As we come to the close of the International Year of the Periodic Table, this column is a reflection on the metals in wine with which I have worked over the last 35 years or so. The column interfaces with ‘Elements of wine’ on page 37. The metals are discussed here in order of their atomic number.
In the early 1990s, there were claims that wine contained toxic levels of aluminium. A survey showed that white wine contained on average a higher Al concentration than red wine. Bentonite, used in white wine production to assist in achieving protein stability, was identified as a source of Al. The acidity of wine is such that the passive oxide layer on aluminium metal is removed, leading to its dissolution. The origin of the extremely high Al concentration in one wine was traced to the remains of an Al ladder that had been left in the tank after cleaning! Al in wine is tightly bound to anions of tartaric acid and citric acid, which raises the issue of its bioavailability. But interest in Al toxicity of wine faded, so we did not pursue the study any further.
The interest in potassium is linked to the precipitation of potassium hydrogen tartrate, which, when it occurs in bottle, can give the appearance to consumers of glass shards. While the precipitation process is fairly well understood, a simple routine analytical measurement of K was not readily available. So, we developed a procedure using a K ion selective electrode (ISE). However, the uptake by wineries was slow, with winemakers expressing concern that ‘an electrochemical method is not appropriate in wine laboratories’, undoubtedly without realising how pH is measured.
The precipitation of calcium L(+) tartrate is an insidious process, sometimes occurring after the wine is bottled and in the marketplace. Using a Ca ISE, we were able to show that molecular calcium tartrate exists in wine, adding to the complexity of establishing a simple precipitation model. Several wine components inhibit the precipitation process, the most efficient of which is the acidic polysaccharide rhamnogalacturonan I. Rhamnogalacturonan I is a component of pectin, but unfortunately it is broken down by pectolytic enzymes used in wine making. A range of other precipitation inhibitors are available commercially.
Iron contamination of wine is minimal, although traces are sufficient to cause oxidative processes, especially when light exposure occurs. Iron(III) tartrate is photoactive, which can lead to enhanced colouration in bottle, especially in clear or pale green bottles. Fe mediates the degradation of tartaric acid into glyoxylic acid that links catechin-type phenolic compounds, with consequent colour enhancement. Fe speciation, both chemical and oxidation state, remains a challenge to be resolved.
Copper is the one metal with which I have had a love–hate relationship over the last 35 years. I have written frequently about Cu in these columns and so will not repeat much here. Its chemistry is highly complex because it has the ability to mediate oxidative reactions and is associated with the removal and re-formation of sulfidic off-odours. Use of stripping potentiometry has been a boon in aiding our understanding because the direct probe into wine allows a direct measure of labile and non-labile Cu and identifies the importance of sulfide in influencing the labile concentration. Nanoparticle tracking analysis led us to an understanding of the frustrations of removing copper sulfide by membrane filtration. The next step is to relate our existing knowledge to the control of copper-induced spoilage.
One of the more fascinating issues that I have encountered was concern regarding a proposal to construct a crematorium in a major wine tourism region. It was suggested that mercury from dental amalgams would be vented to the atmosphere and then taken up by grapes. Putting aside the fact that the amount of Hg that might reach the atmosphere would be negligible, we were able to show that Hg in grape juice was readily removed by yeast cells during fermentation, resulting in a concentration below the level of detection. This was sufficient to calm the concern.
Lead is another metal about which there is considerable emotion in terms of its potential toxicity. The concentration of Pb in wine nowadays is very low, considerably lower than when lead solder or lead lining was used in wineries and miniscule in comparison to Roman times when lead acetate was used as a sweetener. We utilised stripping potentiometry with medium exchange to examine both the total Pb concentration and its speciation. This technique showed that there was essentially no labile lead and the binder was identified as the acidic polysaccharide rhamnogalacturonan. Using classical complexation capacity plots, we determined the binding constant for the Pb/rhamnogalacturonan II reaction. Plots for actual wine samples resembled those for model systems, confirming the significance of this interaction and underscoring its significance in addressing the bioavailability of lead.
I have not given any references here, but would be happy to provide details of the issues that I have discussed. I also acknowledge the contribution of the many co-workers and postgraduate students with whom I have worked over the years.