Characterization of zinc chelating compounds in instant coffee.

The complex formation of zinc with constituents in instant coffee was studied by complexometry, using the modified tetramethyl murexide (TMM) method. The activity of Zn2+ binding was measured in a pH 5.0 hexamine buffer containing 1 mM ZnCl2. Instant coffee to which Zn2+ had been added was separated into three color components by Sephadex G-25 gel permeation chromatography. Among these components, the second was the strongest in Zn2+ binding activity. Instant coffee with Zn2+ was also chromatographed on an anion exchange column of Amberlite IRA-410 (OH-), the zinc adsorbed on the column being eluted with 0.01 M acetic acid. A mixture of zinc chloride and coffee was subjected to paper electrophoresis, and Zn2+ moved toward the anode at pH 8.5 and pH 6.4. Even at pH 3.2, some compounds carrying Zn2+ moved toward the anode. It follows that the ligands in instant coffee to bind Zn2+ are acidic in nature and that their molecular size is less than 5, 000.