Ferric casse

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. 

Phosphorus is naturally present in wine in both organic and inorganic forms. Ferric casse in white wine, known as white casse , is caused by ferric... 

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). 

The (Fe +)/(Fe +) ratio in wine depends on storage conditions, especially the free sulfur dioxide concentration. For this reason, wine is more susceptible to ferric casse after aeration, as this increases the proportion of the Fe + form responsible for this phenomenon. 

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. 

Ferric casse occurs in white wines due to the formation of an unstable colloid resulting from a reaction between Fe " " ions and phosphoric acid (white casse). This colloid then flocculates and precipitates, in a reaction involving proteins. 

Acidity has a complex effect on ferric casse, not only due to the quantity of acids but also... 

These ferric colloids are less soluble at low temperatures, which tends to facilitate casse, especially in winter. For example, a wine may be aerated at 20°C without developing turbidity, while slight turbidity occurs at 15°C and serious ferric casse at 10°C. 

It was easy to devise a test to assess the risk of ferric casse and the effectiveness of various treatments. A clear glass bottle, half filled with wine, is injected with oxygen. It is corked, agitated and placed, cork down, in a refrigerator in the dark. A wine highly susceptible to casse will become turbid within 48 hours. If the wine stays clear for a week, it will not suffer from ferric casse. 

In view of the ferric casse mechanism (Figure 4.1), there are various treatment processes based on different principles ... 

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. 

Citric acid treatment to prevent ferric casse may be reinforced by adding gum arabic, which acts as a protective colloid (Section 9.4.3). This is especially effective in preventing the flocculation of colloidal ferric phosphate. The doses of gum arabic generally nsed are on the order of 5-20 g/hl. This additive is available in aqueous solutions, at concentrations ranging from 15-30%. Gum arabic must be used in perfectly clear wines that are ready for bottling. It not only stabilizes clarity, but also turbidity, and has a very high capacity for fouling filter surfaces. 

The use of standard doses, for example 10 g/hl to treat ferric casse and 3 g/hl for copper casse, is... 

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. 

Calcium phytate is an efficient treatment for ferric casse in white, but above all red wines. Its effectiveness may be enhanced by adding citric acid or gum arabic (Section 4.6.3). If the above procedure is properly implemented, no residue is left in the wine, and so there can be no objections on health grounds. 

Copper is in an oxidized state, divalent Cu +, in aerated wines. However, when white wines are kept in the absence of air and the oxidation-reduction potential reaches a sufficiently low level, the copper is reduced to Cu+ in the presence of sulfur dioxide. This is likely to cause turbidity at concentrations of around 1 mg/1. Unlike ferric casse, copper casse develops after a long period of aging in the absence of air, at high temperatures and in bright light. It may disappear in contact with air. 

Zinc concentrations in wines range from 0.14 to 4 mg/1. Prolonged maceration of grape solids leads to an increase in zinc concentrations. The use of potassium ferrocyanide to treat ferric casse reduces a wine s zinc content (Table 4.3). 

Metallic precipitation (ferric casse and copper casse)... 

Gnm arable is less effective in preventing ferric casse in white wines. Indeed, the nnstable colloidal ferric phosphate that is precipitated has a much greater mass than the copper snlfide involved in copper casse. A much larger quantity of gum arable would therefore be required to provide proper treatment, and this is likely to affect the wine s turbidity. Gum arable is effective to a certain extent, but the effect is variable from one wine to another and is, in any case, insnfficient to provide total protection. Recommended doses range from 20 to 25 g/hl as a supplementary treatment (Section 4.6.3). 

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). 

In view of the involvement of a protein snpport in the colloid floccnlation occurring in copper casse in white wine, bentonite may be nsed to treat this problem (Section 4.7.3), provided that the copper concentration does not exceed 1 mg/1. The same is not tme, however, of ferric casse (Section 4.6.2) as proteins are not involved, so bentonite is ineffective. 

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. 

Cation exchangers, which are likely to improve tartrate stability by removing K+ and Ca +, acidify wine by adding H" and, possibly, prevent ferric casse by reducing Fe +. 

---