Iron casse

The effectiveness of ascorbic acid in the prevention of iron casse which is exclusively caused by Fe + ions is explained by the above reactions. 

Sulfur dioxide and ascorbic acid therefore have different antioxidant properties. The first has a delayed, but stable, effect which continues over time even in the presence of a subsequent oxygenation. It cannot prevent iron casse, which rapidly appears after an aeration. The second has an immediate effect it can instantaneously compensate the damage of an abrupt and intense aeration (iron casse), but it acts only as long as the wine is not in permanent contact with air. 

The aeration of wine oxidizes iron. The amount of ferric iron formed (several milligrams per liter) can be sufficient to induce iron casse. Protection against iron casse can be ensured if the wine receives 50-100 mg of ascorbic acid per liter beforehand (Table 9.7). 

Table 9.7. Protection from iron casse by the addition of ascorbic acid before aeration (Rib6reau-Gayon et al 1977)...

Ascorbic acid effectively protects against iron casse, which can occur after operations that place wine in contact with air, such as pumping-over, transfers, filtering and especially bottling. In the same conditions, sulfur dioxide acts too slowly to block the oxidation of iron. But, if the wine must be aerated again after a treatment following a first aeration, the ascorbic acid no longer protects the wine. When a wine that has received 100 mg of... 

Table 9.8. Reduction of iron casse by addition of ascorbic acid, 48 h after the aeration cansing the haze (Riberean-Gayon et al., 1977)...

VDN are subject to the same clarification and stabilization problems as other wines. Iron casse, proteic casse, tartrate deposits and colored matter can cloud the wines. Standard preventive measmes can help to avoid these problems. Oxidasic casse is another accident linked to grape rot. 

Hua et al. [595] have described an automated flow system for the constant-current reduction of uranium (VI) onto a mercury film-coated fibre electrode. Interference from iron (III) was eliminated by addition of sulfite. The results obtained for uranium (VI) in two reference seawater samples, NASS-1 and CASS-1, were 2.90 and 2.68 g/1, with standard deviations of 0.57 and 0.75 g/1, respectively. 

Solomon, P. A., S. M. Larson, T. Fall, and G. R. Cass, Basinwide Nitric Acid and Related Species Concentrations Observed during the Claremont Nitrogen Species Comparison Study, Atmos. Em -iron., 22, 1587-1594 (1988). 

Before the wine is bottled, it must be rendered stable to qualitydegrading changes in the bottle. Malic acid stability has been discussed already. Other changes that the wine must be stabilized against are precipiation of cream of tartar, unstable color deposits, iron and copper casse, oxidation, and, of course, microbiological breakdown. 

A very successful mediator-chemically modified electrode (MCME) has been developed by using ferrocene derivatives (Cass et al., 1984). Ferrocene (bis-cyclopentadienyl iron) and its derivatives (FecpR) combine the good electrochemical behaviour of ferrocyanide with the possi-... 

Ferric casse is also affected by pH. Indeed, iron has a degree of oxidation of three and prodnces soluble complexes with molecules such as citric acid. These complexes are destabilized by increasing pH to prodnce insolnble salts, snch as ferric phosphates (see white casse ) or even ferric hydroxide, Fe(OH)3. 

Iron and copper are present in small qnantities, bnt they are significant causes of instability (ferric casse and copper casse), so they are described in separate sections (Sections 4.6 and 4.7). Heavy metals, mainly lead, even in trace amounts, also affect toxicity and deserve a separate description (Section 4.8). 

Wine always contains a few mg/1 of iron. A small percentage comes from grapes (2 to 5 mg/1). The rest comes from soil on the grapes, metal winemaking, handling and transportation equipment, as well as improperly coated concrete vats. The general use of stainless steel has considerably reduced the risk of excess iron and, consequently, of ferric casse. 

Besides the iron concentration, a wine s oxidation-reduction state and the possible presence of oxidation catalysts are also involved. The quantity of soluble complexes with organic acids, i.e. iron that is not involved in the casse mechanism, is also significant. This factor is, however, impossible to measure. 

Precipitating the iron through deliberate ferric casse caused by oxygenation. This process is too brutal and affects wine quality. It is no longer used. 

Citric acid solnbilizes iron, forming soluble iron citrate. Citric acid is an anthorized additive at doses np to 0.5 g/1. The total concentration must never exceed 1 g/1. This treatment may only be envisaged for wines that have been sufficiently sulfured to protect them from bacterial activity that would otherwise break down the citric acid, producing volatile acidity. In practice, this treatment is used exclusively for white wines that are not very susceptible to ferric casse (with no more than 15 mg/1 of iron) and that will not be damaged by this acidification. Doses of 20-30 g/hl are usually sufficient. 

Phytic acid (Figure 4.3) is the hexaphosphoric ester of me o-inositol. The affinity of ferric iron for phosphoric anions, already described in connection with the ferric casse mechanism, is responsible for calcium phytate s effectiveness in eliminating iron from red wines. Under these conditions, phytic acid produces a mixed calcium-iron salt, known as Calciphos, with the following composition Ca, 20%, P, 14% and Fe +, 2%. This mixed salt is not very soluble in water and easily precipitates, thus eliminating the excess ferric iron. Phytic acid is very widespread in plants. It acts as a phosphorus reserve, located in the seed coat, i.e. in wheat, rice and corn bran. Wheat bran may be used directly to eliminate iron from wine. 

Gum arable is also at least partially effective as a treatment for ferric casse in red wines. It does not prevent the appearance of a dark, blnish color, due to the formation of colloidal ferric tannate, but it does stop the colloid flocculating. It acts differently from citric acid, which prevents color from changing, as it produces a soluble complex with iron that is no longer capable of reacting with tannins (Section 4.6.2). These two treatments are often complementary (Section 4.6.3). 

Refrigeration is widely used for stabilization to prevent tartrate precipitation. This treatment alone may be adequate to ensure the stability of red wines. However, if bentonite is not used to treat white wines, prior heating is necessary to prevent protein precipitation. In some countries, equipment capable of applying both heat and cold is used. These processes give satisfactory results in stabilizing white wine with a low iron content, as they have only a limited effect on ferric casse. 

Cold stabilization is also partially effective in preventing other types of colloidal precipitation. It helps to prevent ferric casse by insolubilizing ferric phosphate in white wines and ferric tannate in reds. However, even after aeration to promote the formation of the Fe + ions involved in these mechanisms, only small quantities of iron are eliminated. Fining at the same time as cold stabilization improves treatment effectiveness but is never sufficient to prevent ferric casse completely. 

When microscopically examined, this group of precipitates lack defined shape and generally assumes color reflective of the wine. Precipitates in this category include protein, phenolics (and complexes of the two), copper, and iron instabilities (casses) as well as paraffin used to coat corks. 

Metal instability, described as casse, is relatively rare today. When encountered, the metals involved are generally copper and iron. The latter may be present as either ferric phosphate ( white casse) or ferric tannate ( blue casse). Even though ferric phosphate casse is described as white casse, it may assume various shades of blue even in white wines (Toland, 1996 personal communication). Copper casse is present as an initially white and later reddish-brown precipitate in bottled or other wines stored... 

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