Effect of copper on the color of grape anthocyanins in a similar wine solution

The influence of a number of metals (K, Ca, Sr, Ba, Mn, Co, Ni, Cu, Zn, Cd, Hg, Al, Fe2+, Fe3+, Mg) in the color of anthocyanins is known from long time. Unfortunately, the ability to form such complexes is related to an ortho-dihydroxyl arrangement on the B ring, so that while cyanidin, delphinidin, and petunidina can form them, those of malvidin, pelargonidin and peonidin cannot. Since malvidin 3-glucoside is the major anthocyanin in most vinifera grapes, it is unlikely that pigment-metal complexes play any significant role in the color of wines The objective of this work was to study the possibility of metals of changing the pigments color in different grape varieties. 3 replications of 20 skins of Cabernet Sauvignon (typical malvidin rich profile) and Sangiovese (Italian variety with a relatively high amount of ortho-dihydroxylated pigments) were extracted in a similar wine solution (3,5 g/L Tartaric acid, 100 mg/L Sodium Metabisulphite, 12.5% Ethanol, buffered at pH 3,2 using NaOH). 3 ml of extract were reacted with 1 ml of 4 different dilutions of cupric sulfate. All the controls were made with dilution with water, or reaction with similar wine. The addition of the salt showed a strong hyperchromic effect (color increase: about 500% in the highest cupric sulfate concentration) in both the grape varieties. No particular bathochromic effect was underlined. More studies should be done to better evaluate the role of metals on the colors of grapes and wines, considering that many cations involved in this kind of complexation are contained in significant amounts.