Introduction
Levels of iron needed to cause an instability problem in wines are intermediate between the very low concentrations of copper associated with copper casse formation and the relatively high concentration of calcium needed to give crystalline calcium L-tartrate deposits. Iron-induced precipitates in wine are often referred to as iron casse. As a simple rule-of-thumb, a concentration of about 6 mg/L iron will render a wine vulnerable to an iron casse problem (Rankine 1989). Two deposits resulting from iron contamination are recognised: a white (ferric phosphate) casse and a blue (ferric tannate) casse. The second of these is much less common than the first (Zoecklein et al. 1995).
Sources of iron
Instability problems resulting from iron contamination are now rare in the Australian industry compared with the on-going incidences of copper and calcium deposits in wines. This is the result of the widespread use of stainless steel in place of iron equipment and pipelines in modern wineries. However, the continued use of iron grape bins and hoppers in vineyards can be a source of iron contamination if these bins are inadequately coated to prevent contact of fruit and juice with the metal. Fortunately, iron contamination at this prefermentation stage is usually eliminated by adsorption of the metal on, and thus removal with, the yeast lees. Other sources are certain bentonites used as protein fining agents and possibly some filter pads. A low iron content is one of the criteria on which bentonite fining agents should be selected (Rankine 1989). In common with the tentative evidence associated with copper contamination from filtration media, documentation concerning wine contamination by iron in filter pads is sparse. Nevertheless, contamination at the latter stages of vinification, whether through fining agents or filtration media, is most likely to cause an instability problem. Rare and sporadic sources of contamination, even of an individual bottle by rust or iron objects, have been recorded.
Factors affecting iron instability
White casse formation is dependent upon the iron concentration, as well as the pH, redox potential, phosphate content and the presence of certain wine acids. As mentioned above, an iron concentration of about 6 mg/L is generally accepted as a level at which casse formation is a risk. However, because of the other variables involved in iron instability, cases have been investigated in which white wines with iron levels less than 6 mg/L have shown instability.
Iron in the Fe+++ (ferric) oxidation state is highly insoluble in the presence of phosphate, and it is ferric phosphate that is responsible for white casse instability. However, in wine, most of the iron is in the Fe++ oxidation state due to the normally low redox potential, and in this ferrous oxidation state it will not precipitate with phosphate. Thus, any change in the redox state of the wine, which results in a higher ratio of ferric to ferrous ions, may induce instability (Zoecklein et al. 1995).
Ferric phosphate casse is reported to occur only in the pH range 2.9 to 3.6 (Zoecklein et al. 1995). As the optimum pH for wines is in this range, it may not be a major variable in iron instability. Nevertheless, any operations, such as blending or acid adjustments, that take a wine into this pH range can cause precipitation if other critical factors are in place (Zoecklein et al. 1995).
Phosphate ions are naturally present in wines and usually at sufficient concentration to be not limiting to white casse formation. Any addition of diammonium phosphate to juice to aid fermentation will significantly augment the phosphate concentration in the resultant wine.
Citric acid will complex with iron rendering it unavailable for reaction with, and precipitation by, phosphate (Zoecklein et al. 1995). Accordingly, the natural level of citric acid and factors that may vary that level, such as MLF, can impinge on the availability of iron to give a white casse.
Physical appearance and characteristics of iron deposits
Most problems observed have been in white wines where the instability is seen as a white or grey floculate or haze: the so-called white casse. The haze can be isolated by centrifuging and is soluble in 0.1M HCl. It can be distinguished from protein haze by spot testing with nigrosine. Care may be needed in the interpretation of this spot test as an iron haze may commonly have some protein associated with it. Microscopic examination of an iron haze shows it as amorphous granular particles, similar to copper-protein complexes.
A diagnostic test for iron haze is the observation of an increase in turbidity in the affected wine on addition of hydrogen peroxide. This is due to the oxidation of iron in the ferrous state to the ferric state which then precipates with phosphate.
Iron instability in red wines manifests itself as ferric tannate and, like that of copper deposits, might occur more that it is reported because in bottled red wines it may not be observable or if seen, mis-diagnosed as tannin deposit.
Predictive tests and avoidance strategies
The presence of iron at a concentration above the generally accepted critical range of about 6 mg/L is a predictive indicator of a likely problem. Atomic Absorption Spectrometry is the method of choice for determining iron in wine. A colourimetric procedure using potassium thiocyanate is also available. At-risk wines can be tested by aeration or by the addition of hydrogen peroxide to a sample and holding at 0ºC for several days to observe any deposit formation.
As with copper and calcium instabilities, eliminating all sources of the metal contamination is the best strategy to avoid iron casse instability.
References:
Rankine, B.C. 1989. Making good wine – a manual of winemaking practice for Australia and New Zealand. Melbourne: Sun Books.
Zoecklein, B.W., Fugelsang, K.C., Gump, B.H., Nury, F.S. 1995. Wine analysis and production. New York: Chapman and Hall.