Copper casse

The mechanism of copper casse and the factors influencing its development in wines have been investigated by Joslyn and Lukton (18, 27), Kean and Marsh (15, 16,17), and Peterson et al. (28). 

In heavily sulfited white wines containing over 0.5 ppm copper and stored in sealed containers, a reddish-brown deposit may form. This occurs in the absence of oxygen and ferric ions but redissolves readily upon exposure to oxygen. Its formation may be accelerated by exposure to sunlight or heat, and it is believed to consist of colloidal cupric sulfide (14, 29). More commonly, copper casse may arise from reactions between copper and sulfur-containing amino acids, peptides, and proteins (15,16,17). 

Lukton, A., Joslyn, M. A., Mechanism of Copper Casse Formation in White... 

Table Wine. I. Relation of Changes in Redox Potential to Copper Casse, Food Res. (1956) 21, 384-396.

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. 

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

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. 

The prolonged aging of wine on its yeast lees causes a significant decrease in oxidation-reduction potential, which favors the reduction of copper and, consequently, the appearance of copper casse. At the same time, the presence of yeast lees promotes the fixing of copper, which tends to prevent copper casse. For example, a copper concentration of 0.1-0.3 mg/1 in a champagne-base wine dropped to zero after second fermentation and aging in the bottle in a horizontal position, as this increased the lees/wine interface and promoted exchanges. 

Once the mechanism of copper casse had been elucidated, a test was developed for predicting this type of instability. White wine in full clear glass bottles is exposed to sunlight or ultraviolet radiation (Section 4.7.3) for seven days. If it does not become tnrbid nnder these conditions, it will remain clear dnring aging and storage. Copper casse also develops after three to fonr weeks in an oven at 30°C. 

Copper casse is a serious problem because it may occur when the wine has been in the bottle for a long time. Affected bottles must be uncorked, the wine treated and then re-bottled. 

Copper casse is specific to white wines. They are not as well protected from oxidation and reduction phenomena as red wines, where phenols have a redox buffer capacity. Furthermore, the colloidal cupric derivative contains proteins, while red wines have a low protein content due to combination reactions with phenols. 

Bentonite treatment (Section 10.9.3) is a simple method for protecting wines from copper casse by eliminating proteins. Gum arabic also has a protective effect, by preventing flocculation of the colloid (Section 9.4.3). This method is effective if the copper concentration is below 1 mg/1, otherwise the excess copper must be eliminated. 

Proteins in must are a well-known cause of instability, affecting the clarity of white wines. When they precipitate, they cause protein casse , reported by Laborde as early as 1904. For many years, this was confused with white casse or copper casse . The turbidity or deposits characteristic of protein casse appear in the bottle, usually when wines are stored at high temperatures. They may also occur when tannin is leached into wine from the cork. Tartrate crystallization and flocculated proteins are responsible for the main problems with clarity in bottled white wines. 

The oxidation-reduction potential of white wine decreases on exposure to natural light. This property is used to reduce copper and assess the risk of copper casse (Section 4.7.3). In the past, there was even a method of preventing copper... 

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

Gum arable is a preventive treatment for many problems involving colloidal precipitation. It is effective in treating copper casse and was widely used when wines often contained excessive amounts of copper due to contact with bronze cellar equipment (Section 4.7.3). Doses of 10-15 g/hl were effective in preventing this problem, provided that wines did not contain more than 1.0 mg/1... 

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). 

Bentonite 20-100 g/hl Average clarification. Treats and prevents protein and copper casse. Facilitates racking with proteins. Avoids overfining... 

Sodium bentonite is most frequently used to treat wines. The flakes are more widely spaced (100 A) than those of calcium bentonite (10 A), so they swell more in wine and have a higher protein adsorption capacity. Sodium bentonite flakes are relatively difficult to mix into suspension in water, but the suspensions have a very stable colloidal character. When added to wine, they produce flocculation and settle out as a flaky deposit, leaving a clear liquid. The natural proteins are completely eliminated and the wine is protected from protein (Section 5.6.2) and copper casse (Section 4.7.3). 

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. 

Preventing Certain Types of Colloidal Precipitation Protein Casse and Copper Casse... 

The same treatment also provides protection from copper casse, by reducing the Cu in the form of colloidal copper sulfide. It is then eliminated by fining and filtration. More intense heating is required, e.g. 75°C for 2 hours, for wines containing 1.5 g/1 of copper. 

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... 

---