White 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... 

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. 

Ribereau-Gayon et al. (1976) emphasized that, in phosphatoferric casse, the wine does not become turbid due to clusters of ferric phosphate molecules, which are rather small and remain in the colloidal state in a clear solution. Turbidity, i.e. white casse, occurs when proteins, positively charged at the pH of wine, neutralize the negative charge of these phosphatoferric clusters, making them hydrophobic and therefore insoluble. Flocculation can only occur under these conditions. 

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. 

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... 

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... 

AGPs also have a protective effect against protein casse in white wines (Pellerin et al., 1994 Waters et al., 1994). They are, however, less effective than yeast mannoproteins (Sections 3.7.1 and... 

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. 

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