Introduction
Copper instabilities are one of the most common metal instabilities found in wine. Wines with levels of copper above 0.5 mg/L are strongly susceptible to instability (Rankine 1989). The precipitated deposit or haze from a copper unstable wine is commonly referred to as copper casse.
Sources of copper in wines
The three major sources of copper in wine are:
- vineyard sprays
- winery equipment
- use of copper salts as fining agents to remove reduced sulfur off odours (Zoecklein et al. 1995).
The first of these is typified by the use of Bordeaux mixture (a copper-based vineyard spray), which can contribute very high levels of copper to a must (in excess of 20 mg/L in two cases investigated at the Institute). Fortunately, fermentation usually eliminates much of this through binding of the metal to yeast and removal with the lees.
The second source is contact of a wine with copper or copper alloys during winemaking. It seems possible that in some wineries the presence of a few unprotected copper or brass fittings may be deliberate to help remove any hydrogen sulfide and related off-odours from wines during the vinification process. In general, however, both of these sources are of diminishing importance as contributors to copper in present day winemaking practices, with the availability of alternative antifungal sprays to Bordeaux mixture and the widespread use of stainless steel for vintage equipment.
The third source, the addition of copper sulfate to correct sulfidic off-odours, is arguably the major cause of copper instability. It appears that as long as reduced sulfur off-odours are a feature of some fermentations, copper instability will be an enduring hazard for winemakers. Many of the cases of copper casse investigated at the Institute have been due to the addition of copper sulfate immediately before bottling, often as ‘protection’ against a sulfide problem that might occur.
A further, and as yet, unconfirmed source of copper contamination is from filter pads. It is unclear whether some filters at purchase have an abnormally high level of copper ions present in the filter medium or if copper ions are deposited on the pads during the pre-filtration preparation. The latter source of copper may be from a hot water service used to supply the pre-wash water for the filter pads. As yet, the evidence implicating filtration equipment as a source of copper contamination is indirect and deduced from observations of relatively high levels of copper in samples taken early in a bottling run but lower in wines bottled later in the run and in tank samples from the winery. Until these circumstances are confirmed, winemakers are cautioned to be aware of the possibility of uncontrolled copper addition to wines from filters.
Appearance of copper deposits and characteristics of the instability
Investigations of copper-induced instability have almost exclusively concerned bottled white wines in which the problem was evident either as a cloud, haze or as a solid deposit. Just as calcium L-tartrate precipitates are often not reported in red wines, the incidences of copper precipitation from red wines may be greater than is recognised or reported. Oak wood storage of red wines, often under near or totally anaerobic conditions and for prolonged periods, provides an opportunity for a potential copper instability problem to be avoided through precipitation in the barrel. Other instances of copper instability in bottled red wines may go unrecognised because the deposit is not evident in dark bottles or it may be passed off as a polymeric phenolic deposit with which it has co-precipitated.
After isolation of a copper deposit by decantation or centrifugation, microscopic examination shows it as amorphous granular particles, often with bright spherical inclusions. Deposits occur in a wide variety of colours which have been variously described as pale yellowish, yellow-green, pale green, grey or off-white, cream and either light or dark brown. They are usually readily soluble in 0.1M NaOH. The deposit is often a copper-protein complex and whilst a positive test for protein (e.g. by nigrosine stain testing) can help in assigning this instability, experience shows that in many cases the isolated material may give only a weakly positive response to that test. There is an accumulation of evidence to indicate that copper (I), ie the cuprous oxidation state, is involved in the precipitate (Scollary 1987).
In only about half of all cases studied was the residual concentration of copper in the wine at or above 0.5 mg/L, which is that reported by some authors as likely to cause a copper instability problem. This can, however, be explained by the fact that the concentration of residual soluble copper will have decreased following the precipitation of the copper casse, and may be at a level only near to or even well below that which was critical.
Similarly, only a small proportion of the affected wines was protein unstable by heat testing.
Factors involved in copper casse formation
Wines at risk of copper instability are those that have experienced conditions likely to expose them to the sources of copper as discussed above. Of wines made vulnerable by these conditions, light bodied white wines are particularly at risk because they have insufficient time to self-stabilise in bulk storage before bottling. The reason for the deposit appearing after bottling is the establishment of reducing conditions in the wine at and after the bottling process. Empirically, it is known that such reducing conditions are highly conducive to the formation of copper casse although the mechanisms of, and even the species involved in, the precipitate are incompletely understood. Nevertheless, the maintenance of reducing conditions in bottled wines, through the absence of oxygen and the presence of sulfur dioxide and ascorbic acid all contribute to the precipitation and persistence of copper deposits in wines with a higher than critical concentration of copper.
Whilst the role of light is not understood, it too seems to be a trigger for copper casse formation if other factors are in place. UV light, including sunlight, may not be an absolute prerequisite for deposit formation, however, it seems to be important to the prevalence of copper casse problems in white wines because these are more commonly bottled in clear glass (Zoecklein et al. 1995).
Heat is also a causative factor, and wines that are subject to storage temperatures higher than optimal can throw a deposit if other conditions favouring copper instability are in place (Zoecklein et al. 1995).
Because of the common involvement of protein in copper casse problems, reducing the level of protein in wines can help in limiting the likelihood of copper instability. This is particularly important in wines vintaged in particular years or harvested under conditions likely to contribute high levels of protein. Careful attention to bentonite fining of these wines can help to alleviate a potential copper induced haze problem.
Predictive test for copper
The copper concentration of a wine is best determined in the winery laboratory by AAS. Other instrumental procedures are possible for the determination of copper but these are usually only available in specialist laboratories. A colourimetric method using diethyl-dithiocarbamate requires spectrophotometric determination of OD at 435nm for the assay.
Steps to minimise the likelihood of copper instability problems
In addition to eliminating sources of copper in the vineyard and from winemaking equipment, it is important to ensure that if copper sulfate additions are made, the amount added is the minimum necessary to remove any sulfide-type off odour. Small scale trials should be conducted to establish this minimum amount. Following copper sulfate additions, sufficient time should be allowed for the wine to self-stabilise in bulk storage. The protein concentration in the wine should be reduced by bentonite fining and normal sulfur dioxide levels should be maintained. Finally, if circumstances permit, bottling the wine in dark or green glass will also help to minimise the risk.
REFERENCES AND RESOURCES:
Boulton, R.B., Singleton, V.L., Bisson, L.F., Kunkee, R.E. 1996. Principles and practices of winemaking. New York: Chapman & Hall.
Coulter, A. 2023. Ask the AWRI: Understanding copper hazes in wine. Aust. N.Z. Grapegrower Winemaker (714): 78-79.
Rankine, B.C. 1989. Making good wine – a manual of winemaking practice for Australia and New Zealand. Melbourne: Sun Books.
Ribéreau-Gayon, P., Glories, Y., Maujean, A., Dubourdieu, D.Y. 2000. Handbook of Enology Volume 2: The Chemistry of Wine Stabilisation and Treatments. Chichester: John Wiley & Sons Ltd.
Scollary, G. 1987. Electrochemical methods for the determination of copper in table wines: identification of two forms of copper. Aust. Grapegrower Winemaker (280): 36, 38-39.
Zoecklein, B.W., Fugelsang, K.C., Gump, B.H., Nury, F.S. 1995. Wine analysis and production. New York: Chapman & Hall.