Prevention of copper and iron turbidities in wine

Abstract

A search was conducted for a satisfactory substitute for potassium ferrocyanide in removing copper from wine to prevent formation of copper casse, at present the most important problem in wine stabilization. The most promising among the relatively nontoxic organic compounds tested was rubeanic acid. Cysteine and related sulfhydryl compounds and ascorbic acid precipitate excess copper, but further tests under winery conditions are needed. Aside from potassium ferrocyanide itself, the only other material that gave satisfactory results was an adsorption complex of potassium ferrocyanide and Prussian blue. Properly prepared and used—possibly with the addition of denatured protein—this complex would avoid contamination of the wine with ferrocyanide. Neither it nor rubeanic acid was as effective as the soluble ferrocyanide in removing iron from wine; but iron casses if troublesome might be dealt with by other means. The rest of the materials tested—a number of organic chelating agents, calcium phytate, phytic acid, chlorophyll, and a number of ionexchange resins—were unsatisfactory either because they did not remove copper adequately or because they had an adverse effect on wine flavor. The results suggested, however, that it might be feasible to develop a special cation-exchange resin for this purpose. The rate of formation and precipitation of copper rubeanate and of copper and iron ferrocyanides in wine was found to depend on the proportion of reagent used, the initial copper content, and the type of wine. Sodium ethylenediaminetetraacetate was found to sequester copper and iron as soluble metal chelates in wine, but it did not bind them tight enough to prevent casse formation in some wines; and since it leaves a residue in the wine it does not appear to offer a practical solution to the casse problem. Some tests were conducted on the corrosion of various metals with and without hard chrome-plating, when stored in wine under oxidizing and nonoxidizing conditions. The preliminary results indicated that such plating would reduce copper and iron pickup during wine processing and might be a practical means of salvaging corrodible winery equipment now in use.