Iron in wines

Iron. Excess iron in wines causes cloudiness, interferes with the color, and can impair flavor. The mechanism of ferric phosphate precipitation has been intensively studied, and numerous colorimetric methods have been developed. For routine purposes the color developed with thiocyanate is adequate (6,9), but many enologists prefer the orthophenanthro-line procedures (3, 4, 6, 22). Meredith et al. (Ill) obtained essentially the same results for iron using 2,4,6-tripyridyl-s-triazine (TPTZ) to develop the color. Atomic absorption spectrophotometry can be used but, as with copper, corrections for reducing sugar and ethanol are necessary (51). 

Melarsen (211) is active against both early and late stages of African trypanosomiasis. The antimony derivative (212) is used similarly (B-61MI22000, p. 715). 2,4,6-Tripyridyl-l,3,5-triazine forms a purple bis(triazinyl) complex with iron(III) ions which provides a spec-trophotometric method for estimating iron in wines (59MI22001). 

E. K. Paleologos, D. L. Giokas, S. M. Tzouwara-Karayanni, M. I. Karayannis, Micelle mediated methodology for the determination of free and bound iron in wines by flame atomic absorption spectrometry, Anal. Chim. Acta, 458 (2002), 241-248. 

The normal oxidation-reduction potential Eq of this redox couple in relation to the hydrogen electrode is 771 mV. The oxidation-reduction potential of still wines, even when young, is often much lower, around 500 mV. This value explains why iron is present in both ferrous and ferric forms. If all the iron in wine were in ion form, the potential would be higher. It is obvious that much of the iron is involved in complexes, and is thus more difficult to identify. 

As regards the redox potential, it is quite predictable that iron in wine is not totally in ion form. Part of the iron is involved in soluble complexes with organic acids, especially citric acid. Ferric iron is much more likely to form complexes than ferrous iron. Ferric and ferrous iron, expressed as Fe and Fe , constitute total iron, in both ions and complexes, i.e. non-reactive forms. A total iron assay therefore requires the complete destruction of these complexes by acidification. The use of potassium thiocyanate, a specific reagent for ferric... 

Oszmianski, J., Cheynier, C., and Moutounet, M., Iron-catalyzed oxidation of (+)-catechin in wine-like model solutions. J. Agric. Food Chem. 44, 1972, 1996. 

The earliest recorded history of the use of iron in the treatment of human disease dates to 1500 B.C. (7). Legend has it (2) that Prince Iphyclus, of Thesally, was cured of his sexual impotence by Melampus, a physician and seer. Melampus removed a knife from an oak tree where it had been stuck by Iphyclus father, scraped the rust from the blade into wine and administered the beverage to Iphyclus. After ten days of this treatment Iphyclus regained his fertility. 

Danilewicz, J. C. (2003). Review of reaction mechanisms of oxygen and proposed intermediate reduction products in wine Central role of iron and copper. Am. J. Enol. Vitic. 54, 73-85. 

Laurie, V. F. and Waterhouse, A. L. (2006b). Oxidation of glycerol in the presence of hydrogen peroxide and iron in model solutions and wine. Potential effects on wine color. J. Agric. Food Ghent. 54, 4668-4673. 

Iron and copper in wines may form complexes with other components to produce deposits or clouds in white wines. Iron clouds generally occur at a pH range from 2.9 to 3.6 and are often controlled by adding citric acid to the wines (2). Copper clouds appear in wines when high levels of copper and sulfur dioxide exist and are a combination of sediments, protein-tannin, copper-protein, and copper-sulfur complexes (169). Further, the browning rate of white wines increases in the presence of copper and iron (143). The results of this study indicate that iron increased the browning rate more than copper. 

Potassium ferrocyanide has long been used to remove excess copper and iron from wines. When not used in excess it appears effective and harmless, but if any ferrocyanide residue remains, cyanide may form. While the amounts produced by a slight excess would pose little danger, the blue precipitate and distinctive odor would be undesirable. Special equipment has been devised to detect free cyanide and ferrocyanide (4, 5, 6) as Prussian blue. The suggested limit is 1 mg/liter as cyanide. Hoppe and Romminger (72) devised a rapid procedure for free and bound cyanide, and Bates (73) gives a qualitative screening method sensitive to 0.05 mg/liter. 

Ajlec, R. and Stupar, J. (1989) Determination of iron species in wine by ion-exchange chromatography-flame atomic absorption spectrometry. Analyst, 114, 137-142. 

Several minerals and trace elements are vital to the human organism and must be ingested with daily food in sufficient amounts. Wine can contribute not only minerals containing potassium, calcium, and magnesium but also numerous essential trace elements, such as chromium, cobalt, iron, fluorine, copper, selenium, and zinc, among others. The contents found in wine are very low ranging from mg L 1 to igL, sometimes even lower. 

Digestion of liquid samples may not be necessary. Analyses of a distilled liquor, beer and wines have been reported [25, 36, 169, 208]. Determination of wines by direct aspiration after filtration if required, comparing absorbances to those of standards prepared in a synthetic solution simulating the wine matrix has been described by Ecrement [36] and is outlined in note 5 to Section IV.B.l. Iron in orange juice has been determined following hydrolysis with HN03 [174]. 

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