigation is required regarding potential Rb biomagnification.

Keywords: copper complexation; organic ligands; trace metals; vinegars.

Figure 1

L_T and TCu concentrations (logarithmic scale) in vinegars (BR: balsamic red vinegar, n = 12; BRH: balsamic red vinegar with honey, n = 5; BW: balsamic white vinegar, n = 3; WR: wine red vinegar, n = 10; WW: wine white vinegar, n = 8; F: fruit vinegar, n = 5).

Figure 2

Plots of trace metals determined for all vinegar samples examined. The mean value is displayed with x. (BR: balsamic red vinegar; BRH: balsamic red vinegar with honey; BW: balsamic white vinegar; WR: wine red vinegar; WW: wine white vinegar; F: fruit vinegar).

Figure 2

Plots of trace metals determined for all vinegar samples examined. The mean value is displayed with x. (BR: balsamic red vinegar; BRH: balsamic red vinegar with honey; BW: balsamic white vinegar; WR: wine red vinegar; WW: wine white vinegar; F: fruit vinegar).

Figure 3

Correlation of L_T (μM) with organic carbon (OC) (mg L⁻¹) (A1) and total phenolics (TPC) (mg GAE L⁻¹) concentrations (A2) and correlation of L_T with pH values in common (B1) and balsamic vinegars (B2).

Figure 4

DPASV voltammograms of Cu ion titrations (A) and Ruzic–van den Berg plot (B) for the balsamic vinegar sample BR5.